Choosing a portable power station starts with three numbers people constantly confuse: watt-hours, watts, and runtime. Get these straight and sizing becomes simple.

Watt-hours is the fuel tank Watt-hours (Wh) measure total energy stored. A 1000Wh station can theoretically run a 100W load for 10 hours, or a 500W load for 2 hours. In practice, inverter losses eat 10 to 15 percent, so plan on about 85 percent of the rated capacity being usable.

Watts is the size of the pipe The inverter's continuous watt rating limits what you can run at once. A 1800W inverter can power a coffee maker or a small power tool. A 300W inverter cannot, no matter how big its battery. Always check that your highest-draw device fits under the continuous rating.

Surge watts matters for motors Fridges, pumps, and power tools spike hard at startup, sometimes two to three times their running watts. Look at the surge rating, not just continuous, or the station will trip the moment a compressor kicks in.

Size to your real loads - Phones, laptops, lights, a fan: 250 to 500Wh is plenty for a weekend. - CPAP, mini fridge overnight, camera gear: 500 to 1000Wh. - Running a full-size fridge through an outage, power tools: 1000Wh and up with an 1800W or larger inverter.

Add a margin List your devices, add their watt-hours for the time you need, then add 25 percent for inefficiency and cold-weather losses. Buying slightly bigger than your math suggests is the difference between power to spare and a dead box at hour four.