Supersize Me: The Rise of the McBattery

Aussies love big things.

After the rebate, batteries are heading the same way.

MC Electrical recently tested battery round-trip efficiency across a few systems. One result got a lot of attention. A Sigenergy battery stack with a 20 kW inverter was charged at 10 kW and discharged at just 700 W. Measured round-trip efficiency was 63%.

That sounds bad, but it’s not surprising.

At small loads, big hybrid systems struggle.

And seven hundred watts is being generous. Many homes draw closer to 300W overnight to power the fridge, lights, and standby loads.

Why Size Hurts Efficiency

Inverter efficiency is not a single number. It’s a curve.

As output drops to a small fraction of rated power, efficiency drops fast. At the same time, the battery system must still operate its control boards, power electronics, DC-DC converters and cooling systems. When output is low, that fixed overhead becomes a large percentage of the energy flowing through the system.

The result is a large battery stack working harder than it should to supply a very small load. More heat. More losses. Less useful energy out the other end.

MC Electrical also tested a smaller setup. A 10 kW Fronius inverter and battery stack discharging at just 300W measured at 82% round-trip efficiency.

That difference matters.

If Battery A is 30% more efficient than Battery B, then 20 kWh of the efficient one can give a similar real-world outcome to about 26 kWh of the less efficient one. Capacity numbers alone can mislead.

Here’s a gentle plea to anyone rushing to lock in the battery rebate before it reduces in May:

Between now and May, your feed will be full of ads shouting urgency. Big batteries. Big stacks. Big numbers. “Get in before the rebate drops by $10,000.”

That message plays straight into panic buying.

Before you sign, pause and consider how your system will operate most of the time. Overnight loads are small. That’s where batteries spend a lot of their life. A smaller, better-matched inverter and battery can outperform a huge system once losses are taken into account.

It can also soak up less of a taxpayer-funded rebate while doing the same job.

(You can check how much battery rebate your choice of battery is gobbling up with our battery rebate calculator – just updated with the latest changes to the scheme.)

Yes, a giant battery with a giant inverter will still cut your bill a lot if it’s working properly. And as a bonus, some of the wasted energy will help warm your garage in winter.

But in my experience with energy systems, good design ages better than brute force.

The McBattery looks great on the menu.

But once you get it home, you may find you paid for size you never use, and losses you didn’t need.

If You Already Went Big

If you’ve already bought a giant battery, don’t panic.

Check your monitoring after a few months. Look at how many kWh of battery sit unused most days. If there’s plenty of slack, consider a Virtual Power Plant or a tariff that pays well for battery exports.

Pushing more energy through the system by exporting to the grid can improve efficiency, reduce heat, and put some money back in your pocket. Big systems tend to behave better when they are kept busy.

Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.