Round-trip efficiency: why you never get back 100% of what you store
Put 100Wh of solar energy into a battery and you won't get 100Wh back out. Some is lost to heat during charging and discharging, and the amount lost is different by chemistry — which matters for sizing the array, not just the battery.
Round-trip efficiency is the share of energy you get back out of a battery relative to what you put in — every charge-discharge cycle loses some to heat and conversion. LiFePO4 typically runs 95-98%, lead-acid 80-85%; lower efficiency means a larger solar array is needed to cover the same usable load.
What round-trip efficiency actually measures
Round-trip efficiency is the ratio of energy you can draw back out of a battery to the energy that went into charging it, expressed as a percentage. It's distinct from usable depth-of-discharge (how much of the rated capacity you can safely use) — round-trip efficiency is about conversion losses within the cycle itself, mostly internal resistance generating heat during both charge and discharge.
Typical figures by chemistry
LiFePO4 and Li-ion run high, commonly around 95–96% round-trip. Lead-acid chemistries (AGM, gel, flooded) run lower, typically around 80–85%, with flooded generally at the low end of that range. This is one of several real trade-offs favoring lithium beyond just cycle life and weight — see the chemistry comparison for the full picture.
Why it changes your array size, not just your battery choice
Round-trip efficiency isn't just a battery spec to note and move on — it's a multiplier in the sizing chain. To deliver a given amount of usable energy out of the battery, you have to put in more than that during charging to cover the round-trip loss, on top of accounting for usable depth-of-discharge and temperature derating separately. A system built on lead-acid's ~80% round-trip efficiency needs a correspondingly larger array (or longer charge window) than the same usable-energy target on LiFePO4's ~95%, all else equal. The round-trip efficiency calculator isolates just this factor; the full system calculator folds it into the complete array and battery sizing math automatically.
What doesn't help round-trip efficiency
Charging or discharging faster than the chemistry's comfortable rate increases internal resistance losses and pushes round-trip efficiency down further — another reason to keep loads within a sensible C-rate rather than just within the pack's absolute maximum rating.
Related tools
Sources & standards
- NREL PVWatts — system-loss and array-output modeling reference.
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — general sizing-practice reference.
- IEC 62619 — Safety requirements for secondary lithium cells and batteries — cell-level safety testing, including thermal limits.
Educational content, not an electrical design. Verify with your equipment's datasheet and a licensed installer for anything permanently wired. See the methodology.