Power describes how quickly a device uses energy. Energy accumulates as a device operates. A watt rating alone does not give total daily use.
The underlying idea
Use measured average power or actual on-time for devices that cycle. Rated maximum power, standby power, and everyday average power can differ.
Measure operating behavior
Use measured average power or actual on-time for devices that cycle. Rated maximum power, standby power, and everyday average power can differ. Choose measurements that describe the same object, system, or period; changing the reference changes what the result means.
Worked example
A 2 kW heater on continuously for 3 hours consumes 6 kWh. If its thermostat operates the heater for only half that period, the heating element uses about 3 kWh.
Put the method into practice
- Write down the quantity you need to find.
- Identify the measurements and their units.
- Convert units before doing the calculation.
- Apply the formula and keep unrounded intermediate values.
- Check whether the result is reasonable for your situation.
Try the Energy Calculator with your own values.
Common mistakes
Mixing units, using the wrong reference value, and rounding too early can all change the result. A calculator cannot repair inaccurate measurements. For project estimates, distinguish the calculated base quantity from allowances and supplier rounding.
What to learn next
Explore this topic’s related answers below for focused examples and alternative ways to check a result.
Sources & references
- Electrical Energy and Power — OpenStax
