Round-Trip Efficiency and Standby Losses in Home Batteries

A home battery never gives back all the electricity put into it. Typical round-trip losses are around 10 to 15% of the energy moved in and out¹, and E.ON Next's own examples show a round-trip efficiency of around 80 to 90%². Round-trip efficiency is the ratio between the energy charged into a battery and the energy discharged from it, expressed as a percentage, so a higher figure means less is lost on the way³.

The number printed on a datasheet is often higher than the number a household experiences. Battery pack figures of 95% or more are common on current lithium iron phosphate (LFP) products, for example the Fox ESS EP11⁴ and the BYD Battery-Max LiteIn⁵. Once the inverter and its conversions are counted, the figure falls: Tesla gives 89% for Powerwall 3 measured from solar, through the battery, to the home or grid⁶. Older chemistries and retrofitted systems that convert the energy more times sit lower still.

Those few percentage points matter because every stored unit is paid for once, either as imported electricity or as solar that could otherwise have been exported. Losses shrink the gap between cheap and expensive electricity that a battery exploits, and a system that draws power while idle loses a little more each day whether or not it cycles.

What round-trip efficiency means: the 10 to 15% you lose going in and out

Round-trip efficiency compares what goes into a battery with what comes out of it³. The losses happen in two places. Inside the battery, energy is lost to chemical reactions, internal resistance and heat³. Outside it, the electricity is converted: solar panels produce direct current (DC), homes run on alternating current (AC), and batteries store DC, so stored solar energy passes through several conversions, each with a small loss. As one maker's guide puts it:

Electricity North West makes the same point for a network operator: batteries are not 100% efficient, so not all the energy put in comes back out¹¹.

The critical detail is what a quoted figure covers. AlphaESS distinguishes two measures. Battery efficiency usually focuses on the battery's internal losses only³. Round-trip efficiency, properly stated, applies to the entire energy storage system, including both the inverter (or power conversion system) and the battery³. Datasheets do not always make clear which is meant, and many headline figures describe the battery pack.

The figures in the table are not directly comparable. The Solarwatt figure is labelled a maximum, the Fox ESS figures are for the battery pack, and the Tesla figure follows the energy from the panels all the way to the socket or the grid. That last kind of figure is closer to what a household sees in practice. The brand pages for Fox ESS home batteries, BYD Battery-Box, Solarwatt home batteries and Tesla Powerwall 3 give the rest of each specification.

Lithium-ion, LFP and lead-acid compared: 80 to 95% by chemistry

Chemistry sets the ceiling for how much energy a battery can return. Maker guidance consistently places lithium-ion above lead-acid, although the published ranges differ from one document to the next.

The lead-acid figures show the widest spread. SolaX gives an average range of 50 to 95% and attributes the losses to internal resistance and heat generation¹⁸, while the same maker's comparison of battery types, and EcoFlow, settle on 80 to 85%⁷. The difference reflects how the battery is used and how the figure was framed rather than a contradiction to be resolved; the documents simply disagree on the lower end.

Efficiency is only one reason lithium dominates new home installations. Independent guidance from Plymouth Energy Community gives lithium-ion around 4,000 charging cycles and a typical life of 10 years or more, against around 700 to 1,000 cycles for lead-acid¹⁹. Flexi-Orb notes lithium-ion batteries weigh two thirds less than lead-acid²⁰. SolaX's worked example of two batteries both rated at 10 kWh nominal capacity finds the LFP battery can deliver more than 12 times the lifetime energy of the lead-acid one¹⁷. Lower losses per cycle, multiplied over many more cycles, compound into a large difference in the energy a household actually gets back over a battery's life.

National Energy Action's best practice guide observed as long ago as 2019 that as battery technologies develop, efficiencies improve and costs reduce²¹. More on each chemistry is set out in home battery chemistries, lithium-ion vs lead-acid solar batteries and lead-acid, AGM and gel batteries.

For independence, the chemistry question is practical: a lead-acid bank at the lower end of its range can lose a sixth or more of what it stores, which in an off-grid home means more generation, or more generator running, to cover the same evening demand.

AC-coupled retrofits: around 90% once the extra conversion is counted

How a battery is wired into the house matters almost as much as what is inside it. A DC-coupled battery connects directly to the solar panels, while an AC-coupled battery connects on the AC side of the home's electrical system¹⁹. In an AC-coupled retrofit, solar energy is converted from DC to AC by the existing solar inverter, back to DC to charge the battery, and to AC again when it is used, which is often described as a triple conversion.

Maker guidance puts the cost of that extra conversion at a few percentage points:

Expressed as losses, AC-coupled systems typically lose 12 to 17% per cycle because of the triple-conversion path²⁵. Estimates of the gap vary. Jackery states that swapping to a DC-coupled hybrid improves round-trip efficiency by about 5% by avoiding double conversion²²; SolaX puts the gain from a hybrid inverter at an extra 3 to 5%²⁶; AlphaESS states AC-coupled systems lose approximately 8% more energy than DC-coupled ones²⁷. Hoymiles describes the difference as minimal²⁸, and Carbon Co-op's case study describes it as "a bit more conversion losses"²⁹.

These are all maker or case-study figures, and they are not measured the same way. The independent view on retrofits concerns practicality rather than efficiency: the Centre for Sustainable Energy describes AC-coupled batteries as easier to retrofit in homes that already have solar panels³⁰. SolaX states that an AC-coupled battery suits 90% of solar retrofits, with a minor energy loss during conversion²⁶.

What this means in practice is a trade-off. A retrofit keeps a working solar inverter in service and avoids the cost of replacing it, and pays for that in a slightly larger loss on every stored unit. A DC-coupled or hybrid system reduces conversions but ties the solar and the battery to one inverter, so a household depends on a single piece of equipment, and on one maker's continued support, for both. AC-coupled and DC-coupled home batteries covers the wiring in detail.

Standby losses: where power goes when nothing is being drawn

Round-trip figures describe energy lost while charging and discharging. They say nothing about what a system uses while it sits idle. Standby electricity is the energy appliances and devices use while switched off but still plugged in³¹, and standby mode is a low-power state that a connected appliance enters when not being actively used³². A home battery system is never fully off: its inverter, battery management system and monitoring stay powered so that it can respond when the sun comes up, the tariff changes or the grid fails. Homey notes more generally that many power adapters draw power even when the device itself is not in use¹⁰.

No standby consumption figure in watts for a specific home battery on sale in the UK has been verified for this page. What is documented is that system design affects it. SolaX, comparing a "battery-ready" system with a hybrid system, lists the battery-ready arrangement as having higher standby consumption, reducing overall system efficiency, and lower charge and discharge efficiency, leading to energy losses³³. That is a maker comparing its own product types, but it confirms that standby draw is a design variable, not a fixed cost.

Two other things reduce what a household can draw from a battery without being losses in the strict sense. The system holds back a reserve: the Energy Saving Trust describes discharging stopping at a set level, "maybe 20% of the total storage capacity"⁹. That energy is not wasted, but it is not available for daily use either; battery capacity and usable capacity explains how nominal and usable capacity differ. And every hour a charged battery waits before being used is an hour of idle consumption by its electronics.

For comparison, heat stores lose energy differently. Homey states a well-insulated water battery loses less than 1 °C per 24 hours, about 2 to 4% energy loss, with a round-trip efficiency of more than 90%³⁴. The Energy Systems Catapult reported in 2022 that the Q-zeta domestic thermal store has an average round-trip efficiency of 97%³⁵. These are heat, not electricity, stores and are not direct substitutes, but they show that "efficiency" and "loss over time" are separate questions for any store.

How efficiency affects what a battery actually saves you

A battery earns its keep by moving energy in time: storing solar that would otherwise be exported cheaply, or charging on a cheap overnight rate and discharging when electricity is dearer. National Energy Action describes the second use, with Economy 7, Economy 10 or other time-of-use tariffs²¹, and the Energy Saving Trust describes a battery without panels charging until full and discharging when the cheap period ends, automatically⁹. Losses eat into both.

For tariff arbitrage, the loss sets a floor on the price gap that is worth exploiting. If 10 to 15% of a stored unit is lost¹, a unit bought at the cheap rate costs more per usable unit than its sticker price, and a narrow gap between cheap and peak rates can be largely absorbed by losses and standby draw. For stored solar, the comparison is with export: each unit stored rather than exported gives up an export payment and returns only part of itself to the home. Stacking a home battery with smart tariffs and Economy 7 set out the tariff side.

The overall financial picture is modest. The Energy Saving Trust found in 2026 that adding a battery to solar panels can increase bill savings, but:

It adds that this is particularly so where households already have a good export tariff³⁶. Efficiency is one reason: every percentage point lost comes straight off the savings a payback calculation assumes. A calculation that uses a pack figure above 95% rather than a whole-system figure nearer 89%⁶ will overstate what the battery returns. Home battery savings and payback and how installers must calculate self-consumption cover the method.

What this means for independence is straightforward. A battery increases how much of a household's own solar it uses, but losses mean a little more generation, or a little more grid import, is needed to deliver each unit into the home. The household still depends on the grid for top-up and on its supplier's tariff structure for the arbitrage value.

Export payments: losses and standby come out of what reaches the meter

Smart Export Guarantee payments are based only on the electricity a smart meter records going back to the grid³⁷. A battery changes that figure in two ways. It deliberately reduces exports, because more solar is stored for use at home: Home Energy Scotland notes that installing a battery lowers export payments, while stating that overall savings are greater than relying on export payments³⁸. Which? states that exports, and therefore payments, reduce with a storage battery, and that a battery "may render you ineligible for some SEG tariffs"³⁹. And any energy lost in conversion, or consumed by the system while idle, is energy that is neither used in the home nor exported.

Feed-in Tariff households face different rules. Ofgem states that where a smart meter is fitted, deemed export payments stop and are replaced by payments for the electricity the meter records as exported⁴⁰. Good Energy notes that on deemed export, payments may remain unchanged depending on how the battery is installed, while on metered export a battery may reduce exported electricity and so reduce payments⁴¹.

Some tariffs are built around exporting stored electricity, where round-trip losses bear directly on the payment:

• Intelligent Octopus Flux charges the battery when power is cheapest and exports between 4pm and 7pm, with matching import and export prices⁴².
• Ecotricity's Battery Booster and Earnings Optimiser modes set different export price thresholds at which the battery discharges to the grid⁴³.
• Which? reported in 2020 that Octopus Energy paid for stored electricity later exported, whereas SSE paid only for renewable electricity⁴⁴.

On such tariffs, a unit bought cheaply, stored and exported returns less than a whole unit to the meter, so the price difference must cover the loss. Virtual power plants and flexibility services covers the wider export-led arrangements.

Chemistry and coupling: what to check on a datasheet

AlphaESS states that round-trip efficiency "can vary significantly depending on the coupling method" between the solar system and the battery³, and that DC-coupled systems generally have the higher figure. Reading a datasheet for efficiency comes down to a few questions:

1. What does the figure cover? A "battery pack" figure, as on the Fox ESS EP5 and CQ7¹³, measures internal losses only. A whole-path figure, as on Powerwall 3⁶, includes the inverter.
2. Is it a maximum or a typical figure? Solarwatt labels its more than 95% as maximum efficiency¹⁴. A maximum reflects ideal conditions.
3. Which chemistry? LFP figures generally sit at 90 to 95% or above; lead-acid at 80 to 85%⁷.
4. How is it coupled? An AC-coupled retrofit adds a conversion, typically costing 3 to 5 percentage points in maker estimates²⁶.
5. Is standby consumption stated? It is often absent, and it affects annual performance³³.
6. How much capacity is held in reserve? A reserve, maybe 20% of capacity⁹, is not lost but is not usable day to day.

A whole-system figure is the one that matches a household's meter readings. Cycle life and degradation, which also shape lifetime energy returned, are covered in home battery cycle life and degradation, and the wider picture in the home battery storage guide and home batteries and energy independence.