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The number printed on the side of a power station is not what powers your devices — it’s a ceiling, and you’ll never touch it. Inverter losses, idle draw, and the efficiency of your specific load all trim it back. But that gap between label and reality isn’t the thing that actually strands people in an outage. What strands them is a fridge compressor kicking on and tripping the inverter in the first ten minutes, on a unit that was perfectly sized for average consumption. The label said 1,800W continuous. The fridge demanded 900W to run and 2,100W to start. The math was never going to work.
Sizing a power station for home backup is really two separate problems that almost everyone treats as one. The first is capacity: how many watt-hours do you need to run your critical loads for the duration you’re planning? The second is delivery: can the unit’s continuous and surge output actually handle the highest-demand moment — not the average, the spike? Get the first right and fail the second, and you have a very expensive paperweight the moment a motor starts. This guide works through both, in the order that matters.
The Two Numbers That Actually Size a Power Station
Before any arithmetic, anchor yourself to two specifications on every unit you’re considering: rated continuous output watts and surge output watts. These live on the spec sheet and they are your hard constraints. The watt-hour capacity is the fuel tank; continuous and surge output are the engine. A full tank doesn’t help if the engine can’t turn the load.
The surge figure is the one people skip. Motor-driven appliances — fridges, well pumps, window AC units — draw roughly two to three times their running wattage for a second or two when the compressor or motor starts up. A fridge that runs at a few hundred watts might briefly demand over a thousand at startup. If that startup spike exceeds your unit’s surge rating, the inverter shuts down as a protection measure. You’re in the dark, the unit is fine, and you have no obvious way to know why it just cut out.
The practical implication: buy for the surge spec of your single largest motor load, not for your average running watts. Manufacturers know this, which is why many mid-tier units specifically list a surge rating roughly double their continuous rating — the EcoFlow Delta 3 Plus, for instance, is spec’d at 1,800W continuous and 3,600W surge. That pairing exists because of compressors, not because you’ll ever sustain 3,600W for more than a moment.
A flat buffer added to your total load estimate — like “add 20% for startup” — doesn’t fix this. Twenty percent of a multi-device total cannot absorb a 200–300% spike on a single motor. Identify your highest-surge device, find its startup draw, and make sure the unit’s surge rating clears it with room to spare. Everything else can be layered on top.
Some Loads Can’t Run at All — Check This First
There’s a class of problem even worse than a tripped inverter: loads that exceed what any standard portable station can deliver, period. A standard North American 120V/15A circuit tops out around 1,800W. Most portable power stations are designed around this — their continuous output sits right at that threshold because that’s what a single standard outlet can handle anyway.
But well pumps, central air conditioners, electric water heaters, and other 240V appliances don’t live in that world. They can’t be run from a standard portable unit regardless of how many watt-hours it holds. Capacity does not equal capability. Before you build a runtime calculation around keeping the well running during an outage, verify whether your well pump is 120V or 240V, and whether its running draw is even in the range a portable station can sustain.
This is worth doing before anything else, because if your critical loads include 240V equipment, you’re in a different category of solution — hardwired whole-home backup or a large generator — and the rest of this guide’s arithmetic doesn’t apply to those loads.
How Many Watt-Hours You Actually Get to Use
Once you’ve confirmed your loads are within reach, you can start on capacity. The label watt-hour figure is not your usable watt-hours. By the time DC power from the battery is converted to AC at your outlet, inverter efficiency trims it. By the time you factor in the station’s own idle consumption when it sits powered-on between load cycles, it trims further.
Two sources examined here land on different numbers for this derate, which is itself informative. One applies a straightforward 15% loss and calls it 85% usable. The other derives inverter efficiency closer to 80% — a 25% effective penalty on AC loads — and notes that idle draw stacks on top of that in real use. Neither figure was measured on actual hardware; both are estimates. The honest planning range is 75–85% of the labeled watt-hours reaching your devices, and where you land in that range depends on:
- Inverter quality — cheaper units tend toward the low end
- Whether loads are AC (through the inverter) or DC-direct (avoid conversion loss entirely)
- How long the unit idles between load cycles — a station left on overnight with a tiny draw bleeds capacity to its own overhead faster than the runtime formula predicts
There’s no measured teardown behind either number in the evidence here, so don’t treat 80% or 85% as a precise specification — treat it as a planning margin. When in doubt, use 75% and let yourself be pleasantly surprised.
What Your Fridge Actually Costs You (It’s Not the Nameplate)
The most practically useful number in home backup sizing is one that almost nobody uses: the fridge’s average draw, not its nameplate draw. A fridge doesn’t run continuously — the compressor cycles on and off, running roughly a third of the time under typical conditions. A unit with a nameplate running draw in the 240–400W range averages something like 80W when you account for that cycling, or around 300 watt-hours per day.
Two independent sources here arrived at approximately that same daily figure from different starting points — one worked from a nameplate and a duty-cycle estimate, the other from an annual energy label. That convergence is reassuring. The fridge daily cost is roughly 300 Wh, and for three days of backup, call it around 900 Wh allocated to the fridge alone.
Why does this matter so much? Because if you size off the nameplate running wattage instead of the duty-cycled average, you overstate daily energy consumption by roughly five times. That translates to massively over-buying on capacity — or, more dangerously, to under-buying because the real load is smaller than you feared and you assumed you couldn’t afford adequate backup.
The counterintuitive part: the surge constraint from the previous section still applies in full. The fridge averages 80W but it starts at two to three times its running draw. You need a small fuel budget and a large surge headroom for the same device. These are genuinely independent requirements, and the surge spec is what determines whether the unit can run the fridge at all.
Estimating Runtime: The Formula and Its Limits
With a derate in hand and a realistic load estimate, runtime arithmetic is straightforward:
(Labeled Wh × derate) ÷ average running watts = approximate hours
The research here works an example: a 2,400 Wh unit derated at 85% yields roughly 2,040 usable watt-hours; at a sustained 400W draw, that’s a little over five hours. Bump the derate down to 75% for a cheaper unit and you’re closer to four and a half.
The formula is sound. The inputs are where reality diverges from the back-of-envelope. A real household doesn’t sustain a flat 400W — it runs a mix of cycling loads, standby draws, and occasional spikes. The runtime number is a planning fiction that’s useful for choosing between a 1,500 Wh and a 3,000 Wh unit, but it shouldn’t be quoted as a guarantee. Use it directionally: does this get me through one night, or three?
For a worked illustration: say your critical loads — fridge, a few lights, phone charging, a CPAP — add up to roughly 600W average running draw. Using a 75% derate as your planning floor, a 2,000 Wh unit gives you something on the order of 1,500 usable watt-hours, or around two and a half hours of sustained load. A 4,000 Wh unit at the same derate gets you closer to five. These are invented inputs to show the method — your actual load profile is what goes into the formula, and you build that from your specific devices, not from a generic list.
Solar Recharge: A Supplement, Not a Safety Net
If you’re planning to extend backup duration with solar panels, build in a heavy discount before you rely on it. A panel’s rated wattage is measured under laboratory-ideal conditions you’ll essentially never see in the field — the right angle, the right temperature, no haze, peak sun. One source here puts real-world production as low as 50% of the rated figure even on a sunny day, and that’s a floor estimate, not a guarantee. Portable and foldable panels generally underperform fixed mounted ones further still.
The practical implication: solar during a short outage (a few days) is a trickle supplement to battery capacity, not a way to offset a smaller battery purchase. If your daily consumption outpaces what your panels realistically produce — and in most home backup scenarios, it will — you’re drawing down the battery with a slow trickle coming back in. Plan the battery capacity to cover your duration outright, and treat solar as a bonus that might buy you another partial day.
Capacity, Cost, and the Weight Problem Nobody Mentions
The rough market picture from the research here: roughly 2,000 Wh runs around $1,500 and weighs approximately 60 pounds; roughly 4,000 Wh runs around $3,500 and weighs approximately 100 pounds. These are ballpark figures from a single consumer source and prices move — treat them as order-of-magnitude, not current quotes. But the shape of the curve matters: cost and weight scale faster than capacity. Doubling your watt-hours more than doubles your price and puts you in the range where “portable” is technically true and practically misleading.
A hundred-pound unit is a unit you stage once and leave there. That’s fine if you’ve planned for it — but if your backup scenario involves moving the station during an emergency, or taking it between locations, the weight is a real constraint that capacity planning often ignores. Above roughly 1,000–2,000 Wh, a DIY battery build can offer meaningful cost savings, though that trades integrated battery management and warranty for hands-on competence and a custom assembly.
Putting It Together
The right mental model for home backup sizing is two sequential gates, not one calculation. The first gate is capability: can the unit’s continuous and surge output actually start and sustain your highest-demand motor load? If it can’t clear that gate, watt-hours are irrelevant. The second gate is capacity: does it hold enough usable energy — derated from the label, built from realistic duty-cycled load estimates — to cover your planned duration?
Most people go straight to the second gate and skip the first. That’s the failure that leaves them in the dark with a fully charged unit that trips the moment the fridge compressor cycles on. Check surge first, check running watts and 240V compatibility second, then do the runtime math. In that order, the numbers actually mean something.