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My newly designed "biomass fired water heater" gives 50 liters of boiled water within 20 minutes by firing 2kg of firewood. Room temperature is 30 deg. centigrade.

Please let me know how to calculate the wattage.

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Water has a heat capacity of 4.1814 J/g which means that 1 gram of water requires 4.1814 Joules of heat to increase 1°C. The definition of Watt is Joules / second so the formula you need is:

heat capacity * mass * temp. increase / time in seconds

I'm assuming the water was at 30 degrees before you started heating it, just like the room. Since water boils at 100 degrees (at sea level, it's lower at higher altitudes), there was a temperature increase of 70°C (probably a bit less).

So for your case:

4.1814 * 50000 * 70 / (20 * 60) = 12196 W or 12.2 kW

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