Off-Grid Solar System Sizing Calculator
Size a complete off-grid solar system — panel wattage and battery amp-hours — from daily energy demand, peak-sun hours, desired autonomy days, system voltage, usable depth of discharge, and overall efficiency.
How to Use
- Enter daily energy demand in Wh. Count every load: lights, fridge (~1000 Wh/day), laptops, pumps, etc.
- Enter peak-sun hours for your location — not total sunshine hours but equivalent full-sun hours (US average 4–6).
- Set autonomy days — how many consecutive cloudy days the battery must cover. Typical: 2–3 for cabins, 5+ for critical systems.
- Pick system voltage: 12 V for small (<1 kWh/day), 24 V for mid-size (1–5 kWh/day), 48 V for 5 kWh/day+.
- Set usable DoD (50% lead-acid, 80–90% LiFePO4) and system efficiency (0.7 for PWM, 0.85 for MPPT with inverter).
Show Work
Formulas
History of Off-Grid Solar
Photovoltaic cells were discovered by Edmond Becquerel in 1839 — he observed that certain materials generated a voltage when exposed to sunlight. Bell Labs built the first practical silicon solar cell in 1954, achieving about 6% efficiency; modern commercial panels hit 20–22%. The Vanguard 1 satellite in 1958 was the first spacecraft solar-powered, and the 1970s oil crisis spurred the first serious investment in terrestrial PV.
Off-grid solar as a mainstream DIY pursuit dates to the 1970s "back-to-the-land" movement and the founding of Real Goods in 1978 — the first US retailer focused on off-grid equipment. Early systems were lead-acid-battery + dump-load-controlled (no true charge controllers), with panel arrays sized by rule of thumb. PWM charge controllers appeared in the 1980s; MPPT controllers became mainstream in the 2000s and delivered 20–30% more energy from the same panels.
Modern off-grid systems mix high-efficiency panels (400–600 W each), MPPT controllers tracking peak power point in real time, lithium iron phosphate (LiFePO4) battery banks rated for 3000–7000 cycles at 80% DoD, and hybrid inverters that seamlessly blend solar, battery, and grid or generator inputs. The math in this calculator — daily load ÷ sun hours for array, daily load × autonomy ÷ (voltage × DoD × efficiency) for battery — has been unchanged since the first off-grid sizing guides of the 1970s.
About This Calculator
Enter daily energy demand, local peak-sun hours, desired autonomy days, system voltage, usable DoD, and overall efficiency. The calculator returns the nameplate array wattage needed, the battery amp-hour capacity, total battery watt-hours, and the expected peak charge current through the controller.
These are starting numbers — real installs need local climate data, panel-specific derating (temp coefficient, shading), charge-controller sizing with margin, and electrical-code-compliant wire sizing for the computed currents. Tools like NREL\'s PVWatts and Solar-Electricity-Handbook refine these estimates. Everything here runs client-side; no values leave your browser.
Frequently Asked Questions
What are peak-sun hours?
The number of hours per day equivalent to full sun (1000 W/m²). A location with 5 peak-sun hours receives the same total solar energy as 5 hours of perfect noon sun, even if it actually has 8–10 hours of partial sun. NREL publishes peak-sun-hour maps for every US county; 4–6 is typical across most of the continental US.
Why autonomy days instead of hours?
Batteries size for the worst-case stretch without solar — consecutive cloudy or snowy days. 2 days is fine for hobby RVs; 5+ is standard for year-round off-grid homes. Less autonomy = smaller battery bank = cheaper, but you risk running out during bad weather.
Why size the battery for higher voltage?
At 12 V a 3 kW load draws 250 A; at 48 V it\'s 62 A. Lower current means smaller wire, smaller breakers, lower I²R loss in cabling. Above 1 kWh/day, 24 V pays off; above 5 kWh/day, 48 V is standard. Large systems go to 96 V or higher.
What efficiency should I use?
PWM charge controllers: 70–75% overall system. MPPT controllers with lithium batteries and a decent inverter: 80–90%. Include inverter loss (5–10%), charge controller loss (3–5%), battery round-trip (90–95% for lithium, 80–85% for lead-acid), and wiring losses.
Do I need an inverter?
Only for AC loads. Pure DC systems (LED lights, DC refrigerators, 12 V pumps) skip the inverter and its 5–10% loss. Most modern homes mix DC and AC, so a pure-sine inverter is standard. Size the inverter for peak simultaneous AC load, not daily average.
Common Use Cases
Remote Cabin
2 kWh/day load (lights, pump, small fridge), 5 peak-sun hours, 3 days autonomy, 24 V LiFePO4 at 80% DoD, 85% efficiency → ~470 W array, ~370 Ah battery. A single 450 W panel and a 400 Ah battery bank covers it.
RV / Van Build
1 kWh/day (LED lights, Starlink, laptop, 12 V compressor fridge), 5 sun hours, 2 days autonomy, 12 V LiFePO4 at 80% DoD → ~235 W array, ~200 Ah battery. Common build: 200 W panel on roof + 200 Ah battery.
Full Off-Grid Home
10 kWh/day, 4.5 sun hours, 4 days autonomy, 48 V LiFePO4 at 80%, 85% eff → ~2600 W array, ~1200 Ah @ 48 V (~57 kWh bank). Typical: 8–10 × 400 W panels and a 60 kWh battery bank.
Tiny Workshop
3 kWh/day (tool charging, lighting, small compressor intermittent), 5 sun hours, 2 days autonomy → 700 W array, 350 Ah @ 24 V. Straightforward DIY scale.
Well Pump Backup
1.5 kWh/day (pump runs 1 hr/day at 1.5 kW), 5 sun hours, 3 days autonomy → 350 W array, 280 Ah @ 12 V. Ensures water availability through multi-day outages.
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