Round-trip efficiency tells you how much of the energy you put into a battery you get back out. If a battery takes in 10 kWh of solar and returns 9 kWh to your home, its round-trip efficiency is 90%. Standby losses are the energy the battery system uses simply to stay on, manage itself and communicate, even when it is neither charging nor discharging. Both reduce the value a battery delivers every day, and both deserve a place in your comparison alongside capacity, warranty and price.

Where the energy goes

Energy is lost at several points between your panels and your kettle:

• Conversion losses. Panels produce DC power and your home uses AC. Every conversion between DC and AC, or between voltage levels, loses a little energy as heat.
• Cell losses. Charging and discharging the cells involves internal resistance. LFP chemistry performs relatively well here.
• Battery management and thermal control. The battery management system, relays, displays and any fans or heaters draw power.
• Standby and communications. The inverter, gateway and monitoring stay powered overnight and on cloudy days.

Manufacturers do not all measure efficiency the same way. Some quote the battery module alone, which looks better; others quote the full path from AC in to AC out, which is closer to what you experience. Always ask which one a figure represents.

AC-coupled vs DC-coupled: why the architecture matters

How the battery connects to your solar affects how many conversions stored energy passes through.

As a typical range, whole-system round-trip efficiency for modern LFP home batteries commonly falls somewhere between the high 80s and low 90s in percentage terms, with DC-coupled designs usually towards the upper end. AC coupling remains a sensible way to add a battery to an existing system, and the difference is rarely large enough to rule it out, but it belongs in the comparison. The hybrid vs string inverter guide explains both architectures.

What efficiency means on your bill

A little arithmetic puts efficiency in perspective. Suppose a battery stores 10 kWh of solar on about 300 days a year, so 3,000 kWh goes in annually:

• At 92% round-trip efficiency, about 2,760 kWh comes back out.
• At 85% round-trip efficiency, about 2,550 kWh comes back out.

That is a gap of around 210 kWh a year, or roughly what a 6.6 kW system in Sydney produces in eight average days. Over a 10-year battery warranty it adds up to around 2,100 kWh. To value it, use the rate you would otherwise pay for that energy from the grid in the evening, commonly 30-45 c/kWh in NSW, rather than the few cents a kWh the solar would have earned as export. On its own the gap is rarely a deal-breaker, but it is a real amount to weigh against a price difference between two batteries.

Standby consumption: the quiet overnight cost

Standby or idle consumption is measured in watts and is often buried in a spec sheet or not listed at all. A system drawing a few watts barely registers. One drawing several tens of watts adds up, because it runs 24 hours a day, every day of the year. To estimate it:

1. Multiply standby watts by 24 to get watt-hours per day.
2. Divide by 1,000 to convert to kWh per day.
3. Multiply by 365 for kWh per year.

For example, a combined standby draw of 30 W uses 0.72 kWh a day, or about 263 kWh a year, which is more than the efficiency gap in the previous example. When the battery is empty, some of that comes from the grid. Ask for the standby consumption of the whole installed system, including the inverter, battery management, gateway and meters, not just a single component.

Reading the spec sheet

When comparing batteries, look for these lines and ask for any that are missing:

• Round-trip efficiency, and whether it is measured at the battery module, DC to DC, or AC to AC.
• Test conditions, including charge and discharge rate and temperature. Efficiency usually falls at high power and at temperature extremes.
• Standby or self-consumption power in watts, for both the battery and the inverter.
• Usable capacity compared with total capacity, because efficiency applies to the energy you can actually use.
• Inverter efficiency, often shown as a maximum and a weighted figure; the weighted figure is more realistic. The inverter buyer's guide covers these ratings.
• Operating temperature range, and whether the battery reduces power or uses heating in cold conditions.

Manufacturers, including Sigenergy, GoodWe, Fox ESS and Sungrow, each present specifications in their own format, so transfer the figures into one table rather than relying on headline claims. Installation location matters as well: a battery in a hot, sunny spot or an unventilated space will usually perform below its spec sheet figures.

Keeping efficiency in perspective

Efficiency is one factor among several. A slightly less efficient battery with more usable capacity, a stronger warranty and good local support can still be the better buy. Right-sizing matters more: a battery that cycles fully on most days turns its efficiency into value, while an oversized battery that rarely fills still pays its standby costs. Confirm current incentive values for the federal Cheaper Home Batteries Program on the official program page before signing, because the per-kWh value steps down every 1 January and 1 July. If you want these numbers modelled on your own usage, the market on this site lists a Battery Feasibility Study from $199 (indicative).

Next steps

Ask each installer for round-trip efficiency and standby figures for the complete system they propose, measured on the same basis, and add them to your comparison table. For a battery design matched to your usage, you can request a free assessment from Blue Energy Solar, and explore battery options and studies in the market.

Frequently asked questions

Does round-trip efficiency get worse as a battery ages?

Capacity fade is the main effect of ageing, but internal resistance also tends to rise over time, which can reduce efficiency slightly, particularly at high charge and discharge rates. Keeping the battery within its recommended temperature range and applying firmware updates helps. Monitoring data showing energy in and energy out over a month is the most practical way to track real-world efficiency as the years pass.

Is efficiency different when the battery runs in backup mode?

During a blackout the system usually runs isolated from the grid and supplies household loads directly. Efficiency can be lower at very light loads, because fixed inverter losses make up a larger share, and near the inverter's maximum output. It is a minor issue compared with having backup available, but it helps explain why a battery can run down faster than expected overnight during an outage.

Can I reduce standby losses after the battery is installed?

Some systems offer settings such as display sleep or reduced communication that trim idle consumption, but options vary and should only be changed through the app or by your installer. Avoid switching equipment off at the isolators to save power, as that can affect battery management and warranty conditions. A review of operating mode, reserve level and tariff settings is usually more worthwhile.