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Thread: How do I convert watts to amps and amps to watts?

  1. #1
    420 Member Medical Marijuana's Avatar
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    How do I convert watts to amps and amps to watts?

    The formula for converting Watts to Amps is : Watts = Amps * Volts

    The formula for converting Amps to Watts is : Amps = Watts / Volts

    SAFTEY WARNING : Only use 80% of the circuit breaker capacity!

    Common Conversions North American and other 110 volt countries

    Lights :
    1000W / 110V = 9.1A
    600W / 110V = 5.4A
    400W / 110V = 3.6A
    250W / 110V = 2.3A

    Circuits :
    10A * 110V = 1100W and 80% safe usage is 880W
    15A * 110V = 1650W and 80% safe usage is 1320W
    25A * 110V = 2750W and 80% safe usage is 2200W
    30A * 110V = 3300W and 80% safe usage is 2640W

    Common Conversions British Commonwealth and other 240 volt countries

    Lights :
    1000W / 240V = 4.1A
    600W / 240V = 2.5A
    400W / 240V = 1.7A
    250W / 240V = 1.1A

    Circuits :
    10A * 240V = 2400W and 80% safe usage is 1920W
    15A * 240V = 3600W and 80% safe usage is 2880W
    25A * 240V = 6000W and 80% safe usage is 4800W
    30A * 240V = 7200W and 80% safe usage is 5760W

    Remember that a circuit services more than one power outlet.
    A two outlet wall plug will use only one circuit.
    There will be more outlets on 240V systems per circuit than there will be in 110V systems.
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  2. #2
    420 Member curlybud's Avatar
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    Re: How do I convert watts to amps and amps to watts?

    Just wanted to add - if you are calculating the amperage draw for a device that uses a power adapter, make sure you use the voltage on said adapter and not your local outlet voltage.

    Example with a 12v fan using 8 watts :

    Amps = 8w /12v = 0.66 a

    and NOT 8w / 110v = .072 a

    Just a common mistake people might make, I think!
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  3. #3
    420 Member Stix's Avatar
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    Re: How do I convert watts to amps and amps to watts?

    A watt is a watt no matter what the voltage is. Just because you lowered the voltage using a transformer doesn't mean the bulb uses any less or more energy. Therefore if the bulb is 8 watts you would be accurate if you took 8 watts/ 110 volts which is .07 amps. The only reason you would use any more energy is due to heat loss by the transformer which is very minimal.

    BTW. Voltage in most homes in the U.S is 120/240 volts.
    Last edited by Stix; 01-15-2009 at 03:45 PM.