W tells us how much power. Wh tells us how much energy. Keep the battery’s rated capacity separate from the output it can supply.
Put time beside the watts
Consider a hypothetical device using a steady 100 watts for two hours. Its energy requirement is 200 watt-hours:
The same 100 W device used for one hour needs 100 Wh. The power has not changed; the amount of energy has. For unit conversion, 1,000 Wh equals 1 kWh.
In the finder, “total usage hours” means hours across your entire selected period. If you enter four laptop hours for a two-day trip, those four hours are not multiplied by two again.
Alternatively, enter an established daily energy value. An illustrative 200 Wh/day over two days gives 400 Wh. This is a separate input route: ChooseWatt uses either watts × total hours or Wh/day × days for each device.
Energy does not settle the power check
Imagine a battery labeled 1,000 Wh with a 300 W normal continuous AC limit. A 500 W load exceeds that output limit, whatever the battery’s stored energy. Those numbers are an illustration, not a product recommendation.
ChooseWatt adds the separate running-power inputs for all listed devices. Its first version assumes they can all require power at the same time, even if you entered different usage hours.
Starting is another check
For a load with a relevant starting requirement, enter that information separately. The calculator considers one device starting while the others run. It does not assess several simultaneous starts.
If a relevant starting value is unknown, the overall result stays unresolved. An advertised power-station surge figure with no matching duration and appliance conditions does not fill that gap.
Use numbers that describe your use
Average watts belong in the energy input. Running watts belong in the output check. Use values at the device’s AC plug, including any adapter. For a cycling appliance, use a measured or documented energy figure over a relevant period; do not assume its maximum label wattage applies continuously.
The battery’s nameplate Wh is also not the energy delivered at its AC outlets. The finder applies visible, editable assumptions for usable charge, conversion and idle use. A separate measured-energy path has different rules to avoid subtracting losses twice; the current catalog contains no ChooseWatt measurements.
Keep values you cannot establish marked unknown. A more complete device list is more useful than a precise-looking answer built from missing information.
Sources and assumptions
- Virginia Cooperative Extension: Estimating Appliance and Home Electronic Energy UseBackground on watts, time, variable consumption and motor starting. Examples and calculator decisions here are ChooseWatt’s own.
- ChooseWatt calculation method