Buy a “13.5 kWh” home battery and you won’t actually get 13.5 kWh of usable backup out of every charge cycle. Some of that energy is lost to heat and conversion every time it goes in and comes back out. That loss is called round-trip efficiency, and it quietly affects everything from your backup runtime estimates to the real payback math on time-of-use arbitrage.
What Round-Trip Efficiency Measures
Round-trip efficiency (RTE) is the percentage of energy you get back out of a battery compared to what you put in. If you charge a battery with 10 kWh from the grid and can only discharge 8.8 kWh of usable energy afterward, that’s an 88% round-trip efficiency. The missing 1.2 kWh didn’t vanish — it was lost as heat in the battery cells, the inverter, and the charging electronics.
Typical Numbers by System Type
| System type | Typical round-trip efficiency |
|---|---|
| Modern AC-coupled LFP battery + hybrid inverter | 85–90% |
| DC-coupled LFP battery (fewer conversion steps) | 90–95% |
| Older lead-acid battery banks | 70–80% |
| Flow batteries | 65–80% |
DC-coupled systems generally edge out AC-coupled ones because the energy makes fewer AC/DC conversions on its way from solar panel to battery to home load. See our DC-coupled vs. AC-coupled guide for how that architecture choice affects your whole system, not just efficiency.
Why It Matters for Sizing
If you’re sizing a battery based on a target backup runtime — say, running your refrigerator and a few circuits for 12 hours — round-trip efficiency doesn’t actually change that calculation, since sizing is based on discharge capacity, not round-trip. Where RTE matters is in two other places: solar self-consumption (how much of your rooftop solar you actually get to use after storing it) and arbitrage math (see our peak shaving and time-of-use arbitrage guide for a full worked example of how RTE eats into daily savings).
What Causes the Losses
Three places energy gets lost on a round trip: the battery cells themselves (internal resistance generates heat during charge and discharge), the inverter (every DC-to-AC and AC-to-DC conversion has a conversion loss, typically 2-5% each way), and the battery management system’s own parasitic draw (a small continuous power draw to run monitoring electronics, which matters more on partial cycles than full ones).
Temperature Affects Round-Trip Efficiency Too
Yes, modestly. As a battery ages and its internal resistance rises, round-trip efficiency typically drops a few percentage points over a 10-year warranty period — this is separate from capacity degradation (how many kWh the battery can hold), which is the number most warranties actually guarantee. See our home battery payback guide for what’s typically covered and what isn’t.
How to Find Your System’s Real Number
Manufacturer spec sheets list a round-trip efficiency figure, but lab-tested numbers often run a few points higher than what you’ll see in daily use, since real-world charging rarely happens at the ideal current and temperature a lab test uses. If your inverter or monitoring app reports daily kWh charged and discharged, you can calculate your own real-world number directly: discharged kWh ÷ charged kWh, averaged over a few weeks. A dedicated energy monitor that logs this automatically makes the calculation far easier than reading it off an app once a month.
Bottom Line
Round-trip efficiency is the quiet tax on every kWh that passes through your battery. It won’t change how big a battery you need for backup power, but it directly affects how much money a battery actually saves you arbitrage or solar self-consumption — budget for 85-90% on a typical modern system, not the 100% the sticker math implies.
Official sources: U.S. Department of Energy — Residential Energy Storage, NREL — Energy Storage Technology and Cost Characterization Report.
John Farmer is a veteran and the founder of Veteran Forge Strategies LLC. He researches home battery backup, solar, and energy storage to help homeowners make confident decisions about energy resilience and lower power bills, and writes Home Power Vault to make backup power simple to understand.
