Understanding the Electricity Bill

Power = 500 W, time = 2 hours. We need energy in kWh using $E=P\times t$.

$P=\frac{500}{1000}=0.5\text{ kW}$

$E=P\times t=0.5\text{ kW}\times 2\text{ h}=1\text{ kWh}$

The washing machine consumes 1 kWh of energy.

We need the energy in kWh for a 500 W device running for 2 hours.

1000 kWh would be an enormous amount — equivalent to running a 1 kW device for 1000 hours. This is far too large for a single wash cycle.

2 kWh would result from a 1 kW device running for 2 hours, but our device is only 0.5 kW.

0.5 kWh would be correct if the machine only ran for 1 hour at 0.5 kW, but it ran for 2 hours.

$E=0.5\text{ kW}\times 2\text{ h}=1\text{ kWh}$, so the answer is 1 kWh.

Previous reading = 12,480 kWh, current reading = 12,743 kWh, tariff = $0.25/kWh.

$\text{Units used}=12,743-12,480=263\text{ kWh}$

$\text{Cost}=263\times \$0.25=\$65.75$

The energy charge for the month is $65.75.

We need to find the cost by subtracting meter readings and multiplying by the tariff.

$263.00 is the number of units used, not the cost. This confuses energy (kWh) with cost (dollars).

**$57.00\ast \ast doesnotresultfromanycorrectcalculationusing263kWhand$0.25/kWh.

**$74.25\ast \ast wouldrequire297kWhat$0.25/kWh — an incorrect reading subtraction.

263 \times \0.25 = $65.75$,sothecorrectansweris\ast \ast$65.75**.