The six numbers that matter
Pull out your most recent Synergy bill (paper or PDF in My Account). You're looking for:
- Total kWh used this period - usually labelled "Electricity usage" or "Total consumption"
- Number of days in the billing period - usually 60–95 days
- Total $ charged for usage (excluding supply charge)
- Solar export kWh (if you have solar) - labelled "DEBS credit" or "Solar export"
- Peak vs off-peak split (if you're on Smart Home Plan) - usually shown as two separate consumption lines
- Your tariff plan name - usually top right of page 1, e.g. "Home Plan A1", "Smart Home Plan", "Sunset Plan"
The maths from those six numbers
Step 1: Daily consumption
Total kWh ÷ days = your average daily kWh. A typical Perth family lands at 20–35 kWh/day. Higher in summer, lower in winter.
Step 2: Evening kWh (the part that matters for battery sizing)
If you're on Smart Home Plan, the peak consumption number on your bill is essentially your evening kWh. If you're on Home Plan A1, multiply daily kWh by 0.4 to estimate the 3pm–10pm portion.
Example: 30 kWh/day total × 0.4 = 12 kWh of evening consumption. That's the size battery you'd want to cover most evenings - round to 13.5 kWh.
Step 3: Solar export check
If your bill shows significant DEBS credits (e.g. 800+ kWh export over the period), you have excess solar that a battery can capture instead of selling at low buyback rates. That makes battery installation particularly attractive - every kWh you stop exporting at 2.5c and instead self-consume at 32c is a 12× return.
What the bill doesn't tell you
Three pieces of info that matter for battery sizing but aren't on the bill:
- Your peak draw. The kW you pull from grid at the moment of highest demand. This matters because some batteries cap their power output, which limits how many appliances they can run simultaneously. You need a smart meter readout to see this.
- Your home's behaviour during outages. Critical loads vs nice-to-haves. A battery sized for evening offset is usually enough for selective backup, but full-home backup needs a different conversation.
- Your future plans. Adding an EV, heat pump, induction cooktop, pool heater - all change the equation. Size for the next 5 years, not just today.
The Synergy My Account hack
If you log into Synergy My Account, you can download your half-hourly consumption data for the last 12 months as a CSV. Open it in Excel. Sum the 3pm–10pm rows. Divide by the number of days in your dataset. That's your true average evening consumption - no estimation needed.
It takes 15 minutes and gives you sizing precision that 95% of installers won't ask for. When you call for quotes, drop this number on them: "I average X kWh during peak hours, what battery do you recommend?" The installer who has a confident, specific answer is the one to pick.
For Horizon Power customers
Horizon billing is similar but the tariff structure varies by microgrid. Look for: total kWh, daily average, any time-of-use split, and (importantly) your export rate - Horizon's varies dramatically. The export rate effectively tells you whether the battery's value is in self-consumption (most Horizon towns) or arbitrage (a few).
Plug it into the calculator
Once you have your average quarterly bill in dollars (just multiply the bill amount by 365/days-in-period and divide by 4), drop it into the calculator with your postcode and current solar size. The combined rebate, savings and payback will be accurate to within ±10%.