How to size a portable power station: watts, watt-hours and the maths in between
Two numbers decide whether a power station works for you: how much it can output at once, and how much energy it stores. Here is the sum.
Portable power stations are sold with big numbers on the box, and the two that matter are easy to mix up.
Watts versus watt-hours#
Watts (W) tell you how much power the unit can supply at one moment. Watt-hours (Wh) tell you how much energy it stores in total. A kettle needs a lot of watts for a few minutes. A phone charger needs very few watts for hours.
Step 1: add up what you will run at once#
List each device and its power draw, usually printed on its label or in the manual. Add the ones that will run together. If the total is 800 W, the inverter must handle at least that, and ideally with some headroom. Devices with motors or heating elements can briefly draw more when they switch on, so check the unit's surge rating.
Step 2: estimate how long you need power#
Multiply each device's watts by the hours you will use it. A 60 W laptop for 5 hours is 300 Wh. A 10 W lamp for 6 hours is 60 Wh. Add them up.
Step 3: add a margin#
Converting stored battery power into mains power wastes some energy, and you will not want to drain a battery to zero. Add roughly 20 to 25 percent to your total as a sensible rule of thumb.
A worked example#
Weekend camping, with a laptop (300 Wh), a small fridge averaging 50 W for 12 hours (600 Wh) and phone charging (50 Wh). That is 950 Wh. With a margin, shop for something around 1,200 Wh.
Things to check on the spec sheet#
Output ports that match your devices: AC outlets, USB-C power delivery, a 12 V socket.
Recharge options. Mains, car and solar input, and how long each takes.
Battery chemistry. LiFePO4 cells usually last more charge cycles than other lithium types but can weigh more.
Weight. Large units are heavy. Check it against how far you will carry it.
Warranty length and whether you can reach support.
Do not skip this#
Never use a unit indoors in a way the manual warns against, and keep it ventilated while charging. Shop by your own sum, not the biggest figure on the box.