Battery Round-Trip Efficiency and Standby Losses Explained for Perth Homes
When you store solar energy in a home battery, you don't get all of it back. Some is lost as heat in the battery cells, some in the electronics that convert power between direct current (DC) and alternating current (AC), and some is consumed just by keeping the system switched on. These losses are usually modest, but they're real, and they affect how much a battery actually saves. Many Perth owners are surprised when their monitoring app shows that the battery put out less than it took in. This guide explains battery round trip efficiency, standby losses and what they mean for your bills.
What Round-Trip Efficiency Means
Round-trip efficiency is the percentage of energy you get back out of a storage system compared with what you put in. If you charge a battery with 10 kWh of solar energy and later draw 9 kWh from it, the round-trip efficiency is 90%. The missing 1 kWh has been converted to heat along the way.
Manufacturers usually quote round-trip efficiency on datasheets, but the figure can describe different things. Some quote the efficiency of the battery cells alone, which is often 95% or higher for lithium iron phosphate. Others quote the efficiency of the whole system, including the inverter, which is lower. When comparing products, check what the figure includes. A system-level figure between about 85% and 92% is typical for modern home systems.
Where the Losses Happen
Energy passes through several stages on its way from your roof to your kettle, and each stage loses a little.
| Stage | Typical efficiency |
|---|---|
| DC to DC conversion into the battery | 97–99% |
| Battery cells (charge and discharge) | 95–98% |
| DC to AC conversion by the inverter | 96–98% |
| Overall system round trip | 85–92% |
Charging
When solar charges a battery in a DC-coupled hybrid system, energy flows from the panels through the inverter's DC converter and into the battery. Losses are small at this stage.
Storage and Cell Losses
Inside the battery, a small amount of energy is lost as heat during charging and discharging. Lithium batteries also self-discharge slowly when idle, though this is minor over a single day.
Discharging
When you draw energy from the battery, the inverter converts it from DC to AC to run household appliances. Inverters are most efficient at moderate loads and less efficient at very low loads. That's why a battery supplying a tiny overnight load, such as a fridge and a few devices, may run at lower efficiency than when it supplies a larger evening load.
AC Coupling vs DC Coupling
How a battery connects to your solar system affects efficiency.
In a DC-coupled system, solar panels and the battery share a hybrid inverter. Solar energy can go straight into the battery as DC, with only one conversion to AC when it's used. This tends to be the most efficient arrangement.
In an AC-coupled system, the solar inverter converts panel output to AC, and a separate battery inverter converts it back to DC to charge the battery, then to AC again when discharging. That extra conversion adds losses, often lowering round-trip efficiency by a few percentage points compared with DC coupling.
AC coupling still has advantages, especially when adding a battery to an existing solar system without replacing a working inverter. The efficiency difference is usually small compared with the overall savings.
Standby Losses
Standby or idle consumption is the energy the system uses just to stay on. Inverters, battery management systems, cooling fans, communication modules and displays all draw some power, even when no energy is flowing. Typical standby consumption ranges from under 10 watts to more than 50 watts, depending on the product.
| Standby draw | Energy per day | Energy per year |
|---|---|---|
| 10 W | 0.24 kWh | 88 kWh |
| 25 W | 0.6 kWh | 219 kWh |
| 50 W | 1.2 kWh | 438 kWh |
At 33 cents per kWh, 438 kWh per year is worth about $145. That's not huge, but it's a real cost that reduces savings, especially for small batteries that store relatively little energy each day. Some systems draw standby power from the battery itself, which reduces stored energy available for the evening; others draw from the grid when the battery is empty.
Heat and Efficiency
Battery efficiency also depends on temperature. Very high temperatures increase losses and accelerate ageing, and battery management systems may reduce charging and discharging power to protect cells. In Perth, a battery in a hot garage on a west wall may run less efficiently in summer than one in a shaded, ventilated location. Very cold conditions can also reduce performance, though Perth rarely gets cold enough for this to matter much.
What Losses Mean for Savings
Let's say your battery stores 10 kWh of surplus solar each day. Without a battery, you'd export that energy at around 2 cents per kWh. With a battery at 88% round-trip efficiency, you get back 8.8 kWh in the evening, replacing grid power at about 33 cents. Add standby losses of 0.5 kWh per day, and net useful energy is about 8.3 kWh.
| Item | Value per day |
|---|---|
| Lost export income (10 kWh at 2c) | -$0.20 |
| Grid power avoided (8.3 kWh at 33c) | $2.74 |
| Net daily benefit | about $2.54 |
| Annual benefit (if fully cycled every day) | about $925 |
Losses reduce savings, but the large gap between export and import rates means a battery still adds significant value. If exports were earning 10 cents during the 3pm–9pm window, the calculation changes slightly, but batteries remain valuable because most stored energy replaces 33-cent grid power.
How to Reduce Losses
- Choose DC coupling for new systems, where suitable.
- Pick efficient products with good system-level round-trip efficiency and low standby draw.
- Install equipment in cool, shaded, ventilated locations.
- Size the battery to your usage, so it cycles fully rather than sitting partly full, where standby losses eat into a smaller amount of stored energy.
- Update firmware, since manufacturers sometimes improve efficiency and control.
Reading Your Monitoring App
Many owners notice that their app shows more energy charged than discharged. That's normal. If the difference is consistently more than about 15–20%, ask your installer to check settings, firmware and installation conditions.
Some apps also show "self-consumption" of the system itself. Look for any unexplained consumption, which might indicate a fault or setting issue.
Efficiency Over the Battery's Life
Round-trip efficiency isn't fixed for the life of the battery. As lithium cells age, their internal resistance rises slightly, which means a little more energy turns to heat during each charge and discharge. The change is usually gradual and modest for good-quality LFP batteries, but over ten years it can lower efficiency by a few percentage points. At the same time, usable capacity declines, so the battery stores less energy each day. Both effects reduce savings over time, which is why sensible payback calculations assume some degradation rather than treating year one performance as permanent.
Warranties usually guarantee a minimum retained capacity, such as 60% or 70% after ten years, but they rarely guarantee efficiency. If efficiency appears to drop sharply, that may indicate a fault worth raising with the manufacturer.
VPP Events and Efficiency
When a battery participates in a Virtual Power Plant event, it discharges energy to the grid, sometimes at high power for several hours. High-power discharge can be slightly less efficient than moderate discharge, and the battery may need to recharge from solar the next day. For Synergy's VPP, which pays 70 cents per kWh for dispatched energy, these small losses are easily outweighed by the payment. But they're worth understanding when you look at your monitoring data after an event and see larger-than-usual differences between energy in and energy out.
Comparing Batteries With Pumped Hydro and Other Storage
For context, round-trip efficiency of around 85–92% is high compared with many large-scale storage technologies. Pumped hydro typically achieves about 70–80%, and hydrogen storage is much lower. Home lithium batteries are among the most efficient ways to store electricity, which is one reason they've become so popular for shifting rooftop solar into the evening.
In everyday terms, the losses inside a home battery are similar to the heat you feel from a laptop charger: a small, unavoidable cost of converting and storing energy. Knowing roughly how large they are helps you set realistic expectations for what your battery will deliver each evening.
Frequently Asked Questions
What is a good round-trip efficiency?
System-level efficiency of around 85–92% is typical for modern home batteries.
Are AC-coupled batteries less efficient?
Usually by a few percentage points, because of extra conversions.
How much power does a battery use on standby?
Typically 10–50 W, depending on the system.
Does heat reduce efficiency?
Yes. Install batteries in cool, shaded spots.
Do losses make batteries not worth it?
No. The gap between import and export prices is large enough that losses only modestly reduce savings.
Independent Information
The independent Perth solar and battery installer Fusion Sun Solar publishes general information for Perth homeowners, including product information on Growatt battery storage. For discussion with other WA solar owners, see the Solar Energy Perth member profile on Elephant Journal.
Final Thoughts
Every battery loses some energy, through conversion, cell resistance and standby consumption. These losses are part of the trade-off of storing energy, and they're usually small compared with the savings. Understanding them helps you compare products more fairly, read your monitoring app with confidence and choose a system that delivers the most useful energy for your Perth home.