Sizing is the step most buyers skip, and it is the reason so many power stations end up either useless or oversized. The arithmetic is simple: one sum for what it must start, one for how long it must last.
This is a research-based explainer using manufacturer-published ratings. No prices or marketplace data appear here.
Step 1: list what runs at the same time
Write down every device you want powered simultaneously and its wattage — usually printed on the appliance or its label. Add them up. That total is your minimum continuous inverter rating.
Add headroom of about 20%. An inverter running permanently at its ceiling is an inverter that gets hot and shuts down.
Step 2: check the biggest start-up surge
Anything with a motor or compressor — fridges, pumps, power tools — draws several times its running wattage for a moment at start-up. Your unit's surge rating must cover the largest of those spikes.
This is why a fridge that runs at 150W can still trip a small inverter. Size for the spike, not the average.
Step 3: convert to watt-hours
Multiply your average draw by the hours of runtime you need. A 100W average over 10 hours is 1000Wh, so you want a 1kWh unit at minimum, and more once you allow for conversion losses.
For a fridge, use its daily energy consumption from the energy label rather than its nameplate wattage — the compressor cycles rather than running constantly.
Step 4: pick the capacity band
Under 500Wh: phones, laptops, cameras, lights, a router, most CPAP machines. Compact and genuinely carryable.
700Wh–1.2kWh: the mainstream band. A camp fridge for a weekend, or a household fridge plus electronics for several hours in an outage.
2kWh and up: outage backup with real endurance, and higher-wattage appliances. Around 50 lb, so plan placement. Above that, 4kWh with 240V output covers well pumps and circuit-level backup.
Step 5: decide whether solar changes the answer
Solar input is capacity you did not have to buy. A modest array can carry a share of daytime load and stretch a mid-size unit across several days.
It does not replace inverter capacity. If the appliance will not start, no amount of solar helps.
Keep reading
Frequently asked questions
Final verdict
Size the inverter for the biggest thing that must start, and the battery for how long the rest must last. Do both sums and most people land in the 1kWh to 2kWh band.
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