Watts, Amps and Watt-Hours in Plain English
Three units cause most of the confusion in off-grid planning: the watt, the amp and the watt-hour. They measure different things. Mix them up and your battery will be the wrong size.
Power and energy are not the same thing
A watt (W) measures power. Power is the rate at which energy is generated or consumed1. Think of it as how hard an appliance is working right now.
A watt-hour (Wh) measures energy. It's an amount, like litres of water in a tank. Run a 100 W light bulb for one hour and you've used 100 watt-hours1.
A kilowatt-hour (kWh) is just 1,000 watt-hours. It's one kilowatt (1,000 watts) sustained for one hour2. Power bills use it.
Here's the same idea side by side.
| Watt (W) | Watt-hour (Wh) | |
|---|---|---|
| What it measures | Power, the rate of use | Energy, the amount used |
| Changes with time? | It's a snapshot of one moment | It grows the longer something runs |
| Where you see it | On an appliance label | On a meter or in your budget |
| Use it for | Sizing an inverter | Sizing a battery and panels |
The same energy can come from different mixes of power and time. That 100 watt-hours would also light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours1. A 12-watt LED lamp left on all day and night uses about 0.3 kWh in 24 hours2.
Never write "watts per hour"
You'll see "watts per hour" a lot. It's wrong for this job. Writing kW/h for the kilowatt-hour is incorrect, because it denotes kilowatt per hour2. The correct unit is the kilowatt-hour, which is power multiplied by time2. Write Wh, and you'll stay out of trouble.
Where amps fit in
An amp (A) measures electric current, the flow through a wire. A volt (V) measures the push behind that flow.
Put them together and you get watts. One watt is what you get when a current of one amp flows across a difference of one volt1. So:
- watts = volts × amps
- amps = watts ÷ volts
That's why a small load on a 12 V battery draws a lot more current than the same load on a higher voltage. Same watts, fewer volts, more amps1.
Amp-hours, the battery label
Batteries are often sold in amp-hours (Ah). One amp-hour is the charge moved by a steady current of one amp flowing for one hour3.
An amp-hour doesn't tell you the energy by itself. You also need the battery's voltage. To get watt-hours, multiply amp-hours by volts. A 12 V battery rated at 100 Ah holds 1,200 Wh on paper, using the watt definition above1 and the amp-hour definition3. That's a label figure, and you won't be able to use all of it. Sizing a battery covers why.
Which number goes where
| Question | Unit to use |
|---|---|
| How big an inverter do I need? | Watts, at the busiest moment |
| How much do I use in a day? | Watt-hours |
| How much can the battery hold? | Watt-hours (or amp-hours plus volts) |
| What does the grid charge me? | Kilowatt-hours |
Your whole plan rests on one number: the watt-hours you use in a day. The next page, how to build a daily energy budget, shows how to get it.
Sources
- Watt, Wikipedia
- Kilowatt-hour, Wikipedia
- Ampere hour, Wikipedia