Battery Round-Trip Efficiency: Where Your Stored Energy Actually Goes

Battery spec sheets tend to quote high efficiencies, and at the cell level lithium-ion batteries are genuinely efficient. Yet when the US Energy Information Administration looked at the country's utility-scale battery fleet in 2019, it found an average monthly round-trip efficiency of 82%. Nothing about that gap is mysterious. It is the sum of every system between the battery cells and the meter, plus the power the site draws whether it is cycling or not. The question for an owner is which of those losses are fixed, which are controllable, and which are being counted in the number they are looking at.

Who this matters to

Battery asset managers reconciling charge and discharge against revenue, owners with round-trip efficiency guarantees in supply or tolling agreements, and trading teams whose bids assume a loss on every cycle.

Efficiency depends on where you measure

Round-trip efficiency is energy out divided by energy in. The answer changes with the boundary: at the battery's DC terminals, at the inverter's AC output, or at the revenue meter where the site connects to the grid. Each step outward adds losses, and whether auxiliary power is included changes the figure again.

Before comparing a contract figure with an operating one, check that both were measured at the same boundary, under the same test conditions, and with the same treatment of auxiliary load.

Where the energy goes

• •The cells. Every cell has internal resistance, and current through it becomes heat. The higher the charge or discharge power, the larger the loss. As cells age, resistance typically rises, so the same battery tends to lose a little more on each cycle as it gets older.
• •Power conversion. The inverter converts AC to DC on the way in and DC to AC on the way out, losing a small share each time. Because conversion happens twice per cycle, small percentages compound.
• •Transformers. Step-up transformers have losses that rise with load, and core losses that continue whenever they are energised, whether the battery is cycling or idle.
• •Thermal management. Batteries need to be kept within a narrow temperature range. Air conditioning or liquid cooling works hardest in hot weather and during heavy cycling, and in a hot climate it can be one of the largest loads on site.
• •Auxiliaries and standby. Battery management systems, controls, communications, fire detection and suppression, and lighting run around the clock. On a battery that sits idle for long periods, this standby draw keeps accumulating.

Why utilisation changes everything

Some losses scale with the energy cycled. Others — standby loads, transformer core losses and baseline cooling — are spent every hour regardless. The less a battery cycles, the fewer megawatt-hours those fixed losses are spread across, and the lower its round-trip efficiency at the meter. Field testing bears this out. In PNNL's evaluation of a utility battery for Snohomish County PUD, round-trip efficiency rose by 3 to 5 per cent when auxiliary consumption during rest periods was excluded, and an NREL study found that auxiliary power consumption dominates at low system utilisation. A battery's efficiency is partly a property of the hardware and partly a property of how it is dispatched.

Same battery, two months: an illustrative case

Numbers illustrative. A 100 MW / 400 MWh battery returns 88% of the energy it cycles before fixed loads are counted, and draws an average of 300 kW for auxiliaries, standby cooling and transformer core losses — 216 MWh over a 30-day month. In a busy month it completes 25 full cycles; in a quiet month, when price spreads are thin, it completes eight.

The hardware did not change. The fixed overhead did not change. Only the utilisation did, and it moved round-trip efficiency by almost four percentage points. If the quiet month is the one measured against a contractual guarantee, the battery looks worse than it is; if the busy month is quoted in a sales discussion, it looks better than it will be.

What it costs, and what you can control

Every lost megawatt-hour was bought at the charging price, so efficiency losses are a direct cost on every cycle and a direct input to every bid. Some losses are fixed by the design. Others are not: cooling set points and schedules, transformer energisation during long idle periods, auxiliary loads left running that do not need to be, and dispatch that pushes the battery at higher power than the revenue justifies. Tracking efficiency month by month, alongside cycles and temperature, separates a battery that is ageing from one that is simply being run differently — and connects directly to the state-of-health picture an owner needs as the battery ages.

Four questions to ask about your battery’s efficiency

At which boundary is the guaranteed figure measured, and is auxiliary power included? Under what test conditions — power level, temperature, rest periods? Is auxiliary consumption metered separately, so you can see it? And is round-trip efficiency tracked every month against cycles and ambient temperature, or only tested once at commissioning?

Frequently asked questions

What is round-trip efficiency?

The share of energy put into a battery that comes back out: energy discharged divided by energy charged, over a defined period and at a defined measurement point.

Why is my measured efficiency lower than the spec sheet?

Usually because the spec sheet measures at a narrower boundary, such as the battery or inverter, under test conditions, while your meter includes transformers, cooling, auxiliaries and standby time.

Does round-trip efficiency fall as a battery ages?

It typically does, because cell internal resistance tends to rise with age, turning more of each cycle into heat. Tracking efficiency over time is one of the signals of battery health.

Do auxiliary loads count against round-trip efficiency?

It depends on the definition. Measured at the grid meter with auxiliaries included, they do. Many contracts and spec sheets exclude them or meter them separately, so check before comparing figures.

The honest close

Sources & Further Reading