Home Battery Standby Use and Everyday Losses: How to Check Your Own
- Some energy is lost whenever power is stored and returned; the Energy Saving Trust says so plainly.
- The inverter and control electronics also draw power while idle.
- Datasheets may list efficiency or standby figures; definitions differ between makers.
- A monthly check of energy in and out shows your real-world result.
- We could not find an independent typical standby figure, so we give none.
Every battery system loses some energy, both in converting power and in keeping its own electronics awake. You can measure how much on your own system with a month of app data and a little arithmetic.
Two kinds of loss
The first is conversion loss. Electricity changes form on the way in and out of a battery, and some turns to heat. The Energy Saving Trust notes that energy taken from the battery is less efficient than using power directly, so some energy is lost (Energy Saving Trust). Our article on round-trip efficiency explains the usual measure.
The second is standby use. The inverter, battery management system, communications and sometimes cooling or heating draw a small amount of power even when nothing is flowing. It is small next to a household's total, but it runs all day, every day. A UK government review points out that systems use open ventilation or, in some cases, fan-assisted or liquid cooling, and that maintenance of vents and fans must be documented in the manual (GOV.UK review, PDF). Those parts are among the things that use power.
What datasheets tell you
Look for a round-trip efficiency figure, a standby or self-consumption figure and the conditions behind them, such as the power level and temperature. Makers define these differently, so a higher number on one datasheet does not always beat a lower one on another. We could not find one independent figure for typical standby use that fits all products, and we would rather say so than quote a number we cannot verify. Our guide on reading a datasheet lists the figures to compare.
Measure your own system
You need four monthly totals, which most apps show: energy charged into the battery, energy discharged from it, and the state of charge at the start and the end of the period. If you have a separate inverter reading, add that too.
Pick a clean month
Choose a period with no outage, firmware reset or holiday that changes use, and note the date and time of start and end.
Record the state of charge
Write down the percentage and, if shown, the energy stored at the start and end.
Note energy in and out
Record total kWh charged and discharged over the month.
Compare
Divide discharged by charged. If the battery ended at a different level, allow for it; a rough result is fine.
A result well below the maker's figure is not proof of a fault. Partial cycles, cold weather and high power levels all affect it. A big drop from month to month, or a battery that empties itself overnight with no load, is worth raising with the installer along with your numbers.
What to do with the result
Treat the figure as one input, not a verdict. A small loss on a battery that saves you money in the evening is acceptable. Our calculator uses conservative round-trip assumptions, so your measured value tells you whether it is too optimistic for your case.
Common questions
Does a home battery use power when idle?
Its electronics draw some power even when idle. The amount varies by product, and we could not find an independent typical figure.
What is a normal round-trip efficiency?
Makers publish their own figures under their own test conditions. Check the datasheet and compare like for like.
Is a poor monthly result a fault?
Not necessarily. Weather, power levels and partial cycles all affect it. Repeated or sudden drops are worth raising with the installer.
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