An energy meter makes 600 revolutions per kWh. If it makes 300 revolutions in 30 minutes, then the load power is
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Think of it proportionally: The meter requires 600 revolutions for 1 kWh. It completed 300 revolutions, which means exactly half a unit of energy (0.5 kWh) was consumed. Since this 0.5 kWh was used over exactly half an hour (30 minutes), the constant power load must be 1 kW.