Watt-Hour (Wh)

A watt-hour (Wh) is a unit of energy equal to one watt of power sustained for one hour. It is the most practical unit for measuring battery storage capacity and daily energy consumption in solar systems because it accounts for both voltage and current, unlike amp-hours which only reflect charge.

Your daily energy consumption is measured in watt-hours. A 60W light bulb running for 5 hours uses 300Wh. A 1,500W microwave running for 10 minutes uses 250Wh. A refrigerator drawing an average of 150W over 24 hours uses 3,600Wh. Adding up all your loads gives your daily watt-hour budget, which is the starting point for sizing your solar array and battery bank.

To calculate a battery's storage in watt-hours, multiply its amp-hour rating by its nominal voltage: a 100Ah 12V battery stores 1,200Wh. A 100Ah 48V battery stores 4,800Wh. This makes watt-hours the fair comparison unit across different battery voltages and configurations.

Portable solar generators and power stations typically advertise their capacity in watt-hours directly, making it easy to estimate how many times they can charge a phone (roughly 10-15Wh per charge), run a laptop (50-80Wh per hour), or power a CPAP machine (30-60Wh per night).

For larger systems, kilowatt-hours (kWh) are used — simply watt-hours divided by 1,000. Your utility bill measures consumption in kWh, and residential battery systems like the Tesla Powerwall are rated in kWh of usable capacity.