The 5-Step Solar Sizing Process
- Audit your loads. List every appliance with its wattage and daily hours of use. Multiply watts by hours for each to get watt-hours. A 60W fridge running 8 hours per day (compressor duty cycle) uses 480Wh. A 10W LED running 5 hours uses 50Wh. Add them all up for your total daily usage.
- Account for system losses. Multiply your total by 1.2 to 1.25 (for 20-25% losses). These losses come from charge controller conversion, battery charge/discharge inefficiency, wiring resistance, and inverter overhead. A 3,000Wh daily need becomes 3,750Wh after accounting for 25% losses.
- Calculate solar array size. Divide loss-adjusted daily usage by your worst-month peak sun hours. 3,750Wh / 4 hours = 937.5W of solar panels. Round up to the next panel — three 400W panels give you 1,200W, which provides a comfortable 28% buffer.
- Size the battery bank. Multiply daily usage by autonomy days, then divide by depth of discharge. For 3,000Wh x 2 days / 0.80 DoD = 7,500Wh of battery capacity. At 12V, that is 625Ah.
- Verify your charge controller. Your charge controller must handle the total panel current. Three 400W panels on a 12V system produce about 33A each at maximum power point — an MPPT controller rated for 100A would work.
Example: Sizing a Weekend Cabin System
Consider a small cabin with these loads: LED lights (40W for 5 hours = 200Wh), a 12V compressor fridge (50W average, runs 10 hours = 500Wh), phone and laptop charging (60W for 3 hours = 180Wh), and a water pump (100W for 0.5 hours = 50Wh). Total daily usage: 930Wh.
The cabin sits in Colorado with 4.5 worst-month peak sun hours. Adding 20% for losses: 930 x 1.2 = 1,116Wh needed from panels. That is 1,116 / 4.5 = 248W of solar. Two 200W panels (400W total) give a solid 60% buffer above minimum.
For batteries, 2 days of autonomy: 930 x 2 / 0.80 = 2,325Wh. At 12V, that is 194Ah. Two 100Ah LiFePO4 batteries (200Ah total) meet this target with a small margin.
Total cost estimate: two 200W panels ($200-300), two 100Ah LiFePO4 batteries ($400-600), a 30A MPPT controller ($100-150), and a 1,000W pure sine inverter ($100-200). Complete system: roughly $800-1,250.
Common Off-Grid System Sizes
| System Size | Daily Output | Battery Bank | Typical Use |
|---|---|---|---|
| 400W panels / 200Ah 12V | 1,200-1,600Wh | 2x 100Ah LiFePO4 | Small cabin, shed, or boat |
| 800W panels / 400Ah 12V | 2,400-3,200Wh | 4x 100Ah LiFePO4 | Full-time RV or van life |
| 2,000W panels / 400Ah 24V | 6,000-8,000Wh | 8x 100Ah LiFePO4 (24V) | Off-grid tiny house |
| 5,000W panels / 800Ah 48V | 15,000-20,000Wh | 16x 100Ah LiFePO4 (48V) | Full-size off-grid home |
Worked Examples
Off-Grid Shed Workshop in Oregon
Context
Calculation
Daily usage = (1800×1) + (80×5) + (15×6) + (10×2) = 1,800 + 400 + 90 + 20 = 2,310Wh
Loss-adjusted: 2,310 × 1.20 = 2,772Wh
Solar array: 2,772 / 3 PSH = 924W → 3 × 400W panels (1,200W)
Battery bank (2 days autonomy): 2,310 × 2 / 0.80 = 5,775Wh ÷ 24V = 241Ah at 24V
Interpretation
Three 400W panels provide a 30% buffer over minimum in Oregon's overcast winters. The 241Ah requirement at 24V means three 100Ah 12V batteries in series pairs (6 batteries total for 24V 300Ah) gives comfortable headroom.
Takeaway
For a workshop with intermittent high-draw tools, make sure your inverter handles the table saw's startup surge. Use our battery size for inverter calculator to confirm the battery bank can deliver peak current without voltage sag.
Grid-Down Emergency Backup in Texas
Context
Calculation
Daily usage = (150×10) + (30×24) + (60×6) + (20×3) = 1,500 + 720 + 360 + 60 = 2,640Wh
Loss-adjusted: 2,640 × 1.22 = 3,221Wh
Solar (winter): 3,221 / 3.5 = 920W → 3 × 400W panels (1,200W)
Battery (3 days autonomy): 2,640 × 3 / 0.80 = 9,900Wh ÷ 12V = 825Ah at 12V
Interpretation
For 3 days without sun, the battery bank is substantial — nine 100Ah batteries at 12V. That's heavy and expensive ($2,200+). Reducing autonomy to 2 days cuts the bank to 6 batteries. The 1,200W array recharges the bank in about 2 sunny days from empty.
Takeaway
An emergency backup system with 3 autonomy days needs serious battery capacity. After sizing, check the solar battery charge time calculator to verify your panels can recharge fully between outage events.
Frequently Asked Questions
Glossary
Autonomy Days
The number of days a battery bank can power your loads without any solar input. Higher autonomy means more batteries and cost, but greater resilience during extended cloudy periods or storms.
System Voltage
The nominal voltage of the battery bank (typically 12V, 24V, or 48V). Higher voltages reduce current for the same power, allowing thinner cables and smaller fuses, which is why large systems use 24V or 48V.
Loss-Adjusted Usage
Your raw daily energy consumption multiplied by a loss factor (typically 1.15 to 1.25) to account for charge controller, battery, wiring, and inverter inefficiencies that consume energy before it reaches your appliances.
Charge Controller
An electronic device between the solar panels and battery bank that regulates charging voltage and current. MPPT controllers are more efficient than PWM, especially when panel voltage differs significantly from battery voltage.
Check whether your planned battery bank can handle your loads with our battery runtime calculator. Try it now →
A properly sized off-grid system balances panel output, battery storage, and realistic loss factors. Undersizing any one component creates a bottleneck that drags down the whole system. When in doubt, round up — a 20% buffer on panels costs far less than running a generator every cloudy week.
Last updated:
Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.