Battery round-trip efficiency: where the kWh go
Put energy into a battery and take it back out, and a little is always missing. The round-trip efficiency on the specification sheet says how much, and it matters more for some homes than others.
In short
Round-trip efficiency is the share of energy put into a battery that comes back out, as stated on the product's specification sheet. The rest is lost as heat in the cells and electronics. It matters little when a battery stores spare solar that would otherwise be exported, and more when it is charged from the grid every night.
What the percentage means
If a battery has a round-trip efficiency of 90%, then for every 10 kWh you put in, about 9 kWh comes back. To get a given amount out, you have to put in that amount divided by the efficiency. The table works it through for one kWh delivered, using example percentages rather than any product's figure.
| Round-trip efficiency | kWh in for 1 kWh out | Lost per kWh out | Lost over a year at 5 kWh a day |
|---|---|---|---|
| 95% (example) | 1.05 kWh | 0.05 kWh | 96 kWh |
| 90% (example) | 1.11 kWh | 0.11 kWh | 203 kWh |
| 85% (example) | 1.18 kWh | 0.18 kWh | 322 kWh |
| 80% (example) | 1.25 kWh | 0.25 kWh | 456 kWh |
Where the energy goes
Energy is lost at each conversion and in the cells themselves. In an AC-coupled system, stored solar is converted from DC to AC by the solar inverter, back to DC to charge the battery, and to AC again for the house, so there are more conversion steps than in a hybrid system that charges the battery directly from the panels as DC. Each step loses a little. The cells also lose a small amount as heat while charging and discharging.
Manufacturers measure and state round-trip efficiency in different ways, sometimes for the battery alone and sometimes for the battery with its inverter. When comparing two products, check the datasheets describe the same thing.
When it matters, and when it hardly does
Storing spare solar
Energy that would otherwise have been exported is energy you were not going to use at home. Losing a small share of it on the way through the battery changes little; the rest is still energy used at home rather than sent out.
Charging from the grid
Here every lost kWh was bought. A battery cycled from the grid every night pays the loss on every cycle, all year. The helper on a battery without panels shows the kWh drawn and lost for your own figures.
Other small losses
A battery system also uses a little power to run its own electronics and monitoring, even when idle. Datasheets sometimes list this as standby or self-consumption power. Over a year it adds up, so it is worth noting when you compare products, especially for a battery that will spend long winter spells near its reserve.
What to do with the figure
- Read it from the specification sheet and check what it includes.
- If you plan to charge from the grid, use it in the grid-charging helper.
- If you mainly store spare solar, give it less weight than usable capacity and power.
- Compare it alongside the other datasheet lines in comparing specifications.
This guide deliberately stays in kWh. Whether a given loss matters in money depends on the prices on your tariff, which a price guide rather than an energy guide should handle. For how efficiency fits into the daily cycle, see how a solar battery works.
An illustrative example across a year
Take a battery that moves 5 kWh into the evening every day of the year, purely from spare solar. At an example efficiency of 90%, it would need about 5.56 kWh of surplus a day to deliver that, and about 200 kWh would be lost across the year. Because that surplus would otherwise have been exported, the house still uses about 1,825 kWh more of its own solar than it would without the battery. Now take the same battery charged from the grid every night: the same 200 kWh or so is lost, but every one of those kWh was bought. Same battery, same losses, very different meaning (our arithmetic, illustrative figures).
That is the whole case for paying attention to the figure if you plan to charge from the grid, and for giving it less weight if you mainly store solar.
Questions to ask about efficiency
- Is the stated figure for the battery alone, or for the battery with its inverter?
- Under what conditions was it measured, and does it change in cold weather?
- How much power does the system use on standby?
Two datasheets that answer these the same way can be compared fairly; two that don't may simply be measuring different things.
For the kWh your own battery would move each day, and the losses that go with them, the grid-charging helper does the arithmetic.
If a datasheet doesn't state round-trip efficiency at all, ask the installer for the manufacturer's figure in writing before comparing products.
What this page is based on
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