Round-Trip Efficiency and Standby Losses in Home Batteries

Round-trip efficiency answers a simple question: if you put 10 units of electricity into a home battery, how many come back out when you need them? The answer is never all ten. Energy is lost at every stage, in the chemistry of the battery itself, in the electronics that convert power between the alternating current (AC) in your home and the direct current (DC) in the battery, and in the small but constant consumption of the system's own control equipment. Electricity North West, a network operator that runs battery storage trials, puts it plainly: "batteries are not 100% efficient, so you will not get all of the energy out that you put in."¹

The definition matters as much as the number. Round-trip efficiency is "the ratio of energy charged into the battery to the energy discharged from it, expressed as a percentage", and it "applies to the entire energy storage system, encompassing both the inverter/PCS and the battery", not just the battery box on the wall². That distinction is where most confusion begins. A manufacturer may quote a figure close to 95% for the battery alone, while the whole system, measured from the solar panels or the grid to the socket in your home, delivers less. Tesla's Powerwall 3 datasheet, for instance, states 89% for solar to battery to home or grid efficiency³, and that is a whole-system figure from a maker that also markets high cell efficiency.

For a household thinking about energy independence, efficiency is the tax every stored kilowatt-hour pays. The rest of this page explains where the losses happen, how the chemistry and the wiring of your system set the size of them, and how the losses feed through into bill savings and export payments.

What round-trip efficiency means, and where the energy goes

Round-trip efficiency is measured across the full journey: energy into the battery, energy back out. Alpha ESS, a storage manufacturer, defines it as "the ratio of energy charged into the battery to the energy discharged from it, expressed as a percentage"². The losses along that journey come from two broad places. The first is inside the battery itself: "chemical reactions, internal resistance, and heat generation within the battery"². No battery chemistry avoids these; they are the cost of storing energy electrochemically rather than leaving the electricity to flow straight from source to appliance.

The second place is everything between the battery and the rest of the house. A battery stores and releases DC electricity, while the grid, your sockets and most appliances run on AC. Every conversion between the two, in either direction, loses a slice of the energy as heat in the power electronics. Because round-trip efficiency "applies to the entire energy storage system, encompassing both the inverter/PCS and the battery"², the honest figure includes those conversions, and it is always lower than the battery's internal efficiency alone.

Real products show what this means in practice. Tesla's Powerwall 3, an all-in-one system with its own solar inverter, is quoted at 89% for the full solar to battery to home or grid path³. For comparison, a different domestic technology, the Q-zeta thermal store trialled in UK homes under an Energy Systems Catapult project, reported an average round-trip efficiency of 97%⁷, but that stores energy as heat rather than in electrochemical cells, so the comparison is limited to the headline number. Among batteries, anything in the high eighties to low nineties for a whole system is a credible, well-engineered figure; anything much higher deserves a careful look at what is actually being measured.

The practical consequence is arithmetic a householder can do. If a system returns roughly nine units for every ten stored, then about a tenth of everything you put through the battery is spent on the way through. That is the price of shifting electricity from the middle of the day to the evening, or from a cheap overnight tariff to the afternoon peak. The electricity you store displaces power that would otherwise be bought at a much higher rate, but the loss is a real cost and it belongs in any savings estimate.

Chemistry compared: lithium-ion against lead-acid

The chemistry inside the battery sets the floor under the whole-system number, because internal losses happen before any conversion electronics come into play. The two families sold for UK homes behave very differently here.

Lithium-ion batteries are the dominant choice, and their advantages compound. Plymouth Energy Community, which advises households on storage, notes that "with around 4,000 charging cycles, a lithium-ion battery can typically last 10 years or more", and that they "contain fewer toxic materials than lead-acid batteries, and like all batteries, can be safely recycled at the end of their life"⁵. Flexi-orb, a certification body for electrical energy storage, adds that lithium-ion batteries "can be more efficient than lead-acid batteries, weigh two thirds less, can discharge more stored energy, have a longer lifespan"⁸. The efficiency and the usable capacity interact: a battery that wastes less internally and lets you draw more of what it holds delivers more useful energy from the same nominal size.

Lead-acid batteries, the older technology, "store less energy than newer technologies and typically provide around 700 to 1,000 charging cycles"⁵. Fewer cycles means the round-trip losses are spread over less delivered energy across the battery's life, which matters if the investment is being judged on lifetime savings. Lead-acid remains a choice where upfront cost is the binding constraint, and it is still used in some off-grid setups, but on efficiency and longevity the gap is wide.

Within lithium-ion, how the battery is treated also affects what it delivers over its life. Guidance on lithium cells warns that "if you charge the battery to 100% it can overheat. This could cause cell degradation, meaning that lithium battery cells lose their ability to charge at their original rate"⁹. Home battery systems manage this automatically, holding a reserve at the bottom and often a ceiling below true 100%, which is one reason usable capacity is lower than nominal capacity. More detail on the chemistries themselves, including LFP and NMC, is on our home battery chemistries page, and the lead-acid options are covered in lead-acid, AGM and gel batteries.

One further point of context for households weighing the embodied energy of the equipment: the Centre for Alternative Technology calculated that a battery "needs to be charged from zero carbon energy and then discharged to replace gas use about 200 times for the carbon cost of making it to be paid back"¹⁰. Efficiency does not change that payback count directly, but a system that wastes less delivers more low-carbon electricity per cycle, which is what the calculation rewards.

AC-coupled retrofits: the extra conversions and what they cost

How the battery is wired to the solar panels changes the round-trip figure, sometimes by more than the chemistry does. The choice is between AC coupling and DC coupling, and it matters most for homes adding a battery to panels that already exist.

A DC-coupled battery "connects directly to your solar panels, creating a simple and efficient energy pathway", and "because there are fewer conversion steps, DC-coupled systems are often highly efficient and make excellent use of the energy your system generates"⁵. An AC-coupled battery, by contrast, "connects on the AC side of your home's electrical system"⁵. In that arrangement, solar electricity is converted from DC to AC to feed the house, and then back from AC to DC to charge the battery, with a further conversion on the way out. Carbon Co-op, which runs community flexibility projects, describes the result: "there are a bit more conversion losses as the solar goes from DC to AC and back again"¹¹.

The size of the penalty is not enormous, but it is measurable. SolaX, a manufacturer, notes that a DC-coupled setup with a hybrid inverter can deliver "an extra 3 to 5% round-trip energy efficiency" over an AC-coupled one⁴, and Alpha ESS states the same pattern from the measurement side: "DC-coupled systems tend to have a higher RTE compared to AC-coupled systems"². SolaX also lists "minor energy loss during AC-DC conversion" as the characteristic trade-off of its AC-coupled option⁴.

Why would anyone accept the extra losses? Because AC coupling is usually the practical retrofit. The Centre for Sustainable Energy advises that "AC-coupled batteries are probably better if you already have PV as they are easier to retrofit"¹², and SolaX presents its AC-coupled system as the straightforward option for the large majority of existing solar installations⁴. If your existing solar inverter is working well, an AC-coupled battery leaves it untouched and sits alongside it, whereas DC coupling generally means replacing the inverter with a hybrid unit. The 3 to 5% efficiency difference is the price of that convenience, and for most retrofit households it is a modest one. The full comparison, including what it means for adding panels later, is on our AC and DC coupled battery systems page.

Standby losses: the power drawn when nothing happens

Round-trip efficiency describes losses while the battery is working. A separate, quieter drain happens the rest of the time: the power the system consumes just by being switched on.

The concept is familiar from household appliances. NICEIC defines standby power as "the energy appliances and electronic devices use while switched off but still plugged in"¹³, and Smart Energy GB describes standby mode as "a low-power state that an appliance, still plugged into the power source, can enter when not being actively used"¹⁴. A home battery never enters that state in any meaningful sense. Its inverter, battery management system, meter interface and monitoring electronics are powered continuously, day and night, whether the sun is shining or not.

The evidence for this comes from the equipment's own behaviour. Growatt's storage inverter documentation, for example, records that the machine "needs to communicate with the battery through the network cable" in lithium battery mode, that under some safety regulations it "needs to be connected to the electricity meter when they work normally", and that a fault appears when "the electricity meter is not connected"¹⁵. A device that must maintain communication with its battery and meter at all times is a device drawing current at all times. That consumption is small in absolute terms, but it runs for every hour of the year, and it comes off the top of the energy the system delivers.

Standby consumption is also why the energy a battery holds is not the same as the energy it will ever give you. Energy Saving Trust describes how a system typically "stops taking electricity from the battery" when it falls to a set floor, "maybe 20% of the total storage capacity"⁶, a reserve that exists partly to protect the cells and partly to keep the system's own electronics alive. The same guidance, in its England-specific version, repeats that floor figure¹⁶.

For the household budget, standby losses behave like a standing charge rather than a unit cost: they are fixed, they accrue whether or not you use the battery, and they weigh more heavily on a battery that cycles rarely than on one that cycles daily. That is worth remembering when comparing tariffs, because tariffs with no standing charges at all are, as Confused.com notes, "best suited for households that use little gas or electricity" and properties left empty for long periods¹⁷, the opposite of a battery household's usage pattern. The Consumer Council for Northern Ireland makes the related point about storage generally: "you can store excess electricity and use it when there is no wind"¹⁸, but storing it costs a little of the electricity every hour it waits.

How efficiency feeds through into savings

Every savings calculation for a home battery rests on the difference between two prices: what electricity costs when the battery charges, and what it costs when the battery discharges. Round-trip efficiency sits between them, shrinking what you get back.

The mechanics are straightforward. Energy Saving Trust describes the tariff-cycling case: with a battery and no solar panels, "you'll stop charging the battery when it's full, and discharge it when the next cheap tariff period ends"⁶. The National Energy Action best practice guide describes the same arrangement, in which "electricity is used to charge the battery during cheap-rate periods and use it later when electricity is more expensive, for example using Economy 7, Economy 10, or more innovative time-of-use (TOU) arrangements"¹⁹. If a system returns roughly nine units for every ten bought at the cheap rate, the arithmetic still works whenever the gap between cheap and expensive rates is wide enough, and smart tariffs are designed to make that gap wide.

With solar, the logic is different and usually stronger. Surplus solar either gets used, stored, or exported. The CPA notes that "with a home battery, that surplus energy is captured and held for when you actually need it, including overnight EV charging", whereas without it the surplus "is wasted or exported to the grid often at a relatively low rate"²⁰. Stored solar displaces electricity that would otherwise be bought at full retail price, so even a system losing a tenth of the energy on the way through beats exporting the same energy for a few pence. Zapmap's guidance for EV drivers points the same way: charging overnight when electricity is cheap "reduces overall fuel costs by more than 70%" compared with fuel²¹, and a battery is what makes overnight solar charging possible.

The honest caveat comes from Energy Saving Trust's own research: "adding a battery to a solar panel system can increase bill savings, but often not enough to recover the cost of the battery within its expected lifetime, particularly where households already have access to a good export tariff"²². Efficiency is one of the variables that decides which way that balance falls. A system with lower round-trip efficiency delivers less usable energy per cycle, so it needs a wider price gap or more cycles to pay for itself. Our battery savings and payback page works through the full calculation, and batteries and smart tariffs covers the tariff side.

Export payments: what the meter receives after standby and losses

Efficiency losses and standby consumption are invisible on their own, but they have one very visible side effect: they change how much electricity crosses your meter in each direction, and export payments depend entirely on that.

The Smart Export Guarantee (SEG) pays for electricity sent to the grid. Uswitch's guidance is precise about the mechanism: "payments are exclusively an export tariff, based solely on how much electricity you export back to the grid, as recorded by your smart meter"²³. A battery changes that flow in two ways. First, as Home Energy Scotland explains, "installing battery storage will lower the amount you export, meaning you'll get less money from export payments", though overall savings are greater than relying on export payments alone²⁴. Second, and more bluntly, Which? warns that "having a storage battery may render you ineligible for some SEG tariffs"²⁵. Some SEG tariffs are open only to households without storage, so the battery that captures your surplus also closes some doors.

The tariffs themselves vary widely. Solar Energy UK's tariff listings include an export rate of 3.0 pence per kWh paid on a 3-month cycle²⁶. Which ?'s survey of the market describes the structure of the smarter options, including Intelligent Octopus Flux, "a two-rate tariff with matching import and export prices, automated battery management charging when power is cheapest and exporting between 4pm and 7pm"²⁷. That last design matters for this page's subject: a tariff that pays you to export stored electricity in the evening peak is a tariff that pays for round-trip losses, because the battery must be charged, held and discharged to deliver them.

Whether stored electricity can be exported at all has been contested between suppliers. Which ? reported in 2020 that "Octopus Energy pays for stored electricity that is later exported, whereas SSE will only pay for renewable electricity" generated directly²⁸, and earlier coverage of the SEG's launch noted a trial by Bulb paying export at the day-ahead wholesale price²⁹. Ofgem adds a further wrinkle for households on older arrangements: "if you currently receive 'deemed export payments', then these will stop and you will instead receive export payments for the amount of electricity recorded by the meter exported to the grid"³⁰. Deemed payments, paid on assumption rather than measurement, cannot survive a battery that changes the actual flow.

For households with smaller or plug-in systems, the picture is still developing: the Centre for Sustainable Energy notes that "some suppliers are planning to provide export tariffs, meaning you might be able to get paid for the excess electricity" from plug-in solar³¹. The general principle holds across all of it: Energy Saving Trust notes that "you can sell the surplus back to your energy supplier through a Smart Export Guarantee (SEG) tariff" once the battery is full⁶, so the battery fills first, exports come second, and export income is what is left over after storage has taken its share.

What to check on a datasheet

Datasheets quote efficiency in ways that are not always comparable, and knowing which figure is which is the single most useful skill when reading one.

The first check is scope. Alpha ESS draws the line explicitly: a battery-only efficiency figure "typically focuses on the battery's internal losses, which occur due to chemical reactions, internal resistance", whereas round-trip efficiency "applies to the entire energy storage system, encompassing both the inverter/PCS and the battery"². If a datasheet quotes a number near 95% or above, check whether it is the internal figure; the whole-system number will be lower. Tesla's Powerwall 3 datasheet shows what a whole-system claim looks like when stated properly: "Solar to Battery to Home/Grid Efficiency 89%"³, with the path spelled out in the label itself.

The second check is coupling. Alpha ESS notes that round-trip efficiency "can vary significantly depending on the coupling method used between the PV system and the battery energy storage system"², and SolaX quantifies the gap at 3 to 5% in favour of DC coupling⁴. A datasheet figure for an AC-coupled unit and one for a DC-coupled unit are not measuring the same journey, so compare like with like.

A short checklist when reading one:

• Which path is measured? Solar to battery to home, or grid to battery to home, or battery alone. The path should be in the label³.
• Whole system or battery only? Battery-only figures exclude inverter losses and will read higher².
• AC or DC coupled? The same hardware can show different round-trip efficiency depending on the coupling method².
• Whose number is it? A maker's own datasheet is the right source for that maker's product, but independent and official guidance is the right source for what the figures mean¹.

None of this makes one system a winner. It means the headline number on page one of a brochure is the start of a question, not the end of one. For a household's energy independence, round-trip efficiency is the running cost of the independence itself: the fraction of every stored kilowatt-hour spent to have electricity on your own terms, in the evening, in a power cut, or at the peak of the price curve. A good system spends that fraction honestly and states it plainly.