Reading Usage Patterns Before Battery Sizing
A home battery is sized correctly when it matches the household's actual evening energy draw — not when it matches a popular capacity number on an installer's price sheet. An oversized battery charged by insufficient spare solar is an expensive underperformer. An undersized battery that runs flat at 10pm does not cover the high-draw morning period. The data to get this right is available before the installer visit, if you know what to read.
- Evening import is the key number: how much grid power does the home draw from sunset to midnight? That is the demand the battery needs to cover.
- Daytime solar export tells you how much spare energy is available to charge a battery each day.
- If export is small, the battery's job is primarily backup, not self-consumption — which changes how to think about sizing and cost.
- Home battery installation requires a licensed electrician and is not a DIY project.
The two numbers that determine battery sizing
Number 1: Evening import (how much the battery needs to supply)
Your electricity bill shows total kWh per quarter, but not when it was used. To get the evening split, you need either:
- Your retailer's portal — most retailers (Origin, AGL, Energy Australia, Amber, etc.) provide half-hourly or hourly usage data online. Log in, find the usage graph and read the evening import pattern.
- A clamp energy monitor (requires professional installation) — measures whole-home import and export in real time.
- Your smart meter data — distributors are required to provide smart meter interval data on request, usually as a CSV download from the DNSP portal.
What to look for: usage from 5pm to 11pm on a typical weekday. That is the peak demand window a battery is designed to cover for most households.
A useful rule of thumb: a battery needs to hold at least 70–80% of the evening import figure to meaningfully reduce grid draw. A household importing 8kWh each evening needs at least 6–7kWh of usable battery capacity.
Number 2: Daytime solar export (how much the battery can charge)
A home battery is charged primarily by spare solar — solar generation that the household is not using and would otherwise export to the grid at the feed-in tariff rate.
Find it on your bill: the export kWh line, if you have solar. If you export 10–15kWh on a typical sunny day, there is plenty of spare solar to charge a 10–13kWh battery. If you export only 2–3kWh per day, a large battery will not fill from solar alone — it will also draw from the grid to charge, which reduces the financial case.
The pattern that changes battery sizing
| Usage pattern | Battery sizing implication |
|---|---|
| High daytime solar export + high evening grid import | Classic battery case: large export, high evening demand — battery pays for itself through self-consumption |
| High daytime solar export + low evening import | Battery fills quickly but has little to do each evening — smaller battery sufficient, payback is longer |
| Low daytime solar export + high evening import | Battery job becomes grid time-shifting, not solar self-consumption — size to evening demand but expect longer payback |
| Low solar export + low evening import | Battery has limited financial case — may be justified primarily for backup, not bill reduction |
What a whole-home energy monitor adds
A plug-in smart meter shows individual appliance data. A whole-home energy monitor (installed on the switchboard by a licensed electrician) shows total household import and export in real time, split by time of day.
This data — 2–4 weeks of half-hourly readings — is the most accurate input for a battery sizing conversation with an installer. It shows the actual evening demand curve, the spare solar window and whether the household's pattern matches the battery investment.
If you are considering a home battery, reading your retailer's usage portal first costs nothing and reveals the pattern that determines whether the investment makes sense.
Before any battery conversation, find your evening import (5–11pm kWh) and your daytime solar export from your retailer's portal. Those two numbers determine the right battery size. If both are low, the financial case is weaker — the battery may still be worthwhile for backup, but the payback period will be longer.
Analyse your bill to understand your usage pattern before comparing battery quotes.