Sizing an off-grid system starts with your real daily load in kilowatt-hours, then works through days of autonomy, battery capacity, array size and inverter surge. The core formula is: battery capacity (kWh) = daily load × days of autonomy ÷ usable depth of discharge (~0.9). Get the load figure right and the rest follows; guess it and you'll either run a generator every second night or pay for capacity you never touch.
Before anyone can size a battery bank or array, you need a genuine daily energy figure in kWh — not watts. List every load in the house and any sheds that run off the system, and estimate hours of use per day.
A rough working example for a modest rural home:
| Load | Typical daily energy |
|---|---|
| Fridge/freezer | 1.5 kWh/day |
| Lighting (LED throughout) | 0.8 kWh/day |
| Water pump (pressure pump, tank water) | 1.0 kWh/day |
| TV, router, general electronics | 1.0 kWh/day |
| Washing machine (a few loads/week, averaged) | 0.7 kWh/day |
| Induction cooktop / oven | 2.0 kWh/day |
| Electric hot water (if not gas/wood) | 4–8 kWh/day on its own |
A modest all-electric rural household without heavy heating or a bore pump typically lands between 8 and 14 kWh/day. Add air conditioning, electric hot water, a bore pump or workshop tools and it climbs quickly — sometimes past 20–25 kWh/day. This is why generic sizing tables are close to useless: your number depends entirely on what's actually plugged in. The most reliable figure comes from checking actual meter data if you're currently grid-connected, or a proper load audit if you're building from scratch.
"Days of autonomy" is how many consecutive days the battery bank carries the full load with zero solar input — cloudy stretches, fog on the Tablelands, or extended winter overcast. It's a judgement call based on climate and risk tolerance:
Pushing autonomy beyond 3 days rarely makes financial sense; a generator as a backstop for rare extreme events is almost always cheaper than the extra battery capacity to cover a one-in-ten-year cloudy week.
Once you have the two numbers above, the formula is straightforward:
Battery capacity (kWh) = Daily load (kWh) × Days of autonomy ÷ Usable depth of discharge
Modern lithium (LFP) battery systems — such as Sungrow's SBR/SBH range, Sigenergy and Fox ESS — typically allow 90–95% usable depth of discharge, so divide by roughly 0.9.
Example: a farmhouse with a 15 kWh/day load and 3 days of autonomy needs 15 × 3 ÷ 0.9 ≈ 50 kWh of usable battery capacity.
This is where modular battery ranges earn their keep for off-grid work. Sungrow's SBR/SBH series, Sigenergy's stackable battery platform and Fox ESS are built to stack from around 5 kWh up to 100 kWh or more, so a 50 kWh bank isn't a special order — it's a standard configuration of the same modules used in much smaller homes. That modularity also means you can expand later (a new shed, an EV, a second dwelling) without replacing the original bank.
The array has two jobs: cover the daily load, and recharge the battery bank after it's been drawn down. Panel sizing in NSW needs to account for real seasonal sun-hour variation, not a single annual average.
Sydney and surrounding regions average around 4.5–5 peak sun hours per day annually, but winter months on the Tablelands (Bathurst, Orange, Lithgow) can drop to 2.5–3.5 peak sun hours, with more frequent fog and frost-related haze. Sizing to the annual average leaves you short every winter — the array needs sizing against the worst reasonably expected month.
Array size (kW) = Daily load (kWh) ÷ Winter peak sun hours ÷ System efficiency factor (~0.8)
Example: the same 15 kWh/day load against 3 winter peak sun hours: 15 ÷ 3 ÷ 0.8 ≈ 6.25 kW, rounded up to a 6.6 kW array as a practical minimum. Many rural off-grid systems end up in the 8–13 kW range once bore pumps, workshops or larger households are factored in — deliberately oversizing the array relative to summer needs is a sensible off-grid trade-off, because the alternative is a system that starves the battery every winter.
The battery and array sizing means nothing if the inverter/charger can't handle peak simultaneous demand — particularly motor-starting loads (bore pumps, workshop tools, air conditioners), which can draw 3–6 times their running wattage for a second or two on startup. This is a common point where DIY sizing goes wrong: the daily energy maths is fine, but the inverter trips every time the pump kicks in. A proper design checks continuous and surge ratings against your actual equipment list. See choosing the right off-grid inverter for detail.
Updated August 2026 — for NSW residents. The federal Cheaper Home Batteries Program (via the Small-scale Renewable Energy Scheme) and the NSW PDRS battery incentive both apply to off-grid and battery-only installs, and the NSW incentive no longer requires existing solar as of 1 July 2026. Honest caveat: a genuinely off-grid system can't join a Virtual Power Plant, so it can't claim the VPP-linked NSW top-up — the base incentive and federal rebate still apply. Confirm current values on the Clean Energy Regulator and NSW Government energy pages, and check batteries against the Clean Energy Council approved products list before budgeting.
The method above gets you within a sensible ballpark, but a real design needs a site-specific load audit, shading assessment and equipment list — particularly with a bore pump, workshop or second dwelling involved. Rough sizing from a web page is a starting point for the conversation, not a substitute for it.
Calculate your needs: run your figures through the solar and battery sizing calculator or the quote wizard at blueenergysolar.com.au — built to handle off-grid and rural cases, not just standard suburban rooftops. Or call 0421 458 217 / email sales@blueenergysolar.com.au.
