<p>An electric water heater is designed to produce 42 kW of heat when it is conn

Question

<p>An electric water heater is designed to produce 42 kW of heat when it is connected to a 240-V source. If the heater is 74 % efficient, how long would it take to heat 45 gallons of water from 20 C to 85 degrees?</p>

Explanation / Answer

A Watt is a Joule per second. It takes 4.18 J to heat 1g of water 1 deg C. One gallon is 8.34 lbs 1 lb is 454 g So we have this many grams of water --> 45gal*8.34lb/gal*454g/lb We have this many degrees --> 85 - 20 C (interesting side note: 85C = 185F) We need 170,386g*4.18J/gC*65C = 46293930.54J The heater supplies (0.74)*42,000J/s need/supply/second = time in seconds = 1489.508705s so the time is 1489.508705s/60s/min = 25 minutes Is the 240V part of this? No, just some added info.

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