Home Energy Storage for Granny Flat: Right-Sizing a Second Dwelling

A granny flat looks like an extension of the main house, but electrically it behaves like a small separate home: its own kitchen, its own heating, its own occupancy pattern, and very often its own meter. Sizing a home energy storage system for one is not a matter of doubling the main house bank. You are covering a second load profile that peaks at different hours and, in a rental or multi-generational arrangement, cannot simply be switched off when the main household is away.

Why a Second Dwelling Changes the Load Picture

The main house and the granny flat rarely peak together. The flat may run a washing machine and a heat pump in the evening while the main house is quiet, or sit nearly empty all week and load up at weekends. If both draw from one bank, the shared system has to cover a combined peak that almost never happens, which is expensive. A dedicated bank sized to the flat’s own profile is usually cheaper and gives the occupant genuine independence during an outage.

Reading the Actual Load Profile

Start from a logger rather than an estimate. Two weeks of interval data will show the base load, the evening peak, and whether there is an air-conditioning or water-heating spike that dominates everything else. A compact one-bedroom flat with induction cooking commonly sits around 1 to 2 kW average with a 4 to 6 kW peak; add a reverse-cycle unit and the summer peak can double. The bank must cover the average over the autonomy window and still deliver the peak without tripping on voltage sag.

Sub-Panel or Whole-Dwelling Backup

Most granny flats are best served by a dedicated sub-panel carrying lighting, refrigeration, communications, the heat pump, and a couple of general-purpose circuits. High-draw appliances such as an electric oven, a clothes dryer, or a workshop circuit stay on the grid side. That keeps the bank small and the outage experience predictable, and it avoids the classic mistake of specifying enough capacity for a load that runs for twenty minutes a day.

Granny Flat Storage Comparison

Metering, Tariffs, and Compliance

If the flat has its own meter, the battery sits behind that meter and the tenant’s consumption stays separate, which matters for both billing and any solar feed-in arrangement. Check whether the local network operator requires notification before a storage unit is connected, and confirm the inverter appears on the approved list for the relevant grid code. Where the flat is let, the installation also has to satisfy the electrical safety regime that applies to rental properties in your jurisdiction.

What to Ask a Supplier

Ask for the continuous and peak power ratings separately, because a headline kilowatt-hour figure says nothing about whether the unit will start a heat pump compressor. Request the usable capacity rather than the nameplate figure, the round-trip efficiency, and the operating temperature range. Confirm what the warranty says about capacity retention after ten years, and ask whether the unit can be expanded later if the flat’s use changes.

People Also Ask

How much storage does a granny flat need? Most one-bedroom flats are comfortable with 8 to 12 kWh of usable capacity covering four to eight hours of evening load. Add capacity if the flat uses electric heating or hot water.

Can one battery serve both the house and the flat? It can, if the cable run and the combined peak allow it. A separate bank is usually simpler to meter and avoids sizing for a peak that rarely occurs.

Does the flat need its own solar array? Not necessarily, but a small dedicated array makes the economics work better and keeps the flat’s generation on its own meter where a feed-in tariff applies.

Will the battery keep a heat pump running? Yes if the inverter’s continuous rating exceeds the compressor starting current. Check the locked-rotor figure with the installer rather than the running power.

Written by Karl at China Battery Technology. Request a quote.