What a backup reserve setting does

A backup reserve is a share of the battery's charge that your system saves for a power outage. You choose the share in the app.

One maker describes it as a way to decide how much of the stored energy is automatically saved as backup power.¹

The split

The reserve is a slice of the percentage. In that maker's design, it's a portion of the stored energy, and the remaining portion goes to self-consumption mode or time-of-use mode.¹ Self-consumption means using your own stored energy at home. Time of use means the battery follows electricity price periods.

So a higher reserve leaves less for daily use. A lower one leaves more. Another maker's guide says a 10% emergency reserve reduces the usable capacity further.²

What happens in an outage

During an outage the reserve becomes available. This maker says the battery may go past the line you set, depending on how long the outage lasts.¹

That behavior belongs to one maker. Other products may act differently, so read your own manual.

Afterward, the battery refills above the reserve level before it goes back to normal operation.¹

The extremes

At a 100% reserve, the battery is held full for outages. The maker says self-consumption and time-of-use modes can't be used then.¹

At a low setting, there's little in hand when the grid fails. The same page gives a stark case. If an outage hits while the reserve is 5%, you lose power to the home immediately.¹

Reserves you didn't set

Some makers also hold back a margin of their own. One guide says manufacturers often reserve a small part of the capacity to protect battery life and keep the system stable.² That's separate from the reserve you pick in the app.

Reading the percentage with a reserve in place

One maker advises reading charge level together with solar output, load, tariff periods and reserve settings.³ A battery sitting at its reserve level may simply be doing what you told it to do.

Choosing a level

A maker's page says that if you want more energy kept for outages, or your area is prone to them, you can set a higher reserve. If you want better performance in self-consumption or time-of-use mode, you can set a lower one.¹

It's a trade. Neither choice is free.

Setting it

This is how one maker, FranklinWH, describes it for its app. Your product's app may differ, so follow your own manual.

1. Open the FranklinWH app and, on the energy home screen, tap Settings.¹
2. Enter Mode and select Self-Consumption.¹
3. Set the reserved energy for outages by adjusting the slider at the top of the screen.¹

FranklinWH shows a warning prompt if the reserve is set below 20%. The prompt says the battery may not have enough stored energy to provide backup support in an outage.¹

The page describes only the Self-Consumption mode slider.

Wiring a battery into a home, or changing how it connects to your switchboard, is licensed electrician work. Don't attempt it yourself.

For how a reserve turns into running time, see the hours-of-backup page.