An outage is not a single-appliance test. Several things run at once, they don't run at the same time as each other, and the ones that matter most — the fridge, the furnace fan — cycle on and off rather than drawing steadily.
That's why home backup maths is different from portable power maths, and why a calculator built for one appliance gives the wrong answer here.
Why the number on the box is optimistic
Loads overlap, but not perfectly. Adding up nameplate wattages for every appliance gives a figure far above reality, because they rarely all run simultaneously. Using average draw over the outage window is much closer to the truth, and much less alarming.
The overnight window is the one that matters. Most household outages are survived rather than lived through. What you actually need to know is whether the battery carries you from evening to morning, not whether it could theoretically run for a stated number of hours at a constant load.
Motor startup decides what runs at all. Fridges, furnace fans, well pumps and sump pumps spike hard for a fraction of a second on start. That spike barely affects runtime, but it determines whether the appliance starts — a constraint no runtime figure will surface.
Cold changes everything. Winter outages are when backup matters most and when batteries hold least, while heating loads simultaneously rise. Plan for the season you'll actually need it in.
What these calculators can and can't tell you
They will tell you reliably whether you're in the right capacity band, and whether a given battery gets your essentials from dinner to breakfast.
They can't model your house. Insulation, outside temperature, how often the fridge is opened, and whether your furnace is running flat out all shift the answer. Treat the output as a planning estimate with real slack in it, and if the result lands close to your requirement, size up rather than hoping.
They also assume your appliance list is complete. The most common error isn't arithmetic, it's forgetting the well pump.
Which calculator you want
For a whole-outage estimate across several appliances at once — the normal home backup question — use the blackout runtime calculator.
If your question is narrower, about how long one specific device runs on one specific battery, the single-appliance version with the loss arithmetic spelled out is the power station runtime calculator.
Once you have a number, turn it into a purchase with my home backup buying guides, narrow the field in the comparisons, and read the verdicts in my home backup reviews.
