The units at a glance
| Unit | Meaning | Example |
|---|---|---|
| W (watts) | Instantaneous power | A lamp draws 10W |
| Wh (watt-hours) | Energy over time | 10W for 5 hours uses 50Wh |
| kWh (kilowatt-hours) | 1,000Wh | 1.2kWh = 1,200Wh |
| V (volts) | Electrical potential | An appliance requires the correct voltage |
Calculate a real load list
Suppose a laptop averages 45W for four hours and a light uses 10W for six hours. The laptop uses 180Wh and the light uses 60Wh, so total delivered energy is 240Wh. If both run together, their combined demand is 55W.
The energy budget chooses battery capacity. Simultaneous demand helps choose inverter output. Neither number replaces a startup-surge check for equipment with a motor.
Why label capacity is not outlet energy
The inverter and other electronics consume energy. Batteries can reserve some capacity, and temperature or age can affect usable energy. Planning with 85% efficiency is an assumption for an example, not a universal measured value.
To leave a reserve, divide load energy by both efficiency and the fraction of capacity you plan to use. At 85% efficiency and 20% reserve, 240 ÷ 0.85 ÷ 0.80 = about 353Wh.
What about amp-hours?
Amp-hours only become an energy estimate when battery voltage is known: Wh = Ah × nominal volts. Two batteries with the same Ah rating can store different energy if their voltages differ.
Compare watt-hours when shopping across products. Do not use a USB output voltage to convert an internal battery’s Ah rating unless that is the voltage the rating actually refers to.
Common mistakes
Watts per hour is not the usual unit for appliance energy. A 1,000W appliance running for half an hour consumes 500Wh. Likewise, a 200W solar panel does not produce 200Wh every hour of every day. Its real output changes with conditions.