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How we work

Solar Backup Lab is a specification and calculation resource. We read the manufacturers' documentation carefully, publish what it says with the source attached, and do the arithmetic that turns a watt-hour figure into an answer about your refrigerator. This page sets out exactly how, so you can judge how much weight our numbers deserve.

Where our figures come from

Published manufacturer documentation and authorized retailer spec tables — never another review site, and never an unattributed number. Each figure is linked and dated on the page it appears on, so a claim you doubt takes one click to check. Runtimes are calculated from those specifications rather than measured, using the formula below.

We publish no ratings — no stars, no scores out of ten. A score compresses a spec sheet into a number that hides the trade-off you actually care about, and the trade-off is the useful part. The comparison table is the verdict.

What we actually do

Pull the published specification from the manufacturer's own site, or from an authorized retailer's spec table where the manufacturer blocks automated access. Every figure is linked and dated at the foot of the page it appears on.
Cross-check it. Manufacturers contradict themselves more often than you would expect — marketing copy and the spec table on the same page routinely disagree about cycle life or weight. When they do, we print both numbers and tell you which one we used.
Calculate runtime from a stated formula, shown on the page so you can check our arithmetic:
usable Wh = rated Wh × 0.85 (inverter efficiency) runtime h = usable Wh ÷ (device watts × duty cycle + 15 W inverter standby)
Say when a number is uncertain. Appliance draw varies by model, age and temperature. Our appliance figures are typical values, not measurements of your fridge.

Why 0.85, why duty cycle, and why standby draw

Rated capacity is not usable capacity. Converting stored DC to household AC costs energy, and these units generally land in the 85–90% range under real loads. We use the conservative end, 0.85, so our runtimes err on the short side rather than flattering the product.

Duty cycle matters more than most buyers realize. A refrigerator does not draw its rated wattage continuously — the compressor cycles on and off, running roughly a third of the time in a well-sealed modern unit. Ignore that and you will calculate a fridge runtime three times shorter than reality.

Standby draw is the correction almost nobody makes, and it runs the other way. An AC inverter consumes power the entire time it is switched on, independently of the load. We budget 15 W. Against a 1,500 W heater that is irrelevant; against a fridge averaging 50 W it adds roughly 30% to the real drain, and against a 30 W CPAP it adds 50%. Published runtime figures that omit it are systematically optimistic, which is the wrong direction to be wrong in when someone is planning for an outage.

The one estimate on this site. That 15 W is not a published specification. Manufacturers almost never disclose inverter standby draw, for the obvious reason that it makes their runtime claims look worse. Everything else we publish is sourced and dated; this figure is a deliberately conservative engineering estimate for this class of unit, and we label it as an estimate everywhere it appears. Independent bench tests of these units generally land slightly below even our corrected figures, so treat our runtimes as an upper bound. If a manufacturer publishes a real standby figure, we will use theirs and cite it.

What would change our mind

If a manufacturer revises a spec, we update the page and change the verification date. If you find a figure here that disagrees with the source we linked, tell us and we will correct it — contact details are here.

How we make money

Affiliate commission, disclosed on every page carrying a link. It does not influence which units we cover or what we say about them — a fabricated recommendation is worth less than the commission it earns. Full disclosure here.