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  Just remember what the units mean. A watt is one joule per second. A kilowatt-hour is therefore one thousand joules per second applied for one hour (3600 seconds), or 3,600,000 joules. 

A 3000W heater is a 3 kW heater. If it operates for 30 minutes, it is using 3 kW * 0.5 hours, or 1.5 kWh (NOT 1500
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