A water picture
Think of a tap filling a bucket. Watts are how fast the water flows. Watt-hours are how much water ends up in the bucket. A battery is the bucket: its size is in watt-hours. An appliance is the tap: its draw is in watts.
The units
| Unit | Measures | Where you see it |
|---|---|---|
| Watt (W) | Power at this moment | Appliance labels, generator ratings, inverter output |
| Watt-hour (Wh) | Energy over time | Power station capacity |
| Kilowatt-hour (kWh) | 1,000 Wh | Electricity bills, appliance energy labels |
| Volt (V) | Electrical pressure | Outlets (120 V), batteries (12 V) |
| Amp (A) | Electrical flow | Breakers, appliance nameplates |
| Amp-hour (Ah) | Battery capacity at a given voltage | Car and deep-cycle batteries |
Four conversions worth knowing
watts = volts × amps
A nameplate reading 120 V and 5 A means up to 600 W.
watt-hours = watts × hours
A 60 W CPAP machine running for 8 hours uses 480 Wh.
watt-hours = amp-hours × volts
A 100 Ah battery at 12 V stores 1,200 Wh. Amp-hours alone do not tell you the energy; you need the voltage too.
average watts = kWh per year × 1,000 ÷ 8,760
A fridge rated at 438 kWh a year averages 50 W. There are 8,760 hours in a year.
Running watts and starting watts
Anything with a motor or compressor needs a short burst of extra power to get moving. A fridge that runs at 180 W might need several hundred watts more for a second or two. Generators and power stations therefore carry two ratings:
- Rated, running or continuous watts: what it can supply steadily.
- Peak, starting or surge watts: what it can supply for a moment.
Average watts and running watts
A fridge compressor runs for part of each hour. While it runs, the fridge might draw 180 W. Averaged over the whole day it might be only 50 W. Which figure you use depends on the question:
| Question | Use |
|---|---|
| How long will the battery last? | Average watts, or daily kWh |
| Can the inverter or generator carry it? | Running watts |
| Will the motor start? | Running watts plus the starting surge |
Why usable capacity is less than the label
A power station stores energy as DC and converts it to household AC through an inverter. That conversion loses some energy as heat, and the electronics use a little just by being on. Planning on 85 to 90% of the labelled watt-hours is realistic for AC loads.
Try it on one appliance
- Find the watts on the label, or volts × amps.
- Estimate the hours per day it actually runs.
- Multiply for daily watt-hours.
- Divide a battery's usable watt-hours by that figure to see how many days it lasts.
For a 20 W modem and router: 20 × 24 = 480 Wh a day. A 1,000 Wh power station at 85% usable gives 850 Wh, enough for about 42 hours.
Related
Reviewed October 2026 by Lee Palmer. This guide is general information, not professional advice. Suggest a correction.