Will Your Backup Power Survive This Outage?

How the runway maths works

The starting point is energy you can actually reach, not energy on a label. Rated capacity times one minus the loss fraction gives the AC energy the battery can deliver. That is the same calculation the runtime calculator makes, and it is the honest number to plan against. A 1,000 Wh station at a 15% loss reaches about 850 Wh at the outlet.

Next comes the load side. Add up every appliance you intend to run at once, in average watts, not peak nameplate figures. That sum is your burn rate. Runway is stored energy divided by burn rate. Solar, if you have any, is not a separate answer. It is more energy added to the same budget, spread across whichever days of the outage have sun. The plan succeeds or fails against the outage length you entered, not against an abstract number of hours.

• Stored energy is fixed. Solar makes that number grow while the sun holds up.
• Runway only means something once you set an outage length to test against.
• The medical load is never part of a shed plan. Size around it first.

Worked example: one night, one small station

Take a 1,000 Wh station, a fridge averaging 60 W, some lighting at 15 W, and a router at 12 W. That is 87 W of continuous load. Usable energy is 850 Wh after a 15% loss. Divide and the runway is close to nine and three quarter hours. If the outage you are planning for is a ten hour night, this plan falls just short, by roughly fifteen minutes. That is the kind of margin a single unplanned kettle boil can turn into a real gap.

Now add a 200 W panel with three peak sun hours expected during the outage. Over one day, that adds about 510 Wh of usable harvest on top of the stored 850. Total available energy climbs past 1,350 Wh, and the runway stretches to roughly fifteen and a half hours. The same battery, the same fridge, and the same router now cover the ten hour night comfortably. There is even room for the lighting load to run longer than the fridge's own duty cycle might suggest.

When the answer says no, what to shed first

If runway falls short of your outage plan, the tool shows the load you could sustain for the full duration instead. The difference between that figure and your entered load is what you have to shed. Work down a priority list rather than guessing. Comfort cooling and heating usually go first, unless a household member's health depends on them. Entertainment and non-essential charging come next. Lighting can often drop to one room.

The fridge is a harder call. Food safety guidance generally says a closed, reasonably full fridge keeps food safe for around four hours without power. A full freezer can hold much longer than that. Shedding a fridge for a short, planned outage is often acceptable. Shedding it for a two day fault is a different decision. Make that call against how full the fridge is, not against the watt figure alone.

This is also the moment to separate a want from a need. A kettle is a want during a blackout, and hot drinks feel essential until the second one. A torch in every room you use is a need, and it draws almost nothing. Working the list this way usually finds more margin than shaving an hour off the lighting schedule does.

How solar changes the plan mid-outage

Solar is the only input here that can turn a fixed budget into a flowing one. If panel output, after losses, exceeds the total load on any sunny stretch, the battery stops being the limit. It can even climb back toward full. If output runs below the load, solar only slows the drain rather than stopping it. The runway figure then reflects a blend of stored and harvested energy, not one or the other alone.

Weather is the honest asterisk on all of this. The sun hours you enter are an expectation, not a guarantee. A storm system sitting over a multi day outage can drop that figure close to zero for the whole event. Plan against the sunny case here, and keep a separate, solar free answer ready for the days the sky does not cooperate. That fallback plan is the one you actually reach for when the forecast turns.

Accuracy, assumptions and limitations

This tool assumes your load stays steady across the whole outage, and that solar arrives at the same rate every sunny day of it. Real behaviour is lumpy. A fridge cycles on and off. A kettle, used once, can eat an hour of careful planning in two minutes. Solar output swings with cloud cover hour to hour, well beyond the average this calculator uses.

It also assumes a healthy battery at a reasonable temperature, and an inverter working at its typical efficiency. It assumes no other household member quietly plugs something in. Treat a shortfall of under an hour as a genuine maybe rather than a clean yes. A plan that only works at the optimistic end of the range usually meets the first real outage with no room left to move.