Energy, Power

Energy and power are used interchangeably in ordinary speech and mean different things in engineering. Power is a rate, measured in watts; energy is power accumulated over time, measured in watt-hours or joules. A 2,000 watt heater running for three hours consumes six kilowatt-hours, and it is the kilowatt-hours that appear on your bill.

Getting the distinction right is the difference between a sensible estimate and nonsense. Solar panels are rated in kilowatts peak, which is a power figure, while your household consumption is in kilowatt-hours per year, an energy figure. Battery capacity, appliance ratings, boiler outputs and engine specifications all mix watts, kilowatts, horsepower, BTUs, joules and calories depending on industry and country.

This converter moves between those units so that specifications from different sources can be compared directly. It is useful for anyone sizing a generator or inverter, comparing heating systems quoted in BTU with ones quoted in kilowatts, or checking the real running cost of an appliance.

How to use the Energy, Power

1. Decide whether your figure is a power rating or an energy quantity.
2. Select the matching source unit, such as watts, horsepower, joules or BTU.
3. Choose the target unit you need for comparison.
4. Enter the value and read the converted result.
5. For running costs, multiply energy in kilowatt-hours by your tariff per unit.

Examples

• A homeowner compares an air conditioner rated in BTU per hour with one rated in kilowatts.
• An installer converts a generator rating in horsepower into kilowatts to check it can carry a workshop load.
• A landlord estimates the annual cost of an electric heater by converting its wattage into kilowatt-hours of expected use.

Frequently asked questions

What is the difference between a kilowatt and a kilowatt-hour?

A kilowatt is the rate of energy use at an instant. A kilowatt-hour is the energy used by one kilowatt running for one hour, and it is the unit you are billed for.

How many watts is one horsepower?

Mechanical horsepower is approximately 745.7 watts. Metric horsepower, used in some European specifications, is slightly lower at about 735.5 watts.

What is a BTU?

A British thermal unit is the energy needed to raise one pound of water by one degree Fahrenheit, about 1,055 joules. Heating and cooling equipment is often rated in BTU per hour, which is a power figure.

Can I work out my electricity bill with this?

You can estimate it. Convert the appliance rating to kilowatts, multiply by the hours of use to get kilowatt-hours, then multiply by your tariff.

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