· 6 min read · By Hotsolar engineering team
It's 3 a.m., you open the app and your battery reads 20 %. It won't go any lower, and the house has started drawing from the grid even though there's charge left. The short answer is that it isn't broken. The inverter has a setting that stops it at that point on purpose, to keep a reserve in case of a power cut and to avoid draining it completely every night. If you don't need that much reserve, the value can be lowered.
What's going on
Hybrid inverters have a value that, depending on the brand, is called minimum discharge, minimum SOC, depth of discharge or reserve. SOC stands for state of charge, the percentage you see in the app.
While the grid is on, the inverter uses the battery to power the house until it reaches that percentage. Then it stops discharging it and the house switches to the grid, even though the battery still holds energy. The next day, when the sun comes up, the panels fill it again and the cycle starts over.
That's why the battery seems stuck at 20 % in the early hours: the inverter is doing what it has been set up to do.
What the reserve is for
Power cuts
If the power goes out in the middle of the night, that 20 % is what keeps the essentials running. For this to work, the installation needs a backup output (shown as EPS or backup on the inverter) with the important circuits connected to it.
An example with a 10 kWh battery: 20 % is about 2 kWh. A fridge, the router and a few LED lights, plus the inverter's own consumption, draw around 100 to 150 W on average. That gives you roughly 12 to 18 hours, enough to get through the night until the grid or the sun comes back.
Rough estimate. It depends on how much your appliances really use, on the inverter and on how low it lets the battery go during a power cut.
Looking after the battery
Lithium iron phosphate batteries (LFP or LiFePO4) handle deep discharges much better than old lead-acid ones. Even so, datasheets usually quote cycles at 80 % depth of discharge, and draining them to 0 % every night wears them more than leaving a margin. We go into more detail in our article on whether charging your battery every day wears it out.
Why it does go below 20 % during a power cut
Many hybrid inverters store two separate values. One is the on-grid minimum, which is what stops the battery at 20 % at night. The other is the off-grid minimum, used when there's no grid, and it's usually lower, for example 10 %.
So while the grid is on, the inverter leaves the reserve alone. When the power goes out, it uses the reserve to keep the house running and only shuts down when it reaches the off-grid minimum. If you ever see the battery below 20 % after a power cut, this is why.
Typical values
From the factory, or after installation, the on-grid minimum is usually set somewhere between 10 and 30 %.
| Your situation | Suggested reserve |
|---|---|
| Power cuts are rare where you live and you want to get the most out of the battery | 10 to 15 % |
| Normal use, the odd power cut a year | 20 % |
| Frequent power cuts or equipment that can't be switched off (water pump, medical equipment, a full freezer) | 30 % or more |
Guide values only. They depend on the inverter, the battery and what the manufacturer says in its manual.
How to change it
The setting is on the inverter's screen or in its app, usually in a menu along the lines of Settings, Battery, Minimum discharge. Every brand names it differently, and some inverters keep the on-grid and off-grid values separate.
Before you change it:
- Check your battery manual for the lowest value the manufacturer allows and don't go below it.
- If the menu asks for an installer password, or you're not sure which value is which, ask your installer to change it.
- Only change that value. Voltage, current or battery type settings are a different matter, and a mistake there can cause real problems.
Frequently asked questions
Am I wasting energy with that 20 %? It isn't lost. It stays in the battery and is there if the power goes out. What happens is that you use a little less each night: with a 10 kWh battery and a 20 % reserve you have about 8 kWh for everyday use, and with 10 % you'd have about 9.
Can I set it to 0 %? It's not a good idea. You lose your backup for a power cut and the battery suffers more. On top of that, the battery's BMS cuts off before it's truly empty to protect it.
Why do I see it below 20 % after a power cut? Because in off-grid mode the inverter lets you use the reserve down to a lower minimum. That's normal.
Does the battery charge from the grid when it reaches 20 %? Usually not. When it reaches the minimum, the inverter just stops discharging it, and the panels charge it the next day. Some inverters can charge from the grid on a schedule or below a set percentage, but that's a separate setting that has to be switched on.
Questions about your battery?
If your battery stops at a certain percentage and you're not sure whether it's the reserve or a fault, tell us what the app shows and we'll tell you what might be going on. More information on our batteries page, or message us on WhatsApp with the word BATTERY.
