What state of charge means on a home battery

The percentage on your battery is its state of charge, often shortened to SoC. It says how much charge remains in the battery at a given moment.¹

The scale

The number is usually a percentage. Zero is empty and 100 is full.¹

Some makers describe the same thing from the other end. Depth of discharge is how much has been used, so it counts up as the battery empties. State of charge counts down.¹

What the number is for

It works like a fuel gauge. One maker's app guide says it helps owners see how much stored energy is available.²

That's all it promises. It's a share, not an amount. Fifty percent of a small battery holds far less energy than fifty percent of a large one. To turn the figure into kilowatt-hours or hours of running time, you need the capacity from your manual. The page on working out hours of backup shows how.

It's an estimate

A battery has no window onto the energy inside. Its electronics work the percentage out. One maker describes the figure as an estimate, not a direct reading.³ The next page, how the percentage is found, covers the methods.

Because it's an estimate, one percentage point doesn't always equal the same usable energy. The same maker says not to assume it does at every temperature and load.³

The number on screen may not be the real number

Some products show a figure that differs from the true charge on purpose. Vehicles give a clear example. On a plug-in hybrid, the gauge or percentage may not match the real level of charge, because a noticeable amount of energy can be reserved for other uses.¹

Home batteries can do something similar with a backup reserve, a slice of charge held back for outages. That setting has its own page: the reserve setting.

Reading it alongside other figures

One maker advises reading the charge level together with solar output, household load, tariff periods and the reserve setting.² ("PV" in many apps means solar panels.) A full battery isn't automatically a well-run one, and a low one isn't automatically a fault.

Quick checks for your own unit

• Find the stated capacity in your product's manual or spec sheet.
• Note whether the manual says the percentage is of total or usable capacity.
• Look for a reserve or minimum setting in the app.
• Follow your own product's manual for anything it says about the reading.

Capacity and charge aren't the same thing

Capacity is how much a battery can hold. State of charge is how full it is right now. The figure is also tied to the battery's age, since the remaining capacity is measured against a given state of ageing.¹ An older battery at 100% holds what it can hold today, which may be less than when it was new.

Short version: the percentage is a share of something, and that something can change.

Odd readings are common enough to have their own guide. Try why readings jump for the usual causes.