Battery Round-Trip Efficiency: The Hidden Spec That Eats Your Stored Power
Batteries are not perfect jars. When your solar panels push energy into a home battery during the afternoon, some of it is lost as heat before it comes back out to run your lights at night. The datasheet's headline capacity — 13.5 kilowatt-hours on the Tesla Powerwall 3, for example [1] — tells you how much the battery can hold; a quieter number, called round-trip efficiency, decides how much of that you actually get back.
What round-trip efficiency actually measures
Round-trip efficiency is a simple ratio: the energy you can usefully get out of a battery divided by the energy you put in, expressed as a percentage. A battery rated at 90% round-trip efficiency returns about 9 kilowatt-hours for every 10 you store; the missing kilowatt-hour is consumed by the battery's chemistry and by the power electronics that convert electricity between direct current (DC) and the alternating current (AC) your home uses [2].
It matters twice over. First, it quietly shrinks the capacity you paid for. Second, it prices every kilowatt-hour you shift from one part of the day to another: at 90% efficiency [2], delivering one kilowatt-hour from storage requires about 1.1 kilowatt-hours of input energy. That last figure is just arithmetic on the efficiency number — 1 divided by 0.90 — not a manufacturer claim. [2]
Where the number lives on a datasheet — and why it looks different everywhere
Battery makers all publish an efficiency figure, but they do not all publish it the same way. Tesla's Powerwall 3 datasheet lists a “Solar to Battery to Home/Grid Efficiency” of 89% [1]. The footnotes carry the fine print: that figure describes a typical solar shifting use case, measured at 25°C, at the beginning of the battery's life, at a modest 3.3 kW charge and discharge rate [1]. One row lower, the same datasheet lists 97.5% for solar power going straight to the home with no battery detour [1].
Do the division on Tesla's own two numbers — 0.89 divided by 0.975 — and the battery detour itself keeps about 91% of the energy that passes through it in that test case [1]. That 91% is our arithmetic on the datasheet's figures, not a number Tesla publishes: Tesla never gives an isolated battery-only efficiency figure [1]. Also worth noting from the same datasheet: the unit charges at a maximum of 5 kW AC even though it can discharge at up to 11.5 kW [1], and the warranty runs 10 years [1] — context that matters more than most shoppers realize, but the efficiency line is where the energy accounting happens.
AC vs DC: the two numbers Enphase publishes
Enphase takes the opposite approach and publishes two figures for its IQ Battery 5P: 90% AC round-trip efficiency and 96% DC round-trip efficiency [2]. The footnote on the AC figure defines the test: energy going from AC into the battery and back out to AC, at 50% of the unit's power rating, at 25°C [2]. The gap between the two numbers reflects the conversion work of the unit's six integrated IQ8D-BAT microinverters — the battery is an AC-coupled design [2] — so the 96% is closer to the bare battery while the 90% is what actually shows up on your side of the meter.
Apply the 90% AC figure to the unit's 5.0 kWh of usable capacity [2], and a full cycle returns roughly 4.5 kWh to your home's wiring. (Again, our arithmetic.) That is the number to size against: nameplate capacity multiplied by round-trip efficiency equals the energy you can truly use.
Why it matters more under time-of-use rates
Efficiency losses are a nuisance under flat electricity pricing; under time-of-use (TOU) rates they are the core of the calculation. The whole point of a battery on TOU pricing is to charge when electricity is cheap and discharge when it is expensive — but every cycle is taxed. At 90% AC efficiency [2], each kilowatt-hour you pull out during the expensive window consumed about 1.1 kilowatt-hours of cheap energy going in, so the spread between your utility's cheap and expensive rates has to clear that roughly 11% overhead before the cycle pays for itself. We cite no rates here because they are yours to look up; the arithmetic is the part that travels.
Even on a flat rate, efficiency decides how far a battery stretches in an outage. A 90%-efficient battery [2] effectively holds 10% less usable energy than its nameplate claims, so size backup plans on capacity times efficiency, not capacity alone.
How to read the efficiency line on any datasheet
The four-step check before you compare batteries
When an installer hands you two competing battery quotes, run this checklist before comparing headline numbers:
- Find the efficiency row. It may be called round-trip efficiency, AC round-trip efficiency, or an end-to-end path like Tesla's “Solar to Battery to Home/Grid Efficiency” [1].
- Check the AC/DC label. An AC figure (Enphase: 90% [2]) includes inverter conversion and is what your meter sees; a DC figure (Enphase: 96% [2]) describes the cells alone and will always run higher.
- Read the test conditions. Both of the datasheets in this article measured at 25°C [1][2]; Tesla's figure is also tied to a gentle 3.3 kW charge/discharge rate at the beginning of the battery's life [1]. Real garages run hotter and harder, so treat these as best-case comparison numbers, not field guarantees.
- Compare like with like. An 89% end-to-end figure and a 90% AC figure are near-twins; a 96% DC figure [1][2] is a different animal. Mixing them is the easiest way to pick the wrong battery.
Round-trip efficiency will never be the headline on a battery brochure — capacity and power get the big type. But it is the number that converts the brochure into the electricity you actually get to use. Ten minutes with the datasheet, one division, and you know exactly how much of your stored sunshine survives the round trip.
Sources: [1] Tesla, Powerwall 3 Datasheet (2025) (https://energylibrary.tesla.com/docs/Public/EnergyStorage/Powerwall/3/Datasheet/en-us/Powerwall-3-Datasheet.pdf); [2] Enphase Energy, IQ Battery 5P Datasheet, document IQB-5P-DSH-00166-2.0-EN-INT-UK (2023-09-21) (https://upvolt-energy.com/wp-content/uploads/2026/05/Enphase_IQ_Battery_5P_5kWh_Battery_DataSheet.pdf).
Verified: 2026-10-14
Disclosure: This article was drafted with AI assistance and reviewed by a human editor. Figures reflect sources available on the verified date; confirm current program rules before acting.