Two large numbers tend to dominate a portable power station listing: watts and watt-hours. They describe different limits. Watts describe power at a moment in time. Watt-hours describe an amount of energy. A station can have enough stored energy for your plan yet still be unable to start the appliance you want to connect.
For a first estimate, multiply a device's power draw in watts by its planned running time in hours. A hypothetical 20-watt load running for five hours uses 100 watt-hours. This is arithmetic for planning, not a promise that a particular battery will deliver that amount through every port. Goal Zero's runtime guidance uses capacity and device consumption as the starting point for the same reason.
Treat the estimate as a ceiling, not a booking
Battery energy must pass through the station's electronics before it reaches an appliance. Conversion losses and the station's own consumption reduce what is available. Temperature, battery age, and the way the device operates also matter. Avoid applying one universal efficiency percentage to every product and load. Use the manufacturer's runtime data where it matches your application, then leave room for uncertainty.
A refrigerator illustrates why a simple label can mislead. It cycles on and off, and its compressor may need more power when starting. A kettle, by contrast, uses a large amount of power for a short time. Both energy and output limits matter, but in different ways. A running-watt estimate alone cannot establish that a station will handle either appliance.
Find the relevant output specification
Check the continuous AC output, supported starting loads, and the limits of the individual socket or USB port you intend to use. Read how the manufacturer defines any temporary boost mode. A marketing figure called “surge” or “boost” should not be treated as unrestricted continuous power.
USB-C ports have their own power profiles, separate from the AC socket rating. A station advertised with a large inverter may still charge a laptop slowly through a lower-powered USB port. The cable and the laptop's requirements are part of that match. Do not use improvised adapters to bypass an incompatible connection.
Include the next charge in the decision
Capacity is only useful if you can replenish it. Read the charging time with the charger actually supplied, not just an optional fastest setup. If you expect solar charging, check the input limits and your available sunlight rather than assuming a panel's rated output lasts all day.
Write down one realistic scenario before comparing prices: a light for the evening, a phone recharge, and a radio, for example. Add their energy needs, note which devices run together, and compare that plan against a candidate station's documented limits. Keep medically necessary equipment in a separate plan agreed with its provider; an estimate on a shopping page is not evidence of suitable medical backup.
A heavier battery may extend runtime, but it also has to be moved and stored. If you cannot lift the unit to its intended location, split the problem into smaller needs or get assistance with a suitable installation. Buy for the loads and circumstances you can name, not for the largest number you can afford.