Three-phase properties get sizing wrong more often than single-phase ones, because the loads are bigger and less predictable. Here is what actually sets the number. Our Complete Guide covers retrofitting batteries generally.
Why is Inverter Sizing Important?
The battery inverter converts DC from your solar panels and battery into the AC your property runs on. Get the size wrong and you pay for it either way:
- Inefficiency: Undersized, it throttles how fast you can store surplus and how much you can draw back. You keep buying from the grid with a full battery.
- Damage: Sustained overload shortens the inverter's life or kills it outright.
- Wasted Investment: Oversized, you have paid for headroom you never use, and inverters run less efficiently well below their rated output.
Understanding the Basics
- Solar System Size (kW): Total panel capacity. Nineteen 350W panels is 6.65kW (19 x 350W = 6650W).
- Inverter Capacity (kW): Maximum AC output.
- Average Daily Electricity Usage (kWh): From your bill. A typical Australian home uses 11 to 23 kWh a day; three-phase properties are usually well above that.
- Battery Capacity (kWh): How much the battery stores.
Factors Affecting Battery Inverter Sizing for 3-Phase Systems
-
Solar Panel System Size: The array sets the ceiling on what you can store. Solar inverters are routinely sized slightly under the array, a 5kW inverter on a 6.6kW array, because panels rarely hit their rated output. Battery inverters follow different logic and need sizing on their own terms.
-
Battery Capacity: A large bank behind a small inverter charges slowly and discharges slowly. The battery's rated charge and discharge rates tell you the minimum inverter that will not bottleneck it.
-
Electricity Consumption Patterns: Peak demand is the number that matters, not the daily total. What is the most you draw at once? On three-phase that often means motors, ovens and air conditioning starting together, and starting loads are far higher than running loads.
-
Backup Power Requirements: If you want the battery to carry the property through an outage, the inverter has to be big enough for the loads you want alive: lighting, refrigeration, and any medical equipment. Backup capability pushes you up a size.
-
Single vs. Hybrid Inverters: AC-coupled keeps solar and battery inverters separate, which suits a retrofit onto an existing array. A hybrid inverter does both in one box, which simplifies a new install and usually costs less, provided it handles your solar and battery loads together.
Sizing Considerations for Australian Conditions
- Abundant Sunshine: Australian irradiance means real midday surplus, so the inverter needs enough capacity to actually capture it rather than clipping it away.
- Time-of-Use Tariffs: On time-of-use, the point is to cover the evening peak from storage. Your inverter has to be able to deliver your peak evening draw, or you are still buying at the expensive rate with a charged battery sitting there.
Getting Professional Advice
Get an installer to size it from your interval data. Your retailer will provide that data, and it shows your real peak demand rather than an estimate. Installers use sizing software; ask what assumptions it ran on, because the output is only as good as the consumption profile fed into it.
Government Rebates and Incentives
The Small-scale Renewable Energy Scheme (SRES) pays rebates on eligible systems, but your total rated panel output has to sit inside the inverter manufacturer's specifications. Exceed the ratio and you can lose the rebate entirely, which is an expensive way to find out.
Final Thoughts
Three numbers decide it: your peak simultaneous demand, your battery's charge and discharge rates, and your array size. Get those from measured data rather than estimates and the inverter size follows.
