A person answers.

Add up the watts of what you want on, times the hours a day each one runs, and divide 13.5 kWh by that. The sample house on this site uses 4.76 kWh a day, so a full charge lasts about 68 hours with no sun. The calculator does the sum for your own list.

No. The backup switch takes the house off the grid and the battery takes over by itself. What helps is deciding beforehand what stays on.

The loads that turn power into heat: space heaters, ovens, dryers and electric water heaters. A 1,500 W heater for four hours uses 6 kWh, almost half the battery, on its own.

The house goes back to the grid and the battery refills, from the roof when the sun is up. The next outage starts from whatever charge it has.

One K13 gives 7.6 kW, enough to run a 3 ton system once the installer fits it with a soft starter. It will not run it for long: at 3,500 W a full battery lasts under four hours. Two or three units, or a roof that refills them, change that sum.

It keeps a CPAP or an oxygen concentrator running like any other load, and the calculator shows how long each one lasts. Ask your doctor and the device maker how long it must run, and keep a plan for when the charge gets low.

A CPAP with the humidifier off draws about 40 W. Eight hours a night is 0.32 kWh, so the CPAP alone would run for 42 nights on one charge.

No, but it recharges from them. Without panels it carries the house on its stored 13.5 kWh. With them, a clear day can fill it again: a 6 kW roof with 4.5 sun hours makes about 21.6 kWh after losses.

Yes. The K13 connects on the AC side of your panel, so it works with any grid-tied solar inverter. The installer checks yours at the site visit.

On a garage wall, a utility room wall or an outside wall: the case is rated NEMA 3R. It needs 29.5 in of wall and 5.9 in of depth. If the wall cannot carry 282 lb, it stands on the floor on steel feet.

About two hours, while the backup switch goes in by the meter. The installers tell you before they turn it off.

One K13 is $7,900. A home also needs one backup switch ($1,150) and installation ($2,600 for the first unit). That is $11,650 for one unit on the wall, before sales tax. The installer confirms the price at the site visit.

No. The visit is free and booking it takes no card. You pay nothing until you have the final price in writing and say yes.

No. The estimate assumes no tax credit, rebate or utility incentive. If your state or utility offers one, your installer will tell you at the site visit.

If the K13 holds less than 70% of its 13.5 kWh within 10 years of the day it was switched on, we repair or replace it. Parts, labor and the trip are covered.

No. The 10 years do not depend on how often it charges. Daily use with solar is what it is built for.

How much is left. Amber rises from the bottom as the battery fills: forty segments, 2.5% of the pack each. Half the bar lit is half the battery.