How Home Battery Software Decides When to Charge and Discharge
- Most systems offer modes, such as maximising self-use or following a tariff.
- Tesla lists Self-Powered, Savings Mode, Backup Reserve and Storm Watch as app features for the Powerwall.
- With solar, a battery typically charges when panels produce more than the home uses.
- Without solar, the battery can charge when rates are low and discharge when they are high.
- How each maker's algorithm works is proprietary, so this guide describes the common logic only.
A home battery does not follow one fixed rule. Its software weighs a handful of inputs, such as solar output, the tariff, your usual use and the reserve you set, and picks a mode. Understanding those inputs lets you check whether the choices suit you.
The basic loop
Tesla's explanation of how Powerwall works gives the simplest version. With solar, the battery charges during the day when the panels produce more than the home needs, then supplies the home at night or during a grid outage. Without solar, it charges when rates are low and discharges when they are high.
Everything else is a refinement of that loop. Newer software adds forecasts and tariff data, but the aim is the same: put energy where it saves the most.
The inputs the software weighs
Different brands label these differently, but the decision usually comes from the same short list.
- Tariff schedule: when electricity is cheap or expensive, including any cheap overnight window.
- Solar generation and forecast: whether the sun is likely to fill the battery tomorrow.
- Your usual use: the evening peak the battery should cover.
- Reserve setting: the share of charge kept for a power cut.
- Export limits and grid signals: rules that restrict what can flow to or from the grid.
Where the goals clash
Several goals can pull in different directions. Keeping a high reserve protects you in an outage but leaves less for tariff savings. Charging from the grid overnight helps on a cheap tariff but is wasteful if the sun would have filled the battery for free the next day.
That is why it is worth knowing which mode your system uses. The same battery can behave very differently on a self-use setting and on a time-based one, and the app does not always say which is better for your tariff. Our look at AI smart charging covers how newer software tries to resolve those conflicts, and our piece on why energy management software may matter more than capacity explains why it deserves your attention.
Proprietary logic
Makers rarely publish their decision rules. If a claim about a battery being smart is not backed by a description of the modes and inputs, treat it as marketing.
How to check it is doing the right thing
You do not need to understand the algorithm to test the result.
Note your settings
Write down the mode, reserve and any scheduled charging in the app.
Watch a typical week
Look at when the battery charges and discharges against your tariff's cheap and expensive periods.
Change one thing
Alter a single setting, such as the reserve, and compare the next week.
Ask the installer
If the behaviour surprises you, ask whether it is intended or a configuration issue.
Common questions
Does the battery learn my habits?
Some systems use your past use to forecast demand. Whether yours does, and how, depends on the brand, so check the manual or ask the maker.
Should I leave the battery on the default mode?
The default is a starting point. If you are on a time-of-use or dynamic tariff, check whether a tariff-aware mode would suit you better.
Why does the battery sometimes not discharge in the evening?
A reserve setting or a mode that holds charge for a power cut can stop discharge. Check the reserve and mode in the app.
Related guides
Want to know which battery size fits your home?
Use our free calculator to get a size, savings and payback estimate.