How to Calculate Heat From a Light Bulb

Written by brian baer
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How to Calculate Heat From a Light Bulb
A standard light bulb is an incandescent light. (Jupiterimages/ Images)

A standard 100-watt light bulb is also called an incandescent light. Incandescent lighting uses metal filaments (very thin wires) and emits light as a consequence of the filament being heated to a high temperature. Usually, close to 90 per cent of the electrical power supplied to light bulbs is converted to heat energy. Heat energy cannot be converted to a temperature, but electrical energy (watts) can be converted to heat energy in BTU (British Thermal Units).

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  1. 1

    Write down the wattage of the light bulb. Typical values are 40 watts, 60 watts and 100 watts. This article uses a 60-watt bulb as an example.

  2. 2

    Divide the bulb's wattage by 1,000 to convert watts to kilowatts. For example, 60 watts is equivalent to 0.06 kilowatts (kW). Check your work with a calculator.

  3. 3

    Multiply the kilowatt value (0.06) by the number of hours the light bulb was used. If the light bulb was used for three hours, the total electrical energy consumed by the light bulb is 0.06 x 3, which works out to 0.18. This is expressed as 0.18 kWh, or 0.18 kilowatt-hours.

  4. 4

    Convert the electrical energy to heat energy by multiplying the kilowatt-hour value by 3,412.14. This conversion means that 1.0 kWh is equal to 3,412.14 BTUs. Continuing from the previous example, the calculation is 0.18 x 3,412.14, which equals 614.2 BTU. Check your work with a calculator.

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