Solar System Size Calculator
Free cabin load sheet — print it, 10 minutes.
Last updated: October 6, 2026 (CT)
Off-grid solar sizing calculator for cabin and van owners.
Add your daily loads. Get the panel count, battery bank Ah, inverter rating, and charge controller amps — worst-month sun hours and derating handled.
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Once you know your system size, matched off-grid solar kits: 400 W class for weekend cabins, 1,000 W+ class for full-time off-grid. 400 W solar kits on Amazon · 1,000 W+ kits on Amazon · 100 W panels on Amazon
The inverter turns battery DC into household AC. Size it by what runs at the same time — plus the startup surge of the biggest motor.
The charge controller sits between panels and battery. Size it by array watts ÷ battery voltage ÷ 0.85.
Have a battery already? Here's how much panel it takes to recharge it in a day — after sun hours and charge losses.
Sizing a whole bank from scratch instead? Use the battery bank sizer.
Frequently asked questions
What size solar system do I need for an off-grid cabin?
Add the watt-hours of everything you run per day, divide by your location's worst-month sun hours, then divide by 0.86 — NREL PVWatts' default 14% system losses (soiling, shading, mismatch, wiring, availability).
How many solar panels does it take to charge a 200Ah battery?
A 200 Ah 12 V battery stores 2,400 Wh. panelsForBattery does amp-hours × volts ÷ sun hours ÷ 0.85. At 4.5 sun hours that is 2,400 ÷ 4.5 ÷ 0.85 = 627.45 W, which the screen rounds to 627 W.
MPPT or PWM charge controller — which do I need?
Victron's SmartSolar MPPT 100/30 and 100/50 datasheet says an ultra-fast MPPT controller improves energy harvest by up to 30% compared with PWM controllers, especially under a clouded sky when light intensity keeps changing. Size it on this page as array watts ÷ battery voltage ÷ 0.85, then round up to the next listed rating (10, 20, 30, 40, 60, 80, or 100 A).
What size inverter do I need for off-grid solar?
Total the continuous watts of everything that could run at once, then apply the surge factor in the inverter tab: 1× resistive, 3× motor, 4× compressor, or 2× mixed. The calculator multiplies that continuous total by 1.25. The refrigerator preset is 150 W; a compressor factor of 4 on that preset is 600 W of surge.
How does off-grid solar sizing work?
Everything flows from one number: daily watt-hours. Panels must replace that number every sun-hour day — array watts = daily Wh ÷ sun hours ÷ 0.86, the derate from NREL PVWatts' default 14% system losses (soiling, shading, mismatch, wiring, connections, light-induced degradation, nameplate, availability). It does not include inverter or battery round-trip losses. The battery bank must carry you through sunless days: bank Wh = daily Wh × days of autonomy ÷ max depth of discharge. LiFePO4's 80% usable depth is why it wins on-paper capacity math even before cycle life.
sizeSystem rounds up to the next 100 W. The 0.86 is Table 6 of the NREL PVWatts Version 5 manual (14% default system losses). Bank watt-hours, not drawn, are daily watt-hours × days ÷ depth of discharge in the same function.How many watt-hours a day do common loads use?
| Load | Watts | Hrs/day | Wh/day |
|---|---|---|---|
| Energy Star refrigerator | 150 | 8 (cycled) | 1,200 |
| LED lights, cabin (10 bulbs) | 80 | 5 | 400 |
| Laptop + phone charging | 100 | 6 | 600 |
| 12 V water pump | 60 | 0.5 | 30 |
| CPAP machine | 40 | 8 | 320 |
| Starlink / internet | 60 | 12 | 720 |
| TV | 100 | 4 | 400 |
| Microwave | 1,000 | 0.25 | 250 |
| Induction cooktop (per burner) | 1,500 | 0.75 | 1,125 |
Draw figures are the preset rows in assets/app.js: watts × hours. Anything with heat in it (cooking, water heating, space heat) is what blows up solar budgets — propane or wood wins there in 2026 economics.
How many sun hours should you plan for (worst month)?
Use the worst-month sun hours for your site. The system-size form opens at 4. PVWatts prints a monthly figure for an address you enter.
Sourced data table + CSV: peak sun hours by US city
Guides
What size solar system for an off-grid cabin?
Array watts = daily watt-hours ÷ sun hours ÷ 0.86. The guide runs the refrigerator preset through that formula.
How many solar panels to charge a battery?
Recharge watts = amp-hours × volts ÷ sun hours ÷ 0.85. The guide's table is that function at 4.5 sun hours.
MPPT and PWM charge controller specs
Maximum PV open-circuit voltage, charge current, and battery voltage, copied from Victron and Morningstar datasheets.
Battery bank sizer ↗
Sizing the storage side in depth — LiFePO4 vs AGM bank builder with parallel-string planning. Generator backup? GeneratorSizer has the fuel-side math.
Common questions
Worked answers for a fixed daily load, a monthly bill, or one appliance. Each page shows the division and the source next to the figure. Enter your own loads in the solar system size calculator.
Daily energy
- What size solar system for 1 kWh per day?
- What size solar system for 2 kWh per day?
- What size solar system for 5 kWh per day?
- What size solar system for 10 kWh per day?
- What size solar system for 30 kWh per day?
Monthly use
- What size solar system for 500 kWh per month?
- What size solar system for 1,000 kWh per month?
- What size solar system for 1,500 kWh per month?
- What size solar system for 2,000 kWh per month?
Appliances and use cases
- What size solar system for an average home?
- What size solar system for a refrigerator?
- What size solar system for a CPAP?
- What size solar system for an electric car?
How the hardware fits the watt-hour result
- How do solar panels work?
- How to mount solar panels
- How to fuse a solar panel array
- How to charge LiFePO4 batteries with solar
- Solar panel degradation
- How to buy solar panels
- How many solar batteries do I need?
- How to size a solar charge controller
- Check a 12V battery open-circuit voltage
- How many solar panels for a heat pump?
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- SolarSizer calculator, assets/app.js — the load table is the preset rows (watts × hours): refrigerator 150 W × 8 h = 1,200 Wh, LED lights 80 W × 5 h = 400 Wh, laptop 100 W × 6 h = 600 Wh, water pump 60 W × 0.5 h = 30 Wh, CPAP 40 W × 8 h = 320 Wh, Starlink 60 W × 12 h = 720 Wh, TV 100 W × 4 h = 400 Wh, microwave 1,000 W × 0.25 h = 250 Wh, induction cooktop 1,500 W × 0.75 h = 1,125 Wh.
sizeSystemuses daily watt-hours ÷ sun hours ÷ 0.86 (1 − the 14% default system losses in the NREL PVWatts Version 5 manual, Table 6), rounds array watts up to the next 100 W, and sets bank watt-hours to daily watt-hours × days ÷ depth of discharge. The form opens at 4 sun hours, 2 days, 24 V, and 80% depth of discharge, with a 50% option.sizeControlleruses array watts ÷ bank volts ÷ 0.85 and the sizes 10, 20, 30, 40, 60, 80, and 100 A.panelsForBatteryuses amp-hours × volts ÷ sun hours ÷ 0.85; 200 × 12 ÷ 4.5 ÷ 0.85 = 627.45 W, andfmtrounds that to 627 W.sizeInvertermultiplies continuous watts by 1.25 and uses surge factors of 1 (resistive), 3 (motor), 4 (compressor), and 2 (mixed). Accessed Oct 6, 2026.
panelsForBattery uses amp-hours × volts ÷ sun hours ÷ 0.85. 200 × 12 ÷ 4.5 ÷ 0.85 = 627.45 W, which fmt rounds to 627 W. sizeInverter multiplies continuous watts by 1.25 and uses surge factors of 1, 3, 4, and 2. The MPPT “up to 30%” sentence is the SmartSolar MPPT 100/30 and 100/50 datasheet. Accessed Oct 6, 2026.