Why summer evenings can cost the most, and what a battery does about it
Verified 17 October 2026
On a hot evening the air conditioner is on, dinner is cooking and the sun is going down. On a time-of-use plan, that is often the most expensive power you buy all day. A battery is built for exactly this, but it has limits. Here is how to see what is happening on your bill.
Peak rates usually sit in the evening
Peak rates usually apply on weekday evenings, and that is when electricity costs the most. The New South Wales (NSW) Government gives 4pm to 9pm on weekdays as a typical peak period, and says peak rates sometimes apply only in winter and summer. Times vary by plan, so check your own peak window in your plan summary and on your bill.
Where the expensive hours fall on a summer weekday
Daylight: solar can fill the battery (roughly)
A typical NSW weekday peak: 4pm to 9pm
If you are on a flat rate plan, power costs the same at every hour, so none of this applies to you.
What a battery does in the peak
The government's solar guide says a battery stores your spare solar power so you buy less from your retailer. A battery that has filled from solar during the day can cover part of the evening. On a time-of-use plan, that is the hours where it does the most for your bill.
Where it runs short
- It can run flat. The government says if you use a lot of power in the morning and evening, you need a large battery to avoid buying from the grid then. A long hot evening can use up a small battery before the peak ends.
- It can only supply so much at once. Each battery has a maximum output. The government says if several big appliances run together, such as a spa bath, the battery can't cover them all and the rest comes from the grid.
- Some energy is lost. The government says charging and discharging a lithium battery typically loses about 10% of the stored energy.
- Its settings may not match your plan. The battery may have been set up around the plan you had at install. If your plan changed, it may need setting up again.
Reading your summer bill
- Find the peak line. That is the power you still bought in the expensive hours.
- Check the dates it covers. A line for a cooler month tells you little about summer.
- Look at the peak rate next to the other rates. A bigger gap makes a full battery at 4pm matter more, as long as it can cover what you are running.
- Compare with the same months last year if you had a battery then.
Moving big jobs out of the peak helps with or without a battery. The NSW Government lists air conditioners, washing machines, dryers, ovens and dishwashers as heavy users you can shift to cheaper hours with a timer or an app.
Is the plan still right?
A plan with a steep summer peak rewards a battery that reaches the evening with charge in it. A plan with free or cheap midday hours changes the picture again. Here is how free midday power works. A bill alone can price your current plan and flat rate plans. For time-based plans, add your meter data file (NEM12), which is free. Here is where to get it.
Start with a recent bill. PowerLens says Stay or Switch, and the Stay answer is free in full. If it says Switch, you pay once to see the plan's name. No saving is guaranteed.
Next: should you switch plans after a battery.