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Watt and Electricity Cost Calculator
Conversion factors from NIST SP 811, quoted as printed
Every conversion on this page is a definition, and they all come from one free US government document. A watt is 3.4121 Btu per hour because a Btu is 1,055.056 joules, and NIST prints that figure. The one thing this page will not do is convert kVA to kilowatts for you, because they are different quantities and the bridge between them is a property of your equipment that no calculator can look up.
Power, energy and cost
V
A
W
kVA
h
days
per kWh
Power factor defaults to 1.0, which is right for resistive loads and wrong for motors. It is an input rather than an assumption.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Apparent, real and reactive power
Worked example
A 1,500 W space heater on 120 V, run four hours a day for thirty days. Entered as volts and amps: 120 V at 12.5 A, power factor 1.0 because a heater is resistive.
Apparent power. 120 × 12.5 = 1,500 VA.
Real power. At a power factor of 1.0, also 1,500 W. A heater is the case where these are equal.
In Btu. 1,500 × 3.4121 = 5,118 Btu/h.
In tons. 5,118 ÷ 12,000 = 0.427 tons of heat.
Energy. 1.5 kW × 4 h = 6 kWh a day, so 180 kWh over thirty days.
Cost. Enter your own rate. At any rate you like, it is 180 kWh.
Now change the load to a motor and set the power factor to 0.8. The same 120 V and 12.5 A still give 1,500 VA, and the real power drops to 1,200 W. The breaker sees the same current either way, which is why circuits are sized on amps and bills are charged on watts. That gap is the entire reason kVA exists as a unit, and it is why converting it without knowing the load is guesswork.
The formula
Four lines, and only the last one needs anything you have to look up:
VA = V × I × (√3 if three phase) W = VA × pf Btu/h = W × 3.4121 cost = kW × h × rate
V, I
voltage and current. Their product is APPARENT power, in VA
pf
power factor. A property of the load between 0 and 1, not a constant
3.4121
3600 divided by 1055.056, the NIST joule value of a Btu
√3
1.732, for a balanced three-phase load
rate
your all-in cost per kWh, from your bill. Not published here
The third term is the only one with a decimal that looks invented, and it is the most solid figure on the page: 3600 seconds in an hour divided by 1,055.056 joules in a Btu, both exact. The one that looks simple and is not is the second.
Two kinds of number on one page
Almost everything here is a definition. A Btu is defined as 1,055.056 joules, a horsepower as 745.6999 watts, a ton of cooling as 12,000 Btu per hour. NIST publishes the first two in Special Publication 811 and the third is the definition of the unit. None of them will change, none of them has a tolerance, and none of them needs a second source.
Two numbers on the page are not like that at all, and they are the two the answer actually depends on.
Power factor is a property of the load. It is 1.0 for a heater and something between 0.6 and 0.95 for a motor depending on how hard it is working, and there is no table for it, because it describes your equipment rather than a class of equipment. The industry habit of assuming 0.8 puts a number where a measurement should be.
The electricity rate is a property of your tariff, and this site publishes no price of its own. It is on your bill, and the figure to use is the all-in one rather than the headline energy rate.
Conversion factors, and which are printed against which derived
Quantity
Equals
How
Source
1 watt
3.4121 Btu/h
3600 / 1055.056
derived
1 kilowatt
3,412 Btu/h
x 1000
derived
1 Btu(IT)
1,055.056 joule
as printed
NIST SP 811
1 horsepower
745.6999 watt
as printed
NIST SP 811
1 ton of cooling
3,516.9 watt
12,000 / 3.4121
derived
1 ton of cooling
12,000 Btu/h
definition
definition
1 kWh
3,600,000 joule
1000 W x 3600 s
derived
1 kWh
3,412 Btu
x 1000, over an hour
derived
Read 2026-09-08. The two rows marked NIST SP 811 are quoted from Appendix B.8. Everything marked derived is arithmetic on those two plus the definition of a ton.
Every number here is a definition, and they come from one document
Power conversions get quoted with a suspicious number of decimal places and no source. They do have a source, it is free, and it is the US government's own guide to units:
Everything else is arithmetic on those two. A watt is a joule per second, so an hour of it is 3,600 joules, and a Btu is 1,055.056 joules:
1 W = 3600 ÷ 1055.056 = 3.4121 Btu/h
Quantity
Equals
How
Source
1 watt
3.4121 Btu/h
3600 / 1055.056
derived
1 kilowatt
3,412 Btu/h
x 1000
derived
1 Btu(IT)
1,055.056 joule
as printed
NIST SP 811
1 horsepower
745.6999 watt
as printed
NIST SP 811
1 ton of cooling
3,516.9 watt
12,000 / 3.4121
derived
1 ton of cooling
12,000 Btu/h
definition
definition
1 kWh
3,600,000 joule
1000 W x 3600 s
derived
1 kWh
3,412 Btu
x 1000, over an hour
derived
The row worth noticing is the ton. A ton of cooling is 12,000 Btu/h by definition, which is 3,517 watts of heat moved. That is the number that connects a cooling load to an electricity bill, and it is why the BTU calculator on this site and this page are two ends of the same arithmetic.
kVA is not kW, and no calculator can convert it for you
This is the question the page refuses to answer the way everyone else answers it.
Volt-amperes are apparent power. Voltage times current, straight off the meter. Watts are real power, the part that actually does work. On a purely resistive load, a heater or an incandescent lamp, they are the same number. On anything with a motor, a transformer or a switching supply in it they are not, because current and voltage stop peaking at the same instant.
W = VA × power factor
Power factor is a property of the load. It is not a constant, it is not 0.8, and it is not something a converter can look up. A generator rated 10 kVA delivers 10 kW into a resistive load and 8 kW into a load at 0.8, and the difference is not the generator.
So the calculator asks for it, defaults it to 1.0, and says what that assumption means. Assuming 0.8 silently, which is the industry habit, invents a property of equipment nobody has described. If you do not know your power factor, the honest answer is that you do not know the watts.
Which is also why generators and UPS units are sold in kVA rather than kW: the limit is the current the windings can carry, and that is apparent power. The watts you get depend on what you plug in.
Why circuits are sized in amps and bills are charged in watts
A breaker does not care about watts. It trips on current, because current is what heats the conductor, and current is set by apparent power rather than real power.
So a 10 kVA load at a power factor of 0.8 draws exactly the same current as a 10 kVA resistive load, and uses 20% less energy. The wiring has to be sized for the first number and the bill is calculated on the second.
That is the practical reason the two units both exist, and why generators, transformers and UPS units are all rated in kVA. Their limit is the current their windings can carry. What that converts to in useful watts depends entirely on what you plug in, which is not something the manufacturer can know.
It is also why a generator that comfortably runs a 5 kW heater may struggle with a 5 kW motor: same watts, more current.
Reading the rate off your bill properly
The number most people quote is the supply rate, the cost of the energy itself. On a lot of bills that is roughly half of what a kilowatt hour actually costs, because delivery, distribution and transmission are charged separately and per kWh as well.
The figure to use is the total bill less any fixed standing charge, divided by the kilowatt hours used. That is what one more kilowatt hour costs you, which is the question a running-cost calculation is asking.
The standing charge itself does not belong in the arithmetic. You pay it whether the appliance runs or not, so it is not a cost of running it.
Time-of-use tariffs break the whole model, because the answer then depends on when the load runs. For those the calculation has to be done per rate period, and a single blended figure will flatter overnight loads and understate afternoon ones.
The conversions people actually search for
Amps to watts needs the voltage, and there is no answer without it. Twelve amps is 1,440 W at 120 V and 2,880 W at 240 V. A converter that answers without asking has assumed a voltage.
Watts to Btu is the one clean multiplication on the page: 3.4121, exactly, from the NIST joule value.
kVA to kW is not a conversion at all. See the section above.
Watt hours to kWh is a factor of a thousand, and the only thing to watch is that a battery rated in amp hours is not rated in watt hours until you multiply by its voltage. A 100 Ah battery at 12 V is 1.2 kWh.
And the running cost, which is the only part that needs your data
Energy is power times time, and cost is energy times a rate:
cost = (watts ÷ 1000) × hours × rate per kWh
That is the whole of it. The only figure the page cannot supply is the rate, because this site publishes no price of its own and an electricity tariff is local, time-of-use in many places, and changes without notice.
It is on your bill, and it is worth reading properly: the number to use is the all-in cost per kWh including delivery and fixed charges spread over your usage, not the headline energy rate. On many bills the delivery half is as large as the supply half, so using the supply rate alone understates running cost by something close to a factor of two.
The standing charge is the part that does not belong in this arithmetic at all. It is paid whether the appliance runs or not, so it is not a cost of running it.
One source is enough here for the same reason it was enough on the voltage drop page: everything on this page is either printed in that document or derived from it by arithmetic you can repeat. The conversions are definitions, not measurements, so there is nothing to triangulate.
What is not from any document is the power factor and the electricity rate. Both are properties of your equipment and your tariff, both are inputs, and neither is defaulted to a plausible-looking number on your behalf.
Frequently asked questions
How do I convert amps to watts?
Multiply by the voltage, and for three phase by the square root of three as well, then multiply by the power factor. Twelve and a half amps at 120 V is 1,500 VA, and 1,500 W only if the load is resistive. There is no answer without the voltage.
How many BTU is 1 kW?
3,412 Btu per hour. The conversion is exact: a watt is a joule per second, an hour is 3,600 seconds, and NIST SP 811 gives a Btu as 1,055.056 joules, so 3600 divided by 1055.056 is 3.4121 Btu per hour per watt.
How do I convert kVA to kW?
You cannot, without knowing the power factor of the load. kVA is apparent power, voltage times current. kW is real power, the part that does work. W = VA times power factor, and power factor is a property of your equipment. Assuming 0.8, which is the common habit, invents a number rather than converting one.
How much does it cost to run a 1500 watt heater?
1.5 kW times the hours you run it gives the kilowatt hours, then multiply by your rate. Four hours a day is 6 kWh a day and 180 kWh a month. Use the all-in rate from your bill, supply plus delivery divided by kWh used, not the headline energy rate.
How many watts is one ton of air conditioning?
About 3,517 watts of heat moved. A ton of cooling is 12,000 Btu per hour by definition, and at 3.4121 Btu per hour per watt that is 3,516.9 W. That is the heat the system moves, not the electricity it draws, which is far less and depends on its efficiency.
How many watts in a horsepower?
745.6999 watts, from NIST SP 811, for the mechanical horsepower defined as 550 foot pounds-force per second. The rounded 746 is fine for almost anything. Note that a motor's horsepower is its output; its electrical input is higher by its efficiency.
What power factor should I use?
1.0 for anything purely resistive: heaters, incandescent lamps, kettles. For motors, transformers and switching supplies it is lower and it depends on the specific equipment and how hard it is loaded, so it should come off a nameplate or a meter rather than a calculator's default.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The conversions are exact; the two inputs that matter are not published here. Power factor is a property of your equipment and this page defaults it to 1.0 rather than assuming 0.8 on your behalf. The electricity rate is a property of your tariff and this site publishes no price of its own. NIST SP 811 was read on 8 September 2026.