Off-Grid Solar & Battery Sizing
Full DIY system sizing: load list, battery bank, solar array, inverter, and AU STC rebate.
Daily Load Calculator
List each appliance, its wattage, and how many hours per day you use it.
System Configuration
Use local solar data for your location (typically 3–6).
2–3 days is typical; northern or cloudy locations (e.g. Canada, Alaska) may need more.
Used to calculate how many panels you need.
Battery/system voltage for MPPT sizing.
Estimates only. Verify with local codes, manufacturer specs, and a qualified installer.
Off-Grid Solar & Battery Sizing
Sizing an off-grid solar system starts with an honest load list: every appliance, its wattage, and how many hours it runs each day. Small errors compound quickly when you are designing for nights, cloudy weeks, and winter sun angles without the grid as a backup.
This calculator walks through the same sequence a qualified installer uses—daily energy, battery autonomy, solar array output, and inverter headroom—so you can compare 12 V, 24 V, and 48 V architectures before you buy hardware. Results are planning estimates; always confirm wire gauges, fusing, and local electrical codes with manufacturer data and a licensed professional where required.
If you are new to off-grid design, work through the load table first, then adjust peak sun hours for your latitude and season. Lithium (LiFePO₄) banks typically allow deeper discharge than lead-acid, which directly changes how many kilowatt-hours you need for the same comfort level.
How this calculator works
- Build a daily load list: for each appliance, multiply watts × hours per day and sum the watt-hours (Wh). That total is your baseline energy demand.
- Choose days of autonomy—the number of days the battery should carry the load without meaningful solar input. Weekend cabins often use 1–2 days; full-time off-grid homes commonly plan for 2–3.
- Set peak sun hours for your site (typically 3–6 equivalent full-sun hours per day). Lower values mean a larger solar array for the same daily recharge.
- Pick system voltage (12, 24, or 48 V) and battery chemistry. Higher voltage reduces current for the same power, which can shrink cable size on larger systems.
- Review battery amp-hours, solar wattage, panel count, and inverter guidance. For Australia, an STC rebate estimate is included when that region is selected.
Core sizing relationships
Daily Wh = Σ (appliance W × hours/day) | Battery Wh ≈ (Daily Wh × days of autonomy) ÷ usable depth of discharge | Solar W ≈ Daily Wh ÷ (peak sun hours × system efficiency)
Daily watt-hours come straight from your load table. Battery capacity scales with how many days you want to run without sun and how much of the pack you are willing to cycle—lithium often allows ~90% usable energy versus ~50% for many lead-acid designs. Solar array size divides daily energy by usable sun hours and real-world losses (cable drop, temperature, charge-controller and battery round-trip efficiency). Inverter sizing should cover simultaneous peak loads, not just average demand.
Formulas used by this tool
- Daily Wh = sum of (watts × hours) for each appliance.
- Battery = daily Wh × days of backup × 1.1 ÷ 0.9 (DoD).
- Battery = daily Wh × 1.1 (efficiency) ÷ 0.9 (DoD).
- Solar = daily Wh × 1.25 (losses) ÷ peak sun hours.
- Inverter continuous = max load; surge = continuous × 2.5 for motors/fridges.
- MPPT amps = (solar W ÷ battery voltage) × 1.25; round up to 20/30/40/50 A.
- Australia: 2026 STC factor 6.8 with tapering (0–14 / 14–28 / 28–50 kWh).
Worked example: small cabin
Suppose your load list totals 2,400 Wh per day (LED lighting, a DC fridge, laptop, and Starlink), you want 2 days of autonomy, 5 peak sun hours, a 24 V lithium bank at 90% depth of discharge, and 400 W panels.
- Required battery energy ≈ (2,400 × 2) ÷ 0.9 ≈ 5,333 Wh. At 24 V that is roughly 222 Ah—round up for inverter efficiency and aging.
- Solar demand ≈ 2,400 ÷ (5 × ~0.85 system efficiency) ≈ 565 W of array. Two 400 W panels (800 W nameplate) gives headroom for winter and partial shading.
- Check inverter continuous and surge ratings against your highest combined load (often fridge compressor + pump or microwave).
- Re-run the table if you add heat, air conditioning, or electric cooking—those loads usually dominate off-grid designs.
Practical tips before you buy
- Measure or verify wattage with a plug-in meter instead of guessing nameplate labels—especially for fridges, pumps, and tools.
- Size solar for the worst month you plan to use the system, not just summer peak sun.
- Keep heavy loads on 48 V or 24 V DC when possible; long 12 V runs at high current waste energy in copper losses.
- Plan MPPT charge-controller headroom above array short-circuit current and battery maximum charge rate.
- Document fuse and disconnect locations early—serviceability matters more when you are your own utility.
Frequently asked questions
How accurate is this off-grid solar calculator?
It provides order-of-magnitude planning numbers from your inputs. Real performance depends on shading, temperature, wiring losses, battery age, and how closely actual usage matches your load table. Use the output to compare scenarios and shortlist equipment, then validate with datasheets and an installer or electrician.
What peak sun hours should I use?
Use location-specific solar data for the month you care about most. Many temperate sites average 3–5 peak sun hours in winter and 5–7 in summer. When in doubt, size for the lower number so the array still recharges the bank on short winter days.
12 V vs 24 V vs 48 V—which is better off-grid?
12 V suits small cabins and RVs with modest loads. 24 V is a common sweet spot for mid-size homes. 48 V reduces current for the same power and is typical once continuous loads exceed roughly 2–3 kW or cable runs are long. The best choice is the voltage your inverter, battery, and charge controller support at a reasonable cost.
How many days of autonomy do I need?
Weekend or fair-weather cabins often plan 1 day. Full-time off-grid homes commonly use 2–3 days to ride through storms. Each extra day increases battery cost linearly; extra solar helps recharge faster after outages but does not replace autonomy if you need overnight and cloudy-day coverage.
Does this include Australian STC rebates?
When you select Australia as the region, the tool estimates Small-scale Technology Certificate (STC) value from system size and zone assumptions. Rebate rules change; treat the figure as indicative and confirm with the Clean Energy Regulator or your installer before purchasing.
Lithium or lead-acid for off-grid?
Lithium (LiFePO₄) offers higher usable capacity, faster charging, and longer cycle life per kilowatt-hour cycled. Flooded or AGM lead-acid costs less upfront but needs larger banks for the same usable energy and more careful state-of-charge management. Match chemistry to budget, temperature, and how often you will cycle the bank.
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