Learn how to calculate watt hours with a simple formula, a worked example, and tips on usable battery capacity and inverter losses.
- Watt-hours equal watts multiplied by hours.
- You will get less than the printed capacity because of losses.
- Add up essentials, multiply by days, and keep a margin.
Watt hours sound technical, but the idea is simple. A watt-hour is a unit of energy: one watt used for one hour. If you can multiply two numbers, you can work out what your appliances use and how long a battery will last. This guide walks through it step by step.
The basic formula
Energy in watt-hours equals power in watts multiplied by time in hours.
- A 10 watt LED light on for 5 hours uses 50 watt-hours.
- A 60 watt device on for 2 hours uses 120 watt-hours.
Battery capacity is also shown in watt-hours. A battery with 1,000 watt-hours could, in theory, supply 100 watts for 10 hours. In practice you get a bit less.
Step 1: find each appliance's watts
Look at the label on the appliance or its power adapter. You might see watts (W) directly. If you see only volts (V) and amps (A), multiply them: watts equals volts times amps. For example, 120 volts and 1 amp is 120 watts. Remember that labels often show the maximum draw, so real use may be lower. A plug-in watt meter can show actual use if you want to measure it.
Step 2: estimate hours of use
Be realistic about how long each item will run. Some things run continuously, such as a router. Others cycle, like a fridge compressor that is on for part of each hour. For cycling items, estimate the fraction of time it runs. If you do not know, a plug-in meter over a day will show the real number, or you can use a conservative guess.
Step 3: add it all up
Write a short table with each appliance, its watts, its hours per day, and the watt-hours. Then add up the column. That total is the daily energy you need.
A worked example
Here is a made-up round-number example for one evening.
- Router and modem: 20 watts for 6 hours is 120 watt-hours.
- Three LED lights: 30 watts total for 5 hours is 150 watt-hours.
- Laptop: 50 watts for 3 hours is 150 watt-hours.
- Phone charging: 10 watts for 2 hours is 20 watt-hours.
The total is 440 watt-hours. That is the energy used before accounting for losses.
Usable capacity and inverter losses
A battery's printed capacity is not all available to you. Power stations lose some energy when they convert the stored direct current to household alternating current. They also use a small amount to run their own electronics and display. Many units also hold back some capacity to protect the battery. The result is that you get noticeably less than the printed number.
The loss is not fixed. It depends on the unit and how heavily it is loaded, so we will not give a single number. As a rule of thumb, plan on a reduced figure and leave a margin. Using the DC and USB ports directly for phones and lights skips the inverter and wastes less. Cold weather can reduce usable capacity too. See portable power station in cold weather.
Check the surge as well
Watt-hours tell you how long a battery lasts. They do not tell you if it can start a motor. For that, you also need to check the output and surge ratings. See running watts vs starting watts.
Turn the math into a plan
- Total your daily watt-hours for essentials.
- Multiply by the number of days you want to cover.
- Divide by the usable fraction of the battery to get the capacity needed.
- Add a margin, and consider solar to recharge. See charging with solar panels.
The what size power station do I need guide and the outage planner do the sums with you, and the how many watts to run a house guide shows the typical whole-home picture. Compare models in the power station hub.
Quick answers
How do I convert amps to watts?
Multiply amps by volts. For example, 2 amps at 120 volts is 240 watts.
Why is my battery not lasting as long as the math says?
Inverter losses, standby use, cold temperatures and a protected reserve all reduce usable capacity.
Is a watt-hour the same as a watt?
No. A watt is a rate of power use, and a watt-hour is the energy used over an hour.