Quick answer
Battery Round-Trip Efficiency is the percentage of the electricity put into a battery that you can take back out. A battery rated at 90% returns 9 kWh for every 10 kWh stored; the missing kWh is lost as heat in the cells and the power electronics.
It is the number that decides whether storing your solar for the evening is worth more than exporting it to the utility.
Quick facts
The key facts about battery round-trip efficiency, with sources:
- What it measures
- Energy out ÷ energy in, over one full charge and discharge 3
- Unit
- Percent (%)
- US utility-scale battery fleet, 2019
- 82% average monthly round-trip efficiency (EIA) 3
- Where to find it
- The battery data sheet, under efficiency or output specifications
- What lowers it
- Extra conversions (DC↔AC), heat, cold, very high or very low power
- Related to
- Usable capacity, depth of discharge, inverter efficiency
Key takeaways
- Round-trip efficiency tells you how much of the energy you store you get back.
- Current home batteries publish about 89% to 90% on the AC side 2 1.
- A DC figure in the mid-90s is not comparable to an AC figure. Check which one a quote uses.
- Where your export credit is far below your retail rate, storing usually beats exporting even after losses.
- No code sets a minimum. It is a maker’s spec, measured under set test conditions.
What is round-trip efficiency?
Every time energy moves into a battery and back out, some of it turns into heat.
Round-trip efficiency (RTE) is the scorecard for that loss: divide the kilowatt-hours you got back by the kilowatt-hours you put in.
EIA puts it simply: it is “the percentage of electricity put into storage that is later retrieved” 3.
For a home system the loss happens in two places. The cells lose a little during charging and discharging. The inverter loses more, because solar panels produce direct current (DC), the house runs on alternating current (AC), and a battery stores DC. Each conversion between the two costs a few percent.
That is why manufacturers publish more than one figure. Enphase lists a DC round-trip efficiency of 96% for the cells and battery electronics alone, and an AC round-trip efficiency of 90% measured from AC into the battery and back to AC 1.
The round trip in one line
- Solar DC → charger or hybrid inverter → battery cells (small loss)
- Cells hold the energy → tiny standby loss
- Cells discharge DC → inverter → AC for the home (small loss)
- Energy out ÷ energy in = round-trip efficiency
How the losses add up
A full cycle on a typical solar-plus-battery home runs through these steps. Losses stack at each one.
- Panels produce DC electricity.
- An inverter or charger conditions it for the battery (DC-to-DC in a DC-coupled system; DC to AC and back to DC in an AC-coupled one).
- The cells store it as chemical energy, losing a little as heat.
- Later, the cells discharge DC.
- The inverter converts it to AC for the house or the grid.
RTE is the product of every stage. Two stages at 98% and one at 94% multiply out to about 90%, which is why the AC figure is always lower than the DC one.
Three figures manufacturers publish
| Figure | What it covers | Published example |
|---|---|---|
| DC round-trip efficiency | Cells and their DC electronics only. The highest figure, and the least useful to a homeowner because the house never runs on that DC. | Enphase IQ Battery 5P: 96% 1 |
| AC round-trip efficiency | AC in to AC out, including the battery inverter. Closest to what your meter sees for AC-coupled storage. | Enphase IQ Battery 5P: 90%, at 50% power 1 |
| Solar-to-battery-to-home | From the array through the battery to the house or grid. Used by integrated solar-and-battery units. | Tesla Powerwall 3: 89% (solar shifting, 25°C, new, 3.3 kW) 2 |
Store it or export it? A Duke Energy Carolinas (SC) example
Take 10 kWh of midday solar your home does not need right now, on Duke Energy Carolinas in South Carolina. Rates retrieved 5 October 2026.
| Choice | Math | Value |
|---|---|---|
| Export it now | 10 kWh × $0.0419 net-excess credit (Rider RSC) 5 | $0.42 |
| Store it at 90% RTE, use it tonight | 9 kWh back × $0.138125 energy charge avoided (Schedule RS, first 1,000 kWh) 1 6 | $1.24 |
| Same, at 80% RTE | 8 kWh back × $0.138125 6 | $1.11 |
Even a less efficient battery returns more value than exporting under this tariff, because the export credit is less than a third of the retail energy charge. That gap, not the efficiency figure alone, decides whether storage pays.
Rider RSC nets exports within each time-of-use period before crediting any excess, so the real split depends on when you use power. The figures ignore battery cost and wear; they compare only what one stored kWh is worth.
The break-even rule
A simple test for any tariff. Storing beats exporting when the retail rate you avoid, times RTE, is more than the export credit. Using the Duke figures above:
| Step | Math | Result |
|---|---|---|
| Retail rate × RTE | $0.138125 × 0.90 | About $0.124 per stored kWh |
| Export credit | Rider RSC 5 | $0.0419 per kWh |
| Lowest RTE where storing still wins | $0.0419 ÷ $0.138125 | About 30% |
Any working battery clears that bar on this tariff. Where the export credit sits close to retail, as under classic net metering, the bar rises close to 100% and storing for savings stops making sense.
Where it shows up on a solar quote
- On the battery data sheet. Check which of the three figures above it is before comparing two products.
- Inside savings and payback estimates. Ask what RTE the installer assumed; a proposal that ignores it overstates what stored solar is worth.
- In battery sizing. To get 9 kWh out at 90% you must put 10 kWh in, which means more panel output set aside for charging.
- In your monitoring app, as energy charged versus energy discharged over a month.
What the number tells you, and what it does not
What it tells you
- One comparable figure for how much stored solar you get to use.
- Lets you check whether storing beats exporting under your utility's credit rate.
- Higher efficiency means less waste heat for the enclosure to shed.
What it leaves out
- Measured at a fixed temperature, power level and age; real results are usually lower.
- Makers measure different paths, so 96% DC and 89% solar-to-home are not comparable.
- Leaves out standby losses, the small constant draw of an idle battery.
- Says nothing about usable capacity or lifespan.
Lab figures vs real life
Data-sheet numbers come from set test conditions. Tesla’s 89% Powerwall 3 figure, for example, is for a new unit at 25°C and 3.3 kW 2. Your garage in July is not a test lab.
Real fleets show the gap. EIA found the US utility-scale battery fleet ran at an average monthly round-trip efficiency of 82% in 2019 3. That figure covers large grid batteries, not home units, but it shows that measured results can sit below spec-sheet values.
Standby use also counts. A battery that sits idle still powers its own electronics, which lowers the figure you see in a month of data.
Ageing, warranties and cost
RTE has no price of its own, but it changes the value of every kWh you store: each point lost is stored solar you paid for and cannot use.
Efficiency can decline as a battery ages, alongside capacity. Warranties track capacity, not RTE: Enphase warrants more than 60% capacity for up to 15 years or 6,000 cycles, whichever comes first; Tesla lists a 10-year warranty for Powerwall 3.
Installed battery prices are on the storage pages linked below; HyreSolar publishes dollar figures only where at least 12 dated quotes support them. See home battery storage.
How to compare two batteries’ efficiency
- Get both data sheets, not just the sales sheet.
- Find the efficiency line and note whether it is DC, AC or solar-to-home.
- Note the test conditions: power level, temperature, age.
- Compare like with like. If one lists only DC, ask the maker or installer for an AC figure.
- Ask the installer which figure the savings model used.
Keeping efficiency up
- Keep the battery inside its rated temperature range. Enphase gives an optimum of 0°C to 30°C (32°F to 86°F); heat and cold both lower efficiency, and a garage wall that bakes in a Carolina summer costs some.
- Keep firmware current; inverter control updates can change charging behaviour.
- Watch monitoring: if energy discharged falls further below energy charged month after month, ask the installer to check it.
Sources: [1]
Mistakes that cost money, and when to call
- Comparing a DC figure on one quote with an AC figure on another, and choosing the "more efficient" battery that is not.
- Accepting a savings estimate that assumes 100% of stored solar comes back.
- Undersizing the charging side: if the battery cannot fill on a cloudy day, RTE matters less than the shortfall.
- Call your installer if the battery feels hot, shows fault codes, or the gap between charged and discharged energy keeps growing. Do not open the unit.
Standards and codes
No US building code sets a minimum round-trip efficiency for home batteries; it is a manufacturer specification.
Codes and listings cover battery safety: the Enphase IQ Battery 5P data sheet cites evaluation to UL 9540A, the fire-propagation test method for energy storage.
Energy storage systems are commonly listed to UL 9540, whose third edition UL Standards & Engagement published on 28 June 2023 4. Your local building department enforces installation rules.
Tesla rates Powerwall 3 solar-to-home efficiency with the California Energy Commission (CEC) weighted efficiency method, the method used to rate grid-tied inverters.
Round-trip efficiency vs related battery specs
| Spec | What it tells you | How it differs from RTE |
|---|---|---|
| Usable capacity | kWh you can draw from a full battery | Size of the tank, not what leaks on the way in and out |
| Depth of discharge | How far the battery may be drained | A limit on use, not a loss |
| Inverter efficiency | Share of DC turned into AC, one direction | One stage of the round trip |
| Capacity retention | Capacity left after years of use | Measures ageing; RTE measures one cycle |
Common misconceptions
- Myth A 96% battery beats a 90% battery.
- Reality Not if the 96% is a DC figure and the 90% is AC. Compare the same path.
- Myth Efficiency losses make batteries a bad deal.
- Reality Where exports earn far less than retail, storing still wins after losses, as the example shows.
- Myth The warranty guarantees the efficiency.
- Reality Warranties guarantee remaining capacity, not RTE 1.
In South Carolina, Virginia and Georgia
The efficiency question turns on your utility's export credit. In South Carolina, Duke Energy Carolinas credits net excess at $0.0419 per kWh and Santee Cooper at $0.0415, both far below retail, so stored solar beats exported solar even after losses (retrieved 5 October 2026) 5 7.
Dominion Energy South Carolina's Solar Choice rider counts a battery only if it charges solely from onsite renewables 8.
In Virginia, Dominion's new tariff keeps annual kWh netting at retail and credits only annual net excess at $0.05829 per kWh, for new non-low-income interconnections from about 1 May 2027; with annual netting, a battery adds more backup than bill value 9. Georgia tariffs were not verified; check your utility.
When round-trip efficiency matters to you
It matters most when you plan to cycle the battery every day, for arbitrage or solar self-use. It matters little if the battery mostly sits full for backup.
Rule of thumb: compare AC figures, and treat a few points of RTE as minor next to the gap between your retail rate and your export credit.
Next step: size the battery with the battery sizing calculator, then check the savings in the solar savings calculator.
Related guides
Questions about battery round-trip efficiency
What is a good round-trip efficiency for a home battery?
Current home batteries publish AC round-trip figures around 89% to 90%. Tesla lists 89% solar-to-battery-to-home for Powerwall 3, and Enphase lists 90% AC for the IQ Battery 5P. A DC figure in the mid-90s is not comparable to these, because it leaves out the inverter.
Is round-trip efficiency the same as inverter efficiency?
No. Inverter efficiency covers one conversion in one direction. Round-trip efficiency covers the whole trip into the battery and back out, which includes the cells and at least two conversions, so it is always lower.
Does round-trip efficiency drop as a battery ages?
It can, as internal resistance rises and more energy turns into heat. Warranties do not guarantee it, though. They guarantee remaining capacity, such as more than 60% for up to 15 years or 6,000 cycles on the Enphase IQ Battery 5P.
Why does my monitoring app show less coming out of the battery than going in?
That difference is the round-trip loss plus standby use. A shortfall of about 10% over a month is consistent with published figures. A gap that grows month over month is worth raising with your installer.
Does DC coupling give better efficiency than AC coupling?
For charging from solar, usually yes, because DC coupling skips the DC-to-AC-to-DC conversion an AC-coupled battery needs. Compare the solar-to-home figures on the data sheets rather than assuming, because the inverter design matters as much as the coupling.
How do I calculate round-trip efficiency?
Divide the energy you got out by the energy you put in, over full cycles.
If your app shows 300 kWh charged and 267 kWh discharged in a month, that is 267 ÷ 300, or 89%.
Standby use is included in a figure like this, so it will read a little lower than the data sheet.
What round-trip efficiency do grid batteries get?
EIA reported that the US utility-scale battery fleet ran at an average monthly round-trip efficiency of 82% in 2019. Pumped-storage hydro plants averaged 79%. Home data sheets quote higher figures, measured under test conditions on new units.
Does cold weather lower battery efficiency?
Yes. Both heat and cold lower efficiency. Enphase gives an optimum range of 0°C to 30°C (32°F to 86°F) for the IQ Battery 5P. Install the battery where it stays near that range, and away from direct afternoon sun.
Is round-trip efficiency worth paying extra for?
Usually only a little. A few points of efficiency change the value of each stored kWh by a few percent. The gap between your retail rate and your export credit, and the battery’s capacity and warranty, usually matter more to savings.
Sources
- Enphase Energy, IQ Battery 5P data sheet (DSH-00010-11.0-EN-2025-05-06), retrieved .
- Tesla, Powerwall 3 Datasheet (2024), retrieved .
- US EIA, Today in Energy: Utility-scale batteries and pumped storage return about 80% of the electricity they store (12 February 2021), retrieved .
- UL Standards & Engagement, ULSE publishes third edition of UL 9540, Energy Storage Systems and Equipment (28 June 2023), retrieved .
- Duke Energy Carolinas (SC), Rider RSC Residential Solar Choice (effective 1 January 2026), retrieved .
- Duke Energy Carolinas (SC), Schedule RS (Docket No. 2025-172-E), retrieved .
- Santee Cooper, Distributed Generation Rider DG-25, retrieved .
- Dominion Energy South Carolina, Residential Solar Choice rider (PSC Order No. 2026-374), retrieved .
- Virginia SCC, Order on Clarification, Case PUR-2025-00079 (20 May 2026), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Efficiency figures are copied from the manufacturers' current data sheets, with each footnote condition stated; the fleet figure is EIA's (read 8 October 2026).
Tariff figures are from the South Carolina schedules held in sc-local/facts.js and the VERIFIED Virginia fact pack row. The worked examples multiply those published numbers and nothing else.
Suggest a correction. We fix errors and say what changed.