Turning a percentage into hours of backup

Percentage alone can't give hours. You need the battery's usable capacity and your load. Then it's one division.

The formula

One maker's runtime guide gives the core formula as usable battery capacity in kilowatt-hours, divided by average outage load in kilowatts. The answer is backup runtime in hours.¹

Usable capacity matters here. The guide describes it as the part you can actually draw during an outage.¹ A kilowatt-hour, or kWh, is a unit of stored energy. A kilowatt, or kW, is how fast energy is being used. Load means the power your home is using at a given time.¹

Steps

1. Find the usable capacity on your product's spec sheet or manual. Don't use the rated figure if the manual gives a usable one. The guide's example is a battery rated at 13.5 kWh with only 12.5 kWh truly available in an outage, so it uses 12.5.¹
2. Check what the percentage means. The percentage is the share of capacity remaining.² None of the sources here give a method for applying the screen percentage to usable capacity. Check your manual for whether the percentage is of total or usable capacity, and how any reserve counts. The guide notes that a 10% reserve lowers usable capacity further.¹
3. Add up your load. List what you'd run and its power draw from labels or manuals. The guide's example is a refrigerator averaging 150 watts over 24 hours, which comes to about 3.6 kWh per day.¹
4. Divide energy by load, in kWh and kW.
5. Trim the answer. For many homes, the guide suggests taking 10% to 15% off the ideal runtime for a more realistic figure.¹

A worked example

The guide's own example takes 12 kWh of usable energy and a 0.75 kW load, which gives 16 hours.¹

The sources give no method for turning a screen percentage into usable energy, so this page gives no hours figure for one. Such a sum would be unsourced. It would also ignore the reserve and the error in the estimate.³

Why the answer is only a guide

The guide says its ranges are not guarantees.¹ The percentage is also an estimate. One maker cautions against assuming a percentage point equals the same usable energy at every temperature and load.³

A reserve setting changes the sum too. If part of the battery is held back, the energy you can draw before it's reached is smaller. Reserve settings explained covers that.

Large appliances change the load quickly. A run time worked out for a refrigerator and lights won't hold if a heavy load joins in.

Where the load figure comes from

The guide points to appliance labels, manuals, utility monitoring apps or smart plugs as ways to find power use.¹ Pick the loads you'd truly keep running. Add them up. A short list gives a longer runtime.

Be honest with the list. Leaving out a heating or cooling load makes the answer look better than it is.

Checking against your own manual

Some makers publish runtime tables or limits for their own products. Use those over any general formula. Where your manual and a formula disagree, follow your manual. If you're unsure what your system can power in an outage, ask the installer. Changes to home wiring or panels are licensed electrician work, not a home project.