Battery Size Optimiser
Check how different battery sizes will impact your annual electricity cost and return on investment. [reset]
About
This tool helps you find the optimal battery size for your home by comparing likely electricity savings with the cost of a battery.
Treat the results as a general guide, not a 100% accurate prediction. Every home is different, and your usage, equipment and electricity prices can change. Use the estimates as a starting point for a discussion with your installer.
Payback period is a simplified estimate based on installation cost and year-1 savings. Factors like battery lifespan and future energy price changes are deliberately left out for simplicity. You can factor these into your decision based on your own expectations.
Methodology
The core principle is to run the following comparison:
- Check a wide range of battery sizes
- Check all smart meter electricity tariffs
- For each battery size find the cheapest electricity tariff + battery schedule combination
- Take this as the annual cost of electricity for that battery size
So overall, the comparison assumes that whatever battery size you install, you compare using a tool like kilowatt.ie which lets you find the optimal tariff/battery schedule combination for your particular electricity conumption profile and battery size.
Electricity usage
For homes with solar, "usage" means total consumption both from solar and from the grid, not just import from the grid.
Battery capacity and performance
All capacities shown are total usable capacity, not additional or nameplate capacity. Maximum charging and discharging power are both assumed to be the smaller of {0.6 × usable battery capacity} and {5 kW}.
Charging schedules
Estimated bills assume you use the charging settings shown for each battery size. Charging times are allowed windows; the battery may fill before the window ends. “Hold” means preventing the battery from supplying your home until the stated time.
Check that your battery supports the charging, hold and solar settings shown.
Existing batteries
If your smart meter data readings already include battery use, we assess grid charging only: savings and payback do not model the effect of storing surplus solar in the battery for use later. Otherwise, we model the full impact of the battery: Load shifting plus storage of surplus solar energy.
Solar generation and charging
Estimated solar generation assumes a 5.5 kW AC inverter. For readings without existing battery use, we compare fixed schedules with surplus-solar charging on and off, including solar charging without grid charging. Export payments forgone are included in the bill comparison.
Where solar generation is simulated, clipping recovery assumes a compatible DC-coupled hybrid inverter. Recorded exports alone cannot reveal clipped generation, so no clipping recovery is estimated for the recorded solar period.
Solar capture is a half-hour approximation, limited by battery capacity and the shared solar/grid charging power limit.
Tariffs and savings
Savings are relative to the cheapest tariff at your current battery capacity (or no battery if you have none), not your current bill.
We compare smart electricity tariffs only; check eligibility with the supplier. Bills include VAT and standing charges. All comparisons assume an urban meter (DG1); rural standing charges are typically higher, but this assumption doesn't generally affect savings or payback period calculations meaningfully.
Installation costs and payback
Installation costs are rough estimates, not quotes.
If you're also installing panels, you can estimate the cost of the full solar installation.
Simple payback is the extra installation cost divided by annual savings. It excludes financing, degradation and future tariff changes.