How-to guide · Portable Power Stations
How to Size a Portable Power Station (Watt-Hours Explained)
Two numbers decide the size: watts for what you run at once, watt-hours for how long it lasts. Here is how to work out both.
By Watt Versus Editorial· Updated · 5 steps
This page was drafted with AI assistance from manufacturers' specs and manuals, and is checked by our editor.
A power station has two ratings that matter. Output in watts (W) is how much it can power at the same moment. Capacity in watt-hours (Wh) is how much energy it stores, and so how long it lasts. You need enough of both.
Two allowances keep the estimate honest. Not all the stored energy reaches your devices: the inverter that makes AC power loses some, and the battery is not drained to absolute zero. BLUETTI, for example, estimates usable energy by assuming 90% depth of discharge and 90% inverter efficiency, which works out to about 80% of the rated capacity. We use the same 80% rule below.
Step by step
- 1
List what you will run
Write down every device and its wattage from its label or manual. For devices that cycle on and off, such as fridges, use the average daily energy (kWh per day) from the label if it is given.
- 2
Add up the watts you will run at once
The total of everything running at the same time must stay below the station's continuous output. For motors and compressors, also check the start-up surge against the station's peak output.
- 3
Work out daily watt-hours
Multiply each device's watts by the hours it runs per day, then add them up. Example: a 60 W laptop for 4 hours is 240 Wh; a 10 W light for 5 hours is 50 Wh.
- 4
Allow for losses
Divide your daily total by 0.8 to cover inverter losses and the reserve the battery keeps. A 600 Wh day needs about 750 Wh of rated capacity.
- 5
Multiply by the days without recharging
For a weekend without sun or an outage you expect to last two days, multiply by two. If you will recharge from solar each day, size for one day plus a margin.
Frequently asked questions
- What is the difference between watts and watt-hours?
- Watts are the power drawn at a moment; watt-hours are energy over time. A 100 W device running for 3 hours uses 300 Wh.
- How long will a 1,000 Wh power station last?
- Roughly 800 Wh is usable after losses, so about 8 hours at a steady 100 W, or 80 hours at 10 W.
- Why can't I use the full rated capacity?
- The inverter that creates AC power loses some energy as heat, and the battery keeps a reserve. Manufacturers such as BLUETTI estimate about 80% of rated capacity is usable.