The sizing formula
Battery size you actually need = evening kWh consumption × 1.3 (headroom) ÷ 0.9 (depth of discharge limit).
That's the engineering answer. The real answer adds context for your roof size, EV plans and how much you'd hate hearing the grid kick in occasionally.
Four household profiles
Profile 1: The small household (1–2 people, ~$400/qtr bill)
Evening consumption typically lands around 5–7 kWh on a normal weekday. Right-sized battery: 5–8 kWh. Going bigger costs more rebate cap, not less. A 5 kWh BYD HVS at $5,500 list nets to roughly $1,800 after rebates - and pays back in 4–5 years for this profile.
Profile 2: The average family (3–4 people, ~$650/qtr bill)
Evening consumption typically 10–13 kWh. Aircon, dinner, devices, hot water boost. Right-sized battery: 10–13.5 kWh. This is the sweet spot for hitting both state and federal rebate caps. Sungrow SBR128 (12.8 kWh) and Tesla Powerwall 3 (13.5 kWh) both work.
Profile 3: The big-home family (4+ people, $800+/qtr bill)
Evening consumption can hit 18–22 kWh. Two-car charging some nights. Pool pump. Big aircon zones. Right-sized battery: 16–20 kWh. Above the rebate cap on capacity, but the federal rebate scales linearly so the maths still works - and bigger systems unlock the cheapest per-kWh installed price.
Profile 4: The EV household (or planning to be)
Add 8 kWh to whatever your base sizing is. An EV charged off-peak at home adds 6–9 kWh of nightly draw. Sizing rule of thumb: 13.5 kWh minimum, 20 kWh if you charge at home overnight.
Why installers oversell
Three reasons batteries get oversized:
- Per-kWh price drops sharply with size. Installers prefer selling 13.5 kWh because the labour is the same and margins are higher.
- The marketing copy targets "average household needs". Manufacturer brochures assume a peak load you probably don't have.
- Customers buy on "what if?" The "what if I get an EV in 4 years?" worry leads to spec inflation. Often you can just add a battery module later - modular systems (Sungrow SBR, BYD HVS) are designed for this.
The "DEBS arbitrage" sizing case
If you're on Synergy SWIS, here's an underrated argument for slightly bigger sizing: any battery capacity beyond your own evening consumption can be charged from cheap midday solar and exported during the 3pm–9pm peak DEBS window at 10c/kWh. That's a 4x markup on your off-peak buyback rate. A battery sized 20–30% larger than your usage can earn that arbitrage on sunny days.
This works best if you have 8 kW+ of solar generating well above your daytime load.
Common sizing mistakes
- Sizing on annual usage instead of evening peak. The battery doesn't need to cover all your annual kWh - just shift evening peak to stored midday solar.
- Forgetting humidity months. Perth summer aircon loads can be 3x winter loads. Size for summer, not the annual average.
- Skipping the headroom. A battery you cycle deeply every day will degrade faster. 30% headroom doubles its useful life.
- Buying the cheapest sizing because of the rebate. A 5 kWh battery hits the rebate cap percentage too, but absolute savings are smaller. Right-size for your home, then look at price.
How to actually figure your evening consumption
Two methods:
- Your bill. Look at total quarterly kWh used. Divide by 90 days. Multiply by 0.4 (roughly the proportion of usage between 3pm–10pm for most homes). That's your daily evening kWh.
- Synergy My Account or your smart meter. Pull the half-hourly data, sum 3pm–10pm. More accurate but needs an account login.
Either way, the answer is usually between 7 and 18 kWh. Multiply by 1.3 for headroom and you've got your right battery size.
Try different sizes in the calculator
The calculator includes a battery size slider - drag it through 5 kWh, 10 kWh, 13.5 kWh, 16 kWh, 20 kWh and watch the payback period change. The sweet spot for your home will become obvious.