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Plain language definitions of electrical terms:
Definition of amps, volts, watts, resistance, current, ohms, electrical phases. We include basic formulas relating amps, volts, resistance, watts, and we explain what these electrical terms mean in practical applications such as for building or appliance electrical power, electrical wiring, and basic troubleshooting.
Photographs and sketches in this article illustrate and help explain concepts and definitions of electrical voltage, electrical resistance, and other electrical wiring concepts.
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In most places in the world, electrical service brought to a building is at either of two voltage levels: 240V or 120V.
These numbers are "nominal," meaning that the actual voltage may be vary. Most modern buildings receive 240V service, a total achieved by the provision of two individual 120V incoming power lines as we discuss below.
Older buildings and electrical services often delivered only 120V. Knowing which voltage level is available is important, but knowing the voltage alone does not indicate the amount of electrical power available inside a building.
For that we need to know both the service voltage at a building, and the service amperage (typically 100A or larger, but historically, 30A, 60A, 100A, 125A, and more recently 150A, or 200A depending on the power requirements at a building). Don't confuse service VOLTS (120/240 V) with service AMPS or WATTS - those terms are discussed next.
Voltage = Current x Resistance
Current = Voltage / Resistance
Resistance = Voltage / Current
Watts = Volts x Amps
Speaking practically, the voltage level provided by an electrical service, combined with the ampacity rating of the service panel determine how much electrical demand, or in another sense how many electrical devices can be run at one time in the building. Sketch courtesy of Carson Dunlop Associates.
Branch circuit wire sizes and fusing or circuit breakers used set the limit on the total electrical load or the number of electrical devices that can be run at once on a given circuit.
If you have a 100 Amp current flow rate available, you could, speaking very roughly, run ten 10 amp electric heaters simultaneously. If you have only 60A available, you won't be able to run more than 6 such heaters without risk of overheating wiring, causing a fire, tripping a circuit breaker or blowing a fuse.
Also see AMPS MEASUREMENT METHODS for a description of using a clamp-on ammeter to make actual electrical usage measurements.
Ampacity, in the electrical code, refers to the current, measured in amperes, that a conductor (a wire) can carry continuously under the conditions of use without exceeding its temperature rating - in other words, the ampacity of a #14 gauge copper wire intended for residential electrical wiring is 15 Amps because that's the amount of current that the wire can carry without getting too hot.
"Too hot" means a temperature that could damage the wire insulation and thus reduce its safety.
What does 20 amps actually mean - Mary Riccio
In a residential electrical circuit, a 20-amp rated circuit means that the electrical wire and it's overcurrent protection (fuse or circuit breaker) are rated for a total load or total current draw of 20 amps. The sum of all of the electrical current drawn by everything connected to that circuit must be 20-Amps or less - else the circuit breaker will trip or fuse will blow to prevent overheating of the wire (and a fire hazard).
The definition of AMPS or ampacity and other electrical terms can be found at InspectAPedia either by using the search box found at the top and bottom of our pages to search for DEFINITION of AMPS
Quoting from "DEFINITION of AMPS Electrical Current",
Amperage or Amps provided by an electrical service is the flow rate of "electrical current" that is available. Mathematically, Amps = Watts / Volts. (Amps = Watts divided by Volts)
Speaking practically, the voltage level provided by an electrical service, combined with the ampacity rating of the service panel determine how much electrical demand, or in another sense how many electrical devices can be run at one time in the building.
Depending on what you are looking at, 20-Amps may be the rating of a particular piece of equipment.
For example a 20-Amp circuit breaker means that that safety device will limit to 20 Amps the current drawn on the circuit that it is protecting.
If you plug three 10-amp electric heaters into that circuit and they all are running simultaneously, their combined current draw (3 x 10 = 30Amps) should trip the circuit breaker and turn off the circuit to protect it from overheating and a possible fire.
This definition is not very helpful to consumers. Using a water-in-pipes analogy, volts is analogous to water "pressure" in the electrical system. Having higher "pressure" in a pipe (or electrical conductor) means that conductor is capable of delivering more energy to the user.
Later in this article we further explain ELECTRICAL POTENTIAL.
Just as a 10 gpm flow rate of water through a pipe provides half the amount of water as a 20 gpm flow rate, 10 amps of current in a conductor provides half the energy as 20 amps of current.
AMPS,as we discussed above, is a measure total current flow (or "gallons per minute" or "gpm" using the popular water analogy) available from an electrical service.
A ten-amp 240V electrical service is capable of delivering, speaking roughly, twice the energy to the end-user than a ten-amp 120V electrical service. So volts is a measure of the strength of an electrical source at a given current or amperage level.
If we bring 100A into a building at 240V, we have twice as much power available as if we bring in 100A at 120V. Volts (continuing the same water analogy) is the "pressure" in an individual electrical conductor.
Some people explain volts as similar to water pressure in a pipe, and amps as water current or total quantity flow. We discuss volts and amps below and in detail at this website. Sketch above courtesy of Carson Dunlop Associates.
The total current ("gpm") that will flow through a conductor is doubled if the pressure is doubled. Twice the power or energy can be delivered on a # 12 conductor by doubling the voltage and holding the current to 20 amps. Doubling voltage and also doubling the amperage will deliver four times the power or energy.
Why we need circuit breakers or fuses: In either case, if we exceed the current rating of an electrical wire, it will get hot, risking a fire. That's why we use fuse devices (or modern circuit breakers), to limit the current flow on electrical conductors to a safe level to avoid overheating and fires. - thanks to Louis Babin for technical review and edits to this text.
Is "240V" really exactly 240 Volts?: Don't expect a "240V" circuit to provide that exact voltage level. We've already said that "120V" and "240V" are "nominal" ratings, meaning that the actual numbers may vary. In a three phase circuit, even if you are using only two phases, the voltage between the phases is 1.732 x 120 = 207.6, or approximately 207 Volts and not 240 Volts
. In various countries the actual voltage level varies around the nominal delivered "voltage rating" and in fact depending on the quality of electrical power delivered on a particular service, voltage will also vary continuously around its actual rating.
Most (but not all) modern electrical equipment can handle small voltage variations and differences without a problem. Sensitive electronic equipment may require that a voltage stabilizer be installed. For example a "240V" appliance can usually handle "208V" just fine.
The technical detail of how "240V" (or 207V if you prefer) is actually delivered to a building may be a bit confusing, so let's follow this carefully. In fact 240V delivered to a building does not mean that the individual service drop wires are carrying that voltage. Rather, 240V in the building is obtained as follows: the two "hot legs" are on different electrical phases provided by a step-down transformer at a neighborhood utility pole or box. Each service conductor on its own phase delivers 120V to the building.
The two (in this case) phases are arranged so that connecting a circuit across the two "hot legs" produces "240V" in for that circuit. An electrician or engineer, trained in safe volt-ohm meter (VOM) or digital multimeter (DMM) use can easily demonstrate this fact.
Connecting a voltmeter from either incoming service conductor to ground will display 120V, and connecting a voltmeter across the two incoming 120V service conductors will display approximately 208V or 240V depending on just how the supplying transformer is designed.
Technical detail about phases of electrical power: Three phase power with the star (Y-connected) connected secondary and the neutral grounded, you get 208 Volts line-to-line, and 120 Volts line-to-neutral.
With a single phase transformer (240 V secondary with a center tap and the center trap grounded), you get 120 Volts line-to-ground (neutral) and 240 Volts phase-to-phase (line-to-line). -- Thanks to N. Srinivasan for these clarifications.
Watts is a measure of the amount of electricity being used - a rate of electrical power consumption. Most people use a very simple mathematical formula to determine how many watts an electrical circuit can carry or how many watts an electrical device will require:
Watts = Volts x Amps
Amps = Volts / Ohms
Given those two equations just cited, we can also write:
Watts = Volts x (Volts / Ohms),
which lets us also write Watts as
Watts = Volts 2 / Ohms
Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]
W = V x I
W = I2 x R
W = V2 / R
W = Watts, V = Volts, I = Current or Amperage or Amps and R = Resistance measured in Ohms
Example: if we have a 50 watt light bulb running on a 120V circuit we can solve for the missing number, I or "Amps"
50 = 120 x I
50 / 120 = I
0.416 = I
Our 50 watt light bulb is drawing .4 amps of current.
Reader Michael V. points out that in the above watts, volts, amps calculations, these simplified formulas are for DC voltage.
In AC electrical systems, V*I=VA not watts. Watts is W=V*I*PF where PF =power factor.
At SEER RATINGS & OTHER DEFINITIONS we include additional examples of calculations of electrical usage by air conditioning equipment, including how we calculate watts, volts, and amperage for an electrical device like an air conditioner.
Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".
Reader Daniel Mann adds that "Watts is correctly shown as Watts-Voltage times Current times power factor. Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power just below.
Various sources offer further definitions of real power P, reactive power Q, complex power S, and apparent power |S|.
If pf - power factor - = 1, then 1 VA of apparent power transferred in a circuit will produce 1 W of real power. If pf = .5 then to produce 1 W of real power we would need to transfer 2 VA of apparent power. (1 W / .5 = 2).
It is interesting that in an electrical circuit, a load that causes a low power factor will draw more current than a load with a high power factor for the same a mount of usable or useful power actually transferred. If an electrical circuit is drawing higher current, there is more energy loss, larger transmission wires are needed, and thus costs of both the circuit and its operation are increased. Electrical engineers may design equipment to include components that improve the power factor to thus lower its operating cost.
Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.
Seer ELECTRICAL POWER ANALYZERS for descriptions of power analysers used to measure electrical energy usage, consumption, efficiency
See SEER RATINGS & OTHER DEFINITIONS and
also A/C DATA TAGS for an example.
What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home).
For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.
If you turn on a 100-watt light bulb for an hour, you've used 100 Wh of energy. Or if you had a one-watt bulb and lit it for an hour, it'd use 1 Wh of energy. Thank James Watt (1736-1819), credited with developing a useable steam engine, for WATT which was named for him in 1882.
Watts is an instantaneous measurement, not related to time.
To factor in time, as the electrical utility wants to do in sending us an electrical bill, the electric company's meter calculates the number. of watt hours (actually kilowatt hours) of electricity we use. If we run our 50 watt bulb for one hour, we've used 50 watt-hours. That's all the electric utility cares about.
As our sketch, courtesy of Carson Dunlop, shows, 240V power delivered to a building in the U.S., Canada, and Mexico, and some other locations, means that the building is receiving two 120V lines which provide 240V for circuits connected across these two incoming wires, and which provides 120V for circuits connected from either of the individual incoming lines to ground.
For heavier and commercial electrical power requirements, three and even four-phase electrical service may be delivered to a building, and in some applications, electrical equipment is designed to be fed directly by multiple phases.
You will not ordinarily see such service at a residential property, but one of the authors (DF) has encountered it in cases where there was a dental office in the basement of a home. The dentist's x-ray equipment required three-phase power.
The tip off was the observation outside of four rather than three service conductors at the masthead, and in the main panel, the main switch was fed by three incoming service conductors rather than the usual two.
(Douglas Hansen's various publications on electricity and electrical inspections and his upcoming book point out variations of these formulae which are useful in discussing the heating of wires carrying current.)
We list these common electrical terms roughly in the order that they are observed, from the electrical utility company's overhead wires and pole to the building receiving electrical power to its electrical panel, and in the panel to individual circuit breakers which provide power to and protect individual electrical circuits that distribute electrical power throughout a building.
ELECTRICAL SERVICE DROP: the overhead electrical service conductors from the last electric utility pole (or other aerial support) to and including splices if any, connecting to the service entrance conductors at the building. These wires usually belong to and are the responsibility of the electric utility company.
Service-Entrance Conductors, overhead system: the service conductors (wires) between the terminals of the service equipment (main electrical panel) and a point usually outside of the building, clear of building walls (usually the electrical masthead, visible in this sketch), where the wires are joined by tap or splice to the service drop (the wires from the utility company).
These are the electrical wires coming down the building exterior from a mast-head connection point to the electrical meter, and continuing inside to the main electrical panel or service switch. These wires normally belong to and are the responsibility of the building owner.
Service Conductors: wires connecting service point (such as the outside electric meter) to the main service disconnect (such as the main breaker in the main electrical panel). These are wires bringing electrical power from the electric meter into the electrical panel.
Service Equipment: usually the circuit breaker(s) or switches and fuses used to connect to the load end of service conductors coming to the building. This is the main electrical switch, fuse, or breaker, usually in the main electrical panel but sometimes installed as a physically separate switch before the main electrical panel.
Circuit Breaker: a device designed to open and close (turn off or on) an electrical circuit by non-automatic means (a physical toggle switch) and to open the circuit automatically (internal trip mechanism) on a predetermined overcurrent without damage to itself when properly applied within its rating. (That is, a 15A circuit breaker is expected to protect a 15A circuit, not something else).
Branch circuit: is the conductors (electrical wires, hot, neutral, ground) between the final overcurrent device protecting the circuit (a circuit breaker or fuse in the electrical panel) and the outlet(s).
A general purpose branch circuit is an electrical circuit that supplies two or more receptacles or outlets for lighting and appliances. In other words, the wires that bring power from the electrical panel to one or more points in the building where it will be used to power an light, power something plugged into an electrical outlet, or to an individual appliance.
Alternating current is almost universally used for home electric power and is, therefore, the kind this article is primarily concerned with. In an AC circuit, the amount of voltage applied to the circuit is constantly changing from zero to a maximum and back to zero in one direction and then from zero to maximum and back to zero in the other direction.
The maximum voltage is set by the generating plant.
Because voltage is the pressure that causes current to flow, the current will also change from zero to maximum to zero and will reverse direction and repeat. The maximum amount of current, however, is determined by the load resistance and can vary as the load resistance varies.
Each complete change from zero to maximum to zero in one direction and then zero to maximum to zero in the opposite direction is called one hertz (formerly cycle).
The term hertz implies "per second." So, 60 hertz means the same as 60 cycles per second. Hertz is abbreviated Hz. Cycles-per-second, which you will still see marked in some electrical devices, is abbreviated cps.
Direct current is most commonly found in homes in the form of electrical energy stored in batteries. In a DC circuit, the amount of voltage and the direction of application are constant. The amount of voltage is determined by the type and size of battery. The direction of current flow is also constant and, as in AC circuits, the amount of current flow is determined by the load resistance.
Batteries convert chemical energy to electrical energy. The chemical energy can be in wet form, as in your car battery, or in dry form as in flashlight and transistor-radio batteries. Some batteries are designed to be recharged from an AC source. The voltage from all batteries, unless recharged, will gradually decrease.
AC power can be converted to DC power for some uses in the home. The conversion is performed by a device called a rectifier or current converter.
15-Amp 120V electrical circuits: typical U.S. 120V household electrical circuit uses #14 gauge copper wire and is protected (and thus limited) by a 15-amp circuit breaker.
Such a circuit can deliver about 350 watts of electrical power to devices plugged into it, and another roughly 10 watts is consumed by the resistance of the circuit and its devices (receptacles and switches).
20-Amp 120V electrical circuits: typical U.S. 120V 20A household circuits use #12 gauge copper wire and are protected and thus limited by a 20-Amp fuse or circuit-breaker. A 20-amp circuit can provide about 2400 Watts. But as some writers have pointed out, for safety, household circuits are intended to carry less current (about 20% less) than their theoretical maximum. 80% of 2400 Watts is 1920 watts - that's about what you should expect to obtain from such a circuit.
How are we figuring these numbers? we use the formula from above, Watts = Volts x Amps and we plug in the nominal voltage and a guess at the resistance over the electrical circuit before we've plugged in anything:
Watts = 120V x 15 Amps so W = 1800 for our 15-Amp electrical circuit.
Watts = 120V x 20 Amps so
W = 2400 for our 20-Amp circuit.
If we have a 1500 Watt electric heater running on "high" and thus drawing a maximum of 1500 Watts, plugged into our 20-Amp circuit above, we've got another 420 Watts available on that circuit. So we could run maybe another four 100-watt lights on the same circuit.
The real world is a little more complex; lots of devices draw more current when they're starting-up, especially air conditioners and refrigerators.
The electrical engineer (during design) or electrician (during house wiring) choose a circuit breaker that can tolerate that temporary high current but that will trip off if high current continues to flow on the wire.
Really? Well not exactly; for safety, electrical circuits are "de-rated" to avoid overheating, so the actual maximum-recommended load in watts for the 15A and 20A circuits would be a smaller number, as Ted, an electrician, comments below.
2017/04/06 Ted, a certified electrician in Washington (USA) said:
It appears there is an error under the heading of "How many Watts can a typical household electrical circuit provide?"
A 15 amp circuit can provide more than 350 watts as you state in the first section.
In the following section on 20 circuits,you got it right for both 15 and 20 amp circuits, ie 15amps x 120volts = 1800watts.
Since residential circuit breakers are heat sensitive devices if they are loaded at 100% for an extended time they are likely to trip out. Essentially, the NEC (National Electric Code) requires circuit breakers to be derated to 80% of their rating in certain cases including heating and motor loads and "continuous loads" (those running 3 hours or more) to avoid this.
So 80% x 15 = 12 amps and 12 x 120 = 1440 watts would be max for a continuous load.
For a 20 amp circuit 1920watts as you stated [in the text above].
Determining just when a breaker will trip is difficult especially when multiple loads are on one circuit, because it depends not only on the amps the devices draw but what the duration of the draw is.
To top it all off circuit breakers are not extremely accurate on their tripping points and vary between brands.
This and other factors are why the NEC does not allow 14 ga wire to be protected at more than 15 amps, and the U.S. National Electrical Code (NEC) requires #12 to be protected at 20 amps, and #10 wire at 30 amps even though today's wiring with higher temperature rated insulations will carry somewhat more than that. It gives us some hedge factor to insure safety against overheating and fires.
Thank you, Ted.
Readers interested in the actual UL standards for testing the current level at which an electrical circuit breaker should trip (or on older systems a fuse should blow or open) should see
UBI FPE CIRCUIT BREAKER TEST RESULTS - UBI Replacements for FPE Stab-Lok® Circuit Breaker Failure-Test Results
Or see ZINSCO CIRCUIT BREAKER TEST REPORT - similar testing on UBI replacement circuit breakers sold for Zinsco electrical panels
There you'll see that typically we test circuit breakers to see that they respond to an overcurrent at several loiad levels. For example, following UL 489, a key test standard for electrical circuit breakers, a breaker has a specific amount of time to trip at a specific overcurrent. Lower overcurrent allows more time to trip.
The word potential is used to explain that the capacity to do work is present, but not that work is necessarily being performed.
Water in the bucket in our sketch at left is a capacity to do work (move water, or exert pressure) but until water actually flows out of the bucket (say when it's tipped), no water is moving and no work is being performed.
Until something has happened, it's just a potential.
Georg Ohm's Law, first published in 1827, I = V / R says that the current (Amps) through a conductor (wire) between two points on a circuit is proportional to the potential difference (Voltage drop) across the two points and that the current (Amps) between the same two points is inversely proportional to the resistance between them (Ohms or Ω).
Details about electrical resistance, Ohms Law, and other versions of the Ohms Law equation above are given at ELECTRICAL RESISTANCE, OHM's LAW
Electrical circuits, alternating current (AC), direct current (DC), short circuits, and other basics of electrical wiring are defined further in our series of articles on electricity for homeowners starting at ELECTRICAL CIRCUITS, SHORTS.
Home inspector Arlene Puentes summarizes distinctions important in understanding the function of electrical grounding at a building:
Here are some elementary electrical ground, grounding, and ground wire definitions to help us get our terms straight when discussing electrical grounds, grounding, and ground bonds.
Please note that referring to "Grounded Conductor" and "Neutral conductor" as synonymous is deprecated by the NEC. There are many installations where the grounded conductor is NOT a neutral conductor, including the very common 3-phase 240/120 "high leg" delta installation, where the grounded conductor is definitely not a neutral.
It is required that the "Grounded Conductor" be white or gray; the neutral conductor is not required to be this color unless it is grounded. I suggest changing the definition of "Grounded Conductor" to omit the present reference to the neutral conductor, and add a sentence or parenthetical comment (usually the neutral conductor on single-phase).
Also see ELECRICAL CIRCUITS, SHORTS, AC/DC.
Also see our ELECTRIC POWER FREQUENCY TABLE [PDF] for a table showing the voltage and frequency for nearly every country in the world, provided courtesy of Paul Galow, Galow Consulting. Sketch courtesy of Carson Dunlop Associates.
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I have a 1967 Airstream International planted in the backyard and want to run an extension cord out to it to run basic electric. It has 5 amber running lights and a porch light on the outside that runs on DC. It is completely gutted and I have access to the single wire from each fixture. What do I need to do to hook up the outside running lights to a power strip. - Rob
At your local electrical supplier or auto supply store or even at Radio Shack you should be able to pick up a small External AC to DC power converter (aka AC to DC Power Supply). What you need is a converter that will have high enough Amps output for the few trailer lights that you cited.
I used the term "External AC to DC Power converter or power supply because you don't want to have to buy a separate cabinet or case and do assembly. When shopping don't rule out existing computer or other electronic power supply "bricks" - just take a look at the DC Wattage output that the supply can provide. If it's big enough you'll be OK.
For just a few running lights and a porch light, most likely you won't even be drawing 10 watts, but to be on the safe side and to allow for expanded use of your power supply once you figure out how useful it is, I'd look at a unit with 35 watts or larger output.
Take a look at our DEFINITIONS of ELECTRICAL TERMS beginning at the top of this page for help with AC, DC, Amps, and Watts.
Run a weather-resistant outdoor-rated extension cord from an outdoor GFCI protected receptacle over to the Airstream and inside it where it will be weather protected. To that 120V (probably 15A) circuit, you'll plug in your AC to DC Converter. You'll then wire the DC output terminals to your Airstream lighting circuit.
If you're going to use this electrical supply system frequently you might want to put together a suitable plug connection feeding the DC powered circuit and a suitable connector on the wires that you run from the AC-DC converter.
For more permanent power conversion of an older Airstream or other mobile home or trailer, I'd look at what's offered by RV suppliers. Certainly I had no trouble retrofitting an AC to DC connector and power supply on an old slide-on pickup truck camper, thus allowing us to "plug in" to any 120V DC power source and run all of the DC devices in the camper.
(May 26, 2015) Mike L said:
Is 1 amp AC equal to 1amp DC? My golf club stores the batteries for member's walking power carts. There are about 20 battery chargers and batteries plugged into one AC circuit that is presumably a 15 amp circuit. My charger has a 2 amp output to the battery and the others are probably the same. If all of the batteries needed charging at the same time, drawing 40 amps, then the batteries would have to share the available 15 amps resulting in a very slow charge.
Am I correct?
Technically AC & DC amps are not precisely equivalent. The DC amperage draw will be slightly less than AC. For example if you are selecting an electrical switch capable of handling 5A at 125VAC, it should be fine to use that switch on a DC current since the actual DC amperage draw will be a bit under 5 amps.
Power = Amps x Volts regardless of AC or DC.
20A at 120VAC = 240 Watts (power)
Also the amperage rating of a switch is increased as the voltage is decreased.
Example: a 120VAC 5-Amp switch can handle about 600 Watts and according to batterystuff.com's calculator, supports a 12-Volt DC amperage of 55.
(June 18, 2015) barry said:
is there a formula i can use to calculate the cost of a water well pump, and a booster pump?
(June 18, 2015) firstname.lastname@example.org said:
hi my name is barry i had ask a question about calculating a well pump and booster pump for cost. i didn't receive an answer so i put my e-mail, by the way you do an excellent job on teaching basic electrical, keep up the good work. thanks barry 2000hrs 6-18-2015
To convert the current draw (from the pump data tagor by actual measurement) to watts and watt hours to relate the electricity usage to what appears on your electric meter and electric bill in watt hours:
Energy E in watt-hours (Wh) = Power P (in watts W) x time period t in hours (h), or
E (Wh) = P(W) x t (h)
(July 5, 2015) layne said:
In selecting inverters for a specific product I know the Volts need to be the same, but as for the amps....if your inverter produces more Amps than needed, is this a problem?
No not as I understand it; the amps rating is the maximum amount of current draw or demand that can be asked of the inverter. If your application draws less current that's ok.
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