Round-trip efficiency: what is lost between charging and using

Put energy into a battery and you won't get all of it back out. Round-trip efficiency is the percentage you do get back¹.

Two things set it apart from other spec-sheet numbers. It's a ratio, so it has no unit. And it describes a whole trip, in and out, rather than one moment. That makes it handy for comparing sheets, as long as the test conditions match.

What the number means

A round-trip efficiency is the percent of energy that can be used relative to the energy that went into charging the battery¹. It covers the whole trip, charging and then discharging.

It can't reach 100 percent. Useful output energy is always lower than input energy, and efficiencies cannot exceed 100 percent².

A made-up example for the arithmetic only: at a round-trip efficiency of 90 percent, 10 kWh put in gives 9 kWh back out. The definition above is what makes that sum work¹.

Where the lost energy goes

The saved sources point to a few places. Internal resistance loss and the chemical processes inside the cells are two of them³. Internal energy losses and limits on how fast ions move through the electrolyte also make efficiency vary³.

A home system has another step. Batteries store electrochemical energy as direct current (DC), while electric power networks are usually run on alternating current (AC)⁴. Converting between the two takes electronics, and that stage can have its own losses. A spec sheet may say whether its efficiency figure covers the battery alone or the whole system with its inverter, so check.

It changes with how you use it

One efficiency number can't describe every situation. Published test results show it moving with the charge and discharge rate. In one experiment on a single cell, round-trip efficiency was reported as 85.5% at 2C and 97.6% at 0.1C¹. "C" is a rate that compares the current to the battery's size, so a bigger number means faster charging or discharging.

Don't read those two figures as what a home battery will do. They come from one cell tested in a lab. They show the pattern: gentler use tends to lose less.

How to use the figure

• Look for the test conditions next to the percentage. Without them it's hard to compare two sheets.
• Check whether the number is for the battery alone or the full system.
• Apply it on top of usable capacity, not instead of it. The usable capacity note explains why.
• Ask whether the figure holds at the power you'll actually draw. The cell results above suggest it can drop at higher rates.
• Remember that the figure is a ratio of energy. It says nothing about how fast the battery can deliver power. That's the continuous power rating.

I won't give a running-cost figure here, since it would depend on your local electricity price. Your utility's own tariff page is the place for that.