How to Size a Portable Power Station: Wh, W and Real Runtime

Choosing a portable power station comes down to two numbers: how much energy it stores and how much power it can deliver at once. Get both right and the unit will run your devices for as long as you need. Get one wrong and you either carry around more weight than necessary or find that your coffee machine trips the inverter. This guide shows you how to work out both, step by step.

Watt-hours and watts: tank and engine

Capacity in watt-hours (Wh) is the size of the tank. A 1,000 Wh station can, in theory, supply 100 W for ten hours or 500 W for two hours.

Output in watts (W) is the size of the engine: the highest load the built-in inverter can supply continuously. A station rated at 1,800 W can run a 1,500 W hair dryer, while a 600 W unit will shut down to protect itself.

You need enough of both. A large battery with a small inverter cannot start a kettle; a powerful inverter with a small battery will start it but run empty quickly.

Continuous and surge power

Most data sheets list two output figures:

• Continuous (rated) output is what the inverter supplies without time limit.
• Surge (peak) output is a short burst, usually for a fraction of a second up to a few seconds.

Surge matters for anything with a motor or compressor. A fridge, freezer, cooler box with compressor, drill, angle grinder or pump typically draws two to three times its running power when it starts, and some motors draw more. Resistive loads such as kettles, heaters and toasters draw roughly the same power from start to finish.

When you check a power station against your devices, add up the running power of everything that will run at the same time and compare it with the continuous rating. Then check that the largest motor load, multiplied by its start-up factor, stays below the surge rating.

Why not all of the capacity is usable

The battery stores direct current (DC). Household devices need 230 V alternating current (AC), so the inverter converts it, and that conversion costs energy. Typical inverter efficiency is around 85–90 %. In addition, the battery management system keeps a small reserve to protect the cells.

As a rule of thumb, plan with about 85 % of the rated capacity for AC loads. Devices you charge through USB-C or the 12 V outlet skip the inverter and use the energy more efficiently, so charging phones and laptops via USB-C is a simple way to stretch your runtime.

Step-by-step calculation

1. List your devices and find their power in watts. The label or manual usually states it. For fridges and freezers, use the annual consumption on the EU energy label instead (see below).
2. Estimate hours of use for each device during the period you need to cover.
3. Multiply watts by hours to get watt-hours per device, then add them up.
4. Divide by 0.85 to allow for inverter losses.
5. Add a margin of 20–30 % for cold weather, battery ageing and the unexpected.
6. Check output: the continuous rating must cover everything running at once, and the surge rating must cover motor start-up.

A note on fridges

A fridge does not run its compressor all the time; it switches on and off. Multiplying its rated power by 24 hours would overstate its consumption considerably. The EU energy label shows consumption in kWh per year. Divide by 365 to get the daily figure: a fridge rated at 150 kWh per year uses about 0.41 kWh, or 410 Wh, per day.

Worked example: fridge, lights and phones for one night

Say you want to keep a small fridge cold, run some LED lights and charge two phones over a 12-hour night, for example during a power cut or at a campsite.

Now apply the steps:

• Inverter losses: 295 ÷ 0.85 ≈ 350 Wh.
• Margin of 25 %: 350 × 1.25 ≈ 440 Wh.

A station of around 500 Wh covers this night comfortably. For output, the loads running together are well under 200 W, but the fridge compressor may draw roughly three times its running power at start-up, so look for a surge rating of at least 300–400 W. Practically every station in the 500 Wh class meets that.

If the same setup has to last 24 hours, double the energy to roughly 900 Wh, which points to the 1,000 Wh class. Examples in our range include the Jackery Explorer 1000 v2 (1,070 Wh, 1,500 W), the UAPOW AP1000 (1,024 Wh, 2,000 W) and the EcoFlow DELTA 3 Classic (1,024 Wh, 1,800 W). Our AI Solution tool does this calculation for you and shows matching models from each brand.

High-power appliances

Some appliances need a lot of power but only for a short time:

• An electric kettle at 2,000 W boiling for five minutes uses about 170 Wh. The energy is modest, but you need a station with a continuous rating above 2,000 W.
• A 1,000 W microwave running for ten minutes uses about 170 Wh.
• An electric heater at 1,500 W running for one hour uses 1,500 Wh, enough to drain a mid-sized station in a single evening.

Some stations offer a mode that runs heating appliances such as kettles at reduced power when their wattage exceeds the inverter rating. This helps in an emergency, but the appliance works more slowly. For regular use, choose a station whose continuous output exceeds the appliance's rating.

Leave room to grow: expansion batteries

Many mid-sized and large power stations accept extra batteries. This lets you buy the inverter output you need today and add capacity later. For example, the EcoFlow DELTA 2 Max (2,048 Wh) can be expanded to 6,144 Wh, and the PECRON E1500LFP (1,536 Wh) to 4,608 Wh. If you expect your needs to grow, check the maximum expandable capacity on the product page and compare models side by side in our comparison tool. Compatible extra batteries are listed separately.

Recharging matters too

Capacity tells you how long you can run; recharging tells you how quickly you can start again. Check three figures:

• AC charging time: how long the station needs on a wall socket. Many current models charge from empty to full in one to two hours.
• Solar input (W): the maximum solar power the station accepts. To refill a battery in one day, the panel output in watts multiplied by the peak sun hours (and by about 0.7–0.8 for real-world losses) should match the energy you used.
• Car charging: the 12 V socket in a car usually delivers only around 100 W, so it is a slow top-up rather than a full recharge.

Battery chemistry and lifetime

Capacity slowly declines over the years. LiFePO4 batteries typically keep 70–80 % of their capacity after 3,000 to 6,000 cycles, while NMC batteries are typically rated for fewer cycles. If you will use the station every day, this difference matters. Our guide LiFePO4 vs NMC explains it in detail.

Quick reference

Summary

Work out the energy you need in watt-hours, allow about 15 % for inverter losses and 20–30 % margin, and check that both continuous and surge output cover your devices. If you are unsure, enter your devices in our AI Solution tool, which suggests matching stations, solar panels and extra batteries from each brand we carry, or browse all power stations.