Townsville solar guide

Battery sizing: why your total Ergon bill is only the starting point

The right battery size depends on when you use electricity, how much spare solar is available, and what you want the battery to do. Your Ergon bill helps, but the bill total alone cannot show how much energy you need after sunset or which appliances must run together.

Why is the bill total not enough?

A bill combines electricity charges, daily supply charges, credits and any other applicable items. That dollar total is not a measurement of battery capacity. Two homes with a similar bill can use power at very different times and need different systems.

Start with energy use in kilowatt-hours, then look for interval meter data or monitoring that shows the pattern across the day. If you already have solar, your grid imports are not the same as the home's total use: some electricity is being used directly from the panels without passing through the import meter.

Which two energy figures matter first?

  • Evening and overnight use: the energy the battery could supply when solar is no longer covering the home.
  • Spare solar: the energy available to charge the battery after daytime household use.

Those figures need to be considered together. A large battery does not create more solar. If it rarely fills, some of its capacity may sit unused. If your daytime use already consumes most of the solar, the design may need more panels or a different operating plan rather than a bigger battery alone.

Exports are a useful starting point, not a complete engineering design. Export limits can hide energy that could otherwise have been generated, and your tariff or operating settings may change when charging is useful. Ask the designer to explain the assumptions.

What is the difference between kW and kWh?

kWh measures stored energy. It helps describe how much energy the battery can deliver over time. kW measures power. It limits how much the system can supply at one moment.

As a simple arithmetic example, a steady 2 kW load uses 8 kWh over four hours. Real equipment uses varying amounts, and battery losses, reserve settings and operating limits mean that example is not a runtime promise.

Check usable capacity, not only the headline nominal capacity. Also check the inverter's continuous power rating and any relevant phase limits. A battery can have energy remaining but still need help from the grid when several large appliances run together.

How do backup needs change the answer?

Backup must be designed into the installation. Some systems support selected circuits; others can support more of the property within the equipment's limits. A three-phase house needs particular attention to which phases and loads are backed up.

Keeping a reserve for outages leaves less energy available for normal bill reduction. Ask for both goals to be explained: what the battery is expected to do on a normal evening, and what it can run during a grid outage. Neither an oversized battery nor a high reserve setting guarantees days of whole-home backup.

How do Townsville air conditioning and future loads fit in?

Use actual patterns where possible. Running one bedroom air conditioner for part of the evening is different from running ducted cooling all night. A pool pump scheduled during solar hours has a different effect from an EV charged after dark.

Tell the designer about planned changes, but separate them from current use. The proposal should explain what the system is sized for now, which future additions it allows for and any compatibility limits. Do not assume every modular battery can be expanded indefinitely.

What should you send when asking for a battery design?

  1. A recent Ergon bill, including usage and export pages.
  2. Interval or monitoring data if you can obtain it.
  3. The make and model of any existing solar inverter and battery.
  4. A switchboard photo and a photo of the proposed battery area, taken safely from the ground.
  5. Your main goal: evening use, blackout backup, a future EV, or a combination.

Our battery systems page shows broad size bands, not a rule that every family needs the same capacity. If you already have panels, read about adding a battery to existing solar. Talk to the team for a design based on your home.

Sources and further reading

Checked 6 November 2026 (Australia/Brisbane). Your installation and product instructions take priority over general guidance.