How to Calculate BTU from Watts

This post came about because I got a lot of hits from people looking for this information on the How To Calculate Server Heat post.

1 Watt will produce 3.412 BTU per hour

To calculate the BTU/heat output of an electrical system, substitute BTUs for Watts and use the following equation:

BTU/h = (V * I) * 3.412

V = Volts
I = Amperes (Amps or Current)

Using the simple math above, we can calculate the heat produced by an appliance on a 120v (US Standard) system drawing 2 Amps for 60 minutes (1 hour).

BTU/h = 3.412 * (120 * 2) = 818.88BTU

Did you find this post useful or have questions or comments? Please let me know!

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13 Responses to How to Calculate BTU from Watts

  1. Ken Sullivan says:

    Thanks for the post. I hope I used it correctly. I am calculating the BTU/hr for a Sun V240. At 90 V I came up with 1280.5236 BTU/hr

  2. admin says:


    A Sun V240 has 400 Watt power supplies. Since Watts a measured of power, they are calculated as Watts = Amps x Volts. If you notice the BTU equation also has Volts * Amps so you can plug your power supply numbers right in.

    If you plug a V240 with a 400 Watt power supply in to a 110 Volt outlet, you can solve for Amps if you want to. Amps = Watts / Volts. For your V240, Amps are 400/110 or 3.636 Amps.

    Now we can plug that 3.636 in to the BTU formula. (110 Volts * 3.636 Amps) * 3.412 = 1364.66352 BTU/hr if your machine is running full blast. It will likely be less.

    Notice that 3.636 * 110 is pretty close to 400 (399.96), that means you can just plug your power supply Watt rating right in. (400) * 3.412 = 1364.8

    So yes, you were pretty much right on!

  3. Martin says:


    I would like to ask you to see if you could help me calculate something. I got a boiler 240 v and 46 Amps with 1 gallon of water tank. The water temp is 80 F. How much energy does it take to boil the water at 212F? Then keep it boiling at 212 for 20 minutes. Thanks in advance!

  4. thigo says:

    boa tarde gostaria de saber a formula com mais clareza

  5. admin says:

    BTU por hora = (volts * ampères) * 3,412

    Exemplo: 120 volts (EUA padrão) do sistema usando 2 ampères para 60 minutos (1 hora).

    (120 * 2) * 3,412 = 818.88BTU

  6. Bruce says:

    I’m a bit thick on this maths thing so if I give you the numbers I have found on the Specs Plate on our Air Conditioner I wonder if you could advise on the cost per hour to run.
    Kw Cooling 6760w
    Kw Heating 6770w
    Amps 14
    Cost charged per Kwh by power company
    31.29 cents per kwh We run on 240v


    Bruce H Dobbie

  7. admin says:


    This calculation is pretty easy. Since you know your Kw power at any given time, we can do KWh with that number. So for cooling you have 6760W so KW is “Thousand Watts”. 6760/1000 is about 6.8 KWh. If you have 31.29 cents per KWh, you multiply that number by 6.8 to get cost per hour for cooling.

    6.8 * 31.29 = 212.8 cents/hour or about $2/hour.

  8. admin says:


    I also want to mention that 6000+ Watts is very high for an AC unit unless this is a large unit or very old one. A Window-mounted AC unit will use 1000-2000 Watts.

  9. nasir says:

    I have room 4m x 5m area in hot country I want to install split unit air conditioning
    please help me how to calculate the btu for this room which has one window

  10. amosi masembo says:

    please adimin help me to find the solution of the following qs. ” what full runing amps of an air conditioner are with models 12000BTU,18000BTU and 24000BTU?”

  11. admin says:


    I = Amps

    BTU = (V * I) * 3.412

    So using simple algebra, solve for I:

    (BTU/3.412) = V * I

    (BTU/3.412) / V = I

    So use that for each. Replace V with the volts from your AC power.

  12. mohamed bably says:

    when we apply this equation to calculate ampere from btu /h and 220v the result is 31 ampere ,this not true because the current from 2400 btu air conditioner is 10 ampere ,you can explain this for me,
    thank u

  13. admin says:


    (BTU/3.412) / 220 = I

    (2400/3.412) / 220 = 3.2, not 31

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