Fast read
Sizing a solar battery for energy self-consumption starts with two numbers: how many kilowatt hours (kWh) you usually export on a sunny day and how many kWh you use from late afternoon until sunrise. A battery that can hold most of that daily surplus—and cover most of your overnight load—will let you run on your own clean power instead of buying from the grid during peak evening rates. Check the battery’s usable capacity (accounting for its depth of discharge), think about future loads such as an EV, and confirm the design with a Clean Energy Council (CEC) accredited installer to ensure compatibility, safety, and a healthy payback period.
How do you size a solar battery for energy self-consumption?
An undersized battery empties before dinner is cooked, leaving you to pay retail rates just when tariffs peak. An oversized unit costs thousands more in upfront capital, stretches the payback period, and may never fill if your solar array is modest. The sweet spot is a battery that regularly charges to near-full from your surplus solar, then discharges through the night, maximising self-consumption and minimising waste. Finding that balance protects both your wallet and the battery, because lithium units last longer when cycled well within their rated depth of discharge.
Gather your energy data first
Pull the last twelve months of bills or smart-meter downloads. Note:
- Average daily consumption (kWh).
- Seasonal swings. Air-con in Darwin looks different from heaters in Hobart.
- Solar exports. Your inverter portal shows what you send to the grid after the household’s daytime demand is met.
This historical data smooths out one-off anomalies and reveals a trustworthy baseline. It also prepares you for an installer’s questions, streamlining quotes and compliance checks such as the STC paperwork.
Map your consumption pattern to the night hours
Self-consumption batteries work hardest between roughly 4 pm and 8 am. Use monitoring software (many retailers provide half-hourly downloads) to total those hours. Example: if you burn 8 kWh between sunset and sunrise, a battery with about 8 kWh of usable storage will cover most nights. Because most lithium iron phosphate units allow 90–100 % depth of discharge, you can aim close to parity, unlike older chemistries that needed 50 % buffers.
Check your solar surplus, not just system size
Panel capacity alone is misleading. A Perth household might have a 6.6 kW array that pumps out 25 kWh on a clear summer day—but if 10 kWh is used by pool pumps, fridges, and the home office, only 15 kWh is left to charge a battery. Choose a battery that can absorb most of that regular surplus. Oversizing beyond the average surplus means winter or cloudy days leave capacity unused, hurting financial returns.
Align battery capacity with self-consumption goals
Decide whether you are chasing:
- Maximum self-consumption. Aim to cover nearly all overnight use, so the battery empties just as the solar array wakes up.
- Load shifting only. A smaller unit covering the 4 pm–9 pm peak can still slash bills where time-of-use tariffs bite.
- Backup power. If blackout protection is a priority, factor in critical loads and inverter power output (kW) as well.
Remember that “off-grid” sizing is a different exercise, usually demanding much larger storage and redundancy.
Real-world battery options and modular growth
Today’s market offers everything from stackable 3 kWh modules to hefty 15 kWh single packs. Modular systems make it easy to start small and expand:
- Sigenergy SigenStor grows in 5 kWh steps and adds AI energy management, handy if an EV is on the horizon.
- Sungrow SBR lets installers clip additional 3.2 kWh blocks without rewiring the cabinet.
- Tesla Powerwall 3 delivers ~13.5 kWh usable in one sleek unit if your data already proves a higher overnight demand.
- Enphase IQ Battery 5P (≈5 kWh) and SolarEdge Energy Bank (≈9.7 kWh) serve homes wanting mid-range capacity with tight inverter integration.
Comparing these products against your calculated 8–10 kWh need keeps the decision objective and avoids brand hype.
Plan for future electric living
Australia’s Cheaper Home Batteries Program (launching 1 July 2025) and state incentives such as WA’s forthcoming $5,000–$7,500 subsidy may tilt the economics toward larger or scalable systems. Also consider:
- Electric vehicles. An EV adds roughly 12–20 kWh for a typical overnight top-up.
- Heat-pump water heaters or pool heaters. Efficient but electricity-hungry, they can shift daytime surplus into night-time demand.
- Family changes. Teenagers and home offices rarely reduce consumption.
If change is imminent, oversizing by a small margin or choosing a modular platform can save on future electrician call-outs and protect your solar self-consumption ratio.
Seal the decision with professional design
A CEC-accredited designer or installer will:
- Model your half-hourly load profile against local solar irradiance.
- Confirm compliance with AS/NZS 5139 (battery safety) and AS/NZS 5033 (PV arrays).
- Check inverter-battery compatibility and DNSP export limits.
- Optimise placement for ventilation, flood clearance, and shading (minimising hotspot risk in adjacent panels).
That expertise ensures warranty validity and access to incentives such as STCs or state battery rebates, and it keeps your system safe.
Making an informed choice for your energy future
Sizing a solar battery for self-consumption is ultimately a data-driven balancing act: match usable storage to typical overnight demand and the solar surplus that fills it. Armed with a year of bills, a clear vision of future loads, and guidance from a trusted accredited professional, you can choose a battery—whether a compact Enphase, a modular Sigenergy, or a single-unit Powerwall— that converts more of your rooftop sunshine into nightly savings and lower emissions.
Need help finding the right installer? Your Energy Answers can connect you with vetted experts who will turn your numbers into a tailored, future-proof battery solution.
