Round-trip efficiency: the energy you lose just by storing it

Storing energy is not free. Every time a battery charges and then discharges, it gives back a little less than it took in. Round-trip efficiency is the number that measures that loss, and it quietly shaves a slice off every savings estimate you will ever read.

What round-trip efficiency means

Round-trip efficiency is the percentage of energy you get back out of a battery, measured against the energy you put in. Push 10 kWh into a battery, get 9 kWh back, and you have a 90 percent round-trip efficiency. The missing kWh is lost mostly as heat, in the cells and in the inverter that converts between your home's AC power and the battery's DC storage.

It is a round trip because the loss happens across the full cycle: the conversion in, the chemistry, and the conversion back out. A single number captures the whole journey.

The real number for 2026 batteries

Modern LFP home batteries are good, but not perfect. Most land in the high 80s to low 90s percent round-trip. Tesla publishes 89 percent round-trip efficiency for the Powerwall 3 (Powerwall 3 datasheet, verified 2026), and Enphase and other makers publish comparable figures in the same band (manufacturer datasheets, verified 2026).

So a useful rule of thumb is roughly 90 percent: expect to lose about one kWh in ten just to the act of storing.

Why it matters to your savings

Here is the part that does not show up in a sales pitch. You pay for energy on the way in, but you only get about 90 percent of it back on the way out. That gap is a quiet tax on every kWh your battery shifts.

On a time-of-use plan, you charge cheap and discharge at peak, so the efficiency loss eats into the spread. If off-peak power costs $0.20 and you only recover 90 percent of it, your effective cost per usable kWh is closer to $0.22. As long as the peak price is far above that, the arbitrage still wins handily. But the loss is real, and it means the headline "store solar and use it later" comes with a roughly 10 percent haircut every time.

What to do with this

Do not panic over it, but do not ignore it either. Round-trip efficiency is a fixed cost of storage, the same way depth of discharge trims your usable capacity. The honest move is to bake it into the math rather than assume a battery returns every kWh you feed it. Our Worth It calculator applies a realistic round-trip loss so the savings figure reflects what comes back out, not what went in. For state rate context that decides whether the spread covers the loss, see the California report.