Off-Grid System Calculator: sizing solar, battery, charge controller and inverter
Cabin, tiny house, hunting lodge or shepherd's hut: an off-grid system has to work in the month with the least sun. That's exactly where this calculator starts – it picks the weakest month of your usage period from real PVGIS monthly data and derives panel power, battery capacity, charge controller current and inverter size from it.
The difference is dramatic: the same system needs several times the panel power for year-round use compared with summer only. Enter your devices and choose when you want to use the system.
Result
Required solar power
224 W
for 685 Wh per day in October
- Battery capacity
- 208 Ah
- at 12 V – equals 2,492 Wh
- Charge controller
- 23 A
- incl. 25 % reserve, MPPT recommended
- Inverter
- not needed
- all devices run directly on 12/24 V
- Daily consumption
- 685 Wh
- 57.1 Ah at 12 V
- Design month
- October · 4.59 kWh per kWp and day
- Reserve without sun
- 3 days
- this drives the battery size
- Sized for October – outside your usage period the system will not be enough.
- Fusing, cable sizes and lightning protection are not part of this calculation. DC circuits with high currents belong in qualified hands.
Show calculation
- 1Cooler: 45 W × 10 h direct = 450 Wh
- 2Laptop: 65 W × 3 h direct = 195 Wh
- 3LED lights: 10 W × 4 h direct = 40 Wh
- 4Daily consumption: 685 Wh = 57.1 Ah at 12 V
- 5Design month: October with 4.59 kWh per kWp and day
- 6Solar power: 685 Wh ÷ (4.59 × 80%) × (1 + 20%) = 224 W
- 7Battery: 685 Wh × 3 days ÷ 85% depth of discharge ÷ 12 V = 208 Ah
- 8Charge controller: 224 W ÷ 12 V × 1.25 = 23 A
How it's calculated
An off-grid system has four parts that have to match: panels, battery, charge controller and inverter. The chain is only as good as its weakest link – and everything hangs on one question: in which month does the system still have to work?
The design month decides everything
A panel in December delivers only a fraction of its June output. Anyone using a cabin in summer only therefore needs a fraction of the panels of a year-round installation. This calculator picks the weakest month of your usage period from the PVGIS monthly data and sizes everything for it.
| Usage | Design month | Yield per kWp and day | Panels for 600 Wh/day |
|---|---|---|---|
| Summer only (May–Aug.) | May or August | approx. 5.5 kWh | approx. 165 W |
| Spring to autumn (Mar.–Oct.) | October | approx. 3.3 kWh | approx. 275 W |
| Year-round | December | approx. 1.8 kWh | approx. 500 W |
solar power [W] = daily consumption ÷ (yield in the design month × system efficiency) × (1 + reserve)
battery [Ah] = daily consumption × reserve days ÷ (depth of discharge × efficiency) ÷ system voltage
charge controller [A] = solar power ÷ system voltage × 1.25
fuse [A] = inverter ÷ (system voltage × efficiency) × 1.25Why system voltage matters
At the same power, current halves when you go from 12 V to 24 V. That means thinner cables, smaller fuses and cheaper charge controllers. Rule of thumb: up to about 1,000 W of inverter power use 12 V, above that 24 V, and from about 3,000 W use 48 V. A 2,000 W inverter at 12 V draws around 190 A – that calls for very heavy cable.
Worked example
Cabin with LED lights, a mobile router and a small fridge on the inverter. Used from spring to autumn, panels tilted south, US Midwest, 12 V system with LiFePO4 and three reserve days.
Inputs
- Devices:
- LED-Licht: 10 W × 4 h/day × 0 1/0
- Mobiler Router: 10 W × 8 h/day × 0 1/0
- Kühlschrank 230 V: 60 W × 8 h/day × 1 1/0
- Usage period: Spring to autumn (March–October)
- Region: US Midwest
- Panel orientation: South
- Tilt: Tilted (30–35°)
- System voltage: 12 V
- Battery type: LiFePO4 (lithium)
- Reserve days without sun: 3 days
- Reserve on panel power: 20 %
- Solar system efficiency: 80 %
- Inverter efficiency: 88 %
Result
217 W
Required solar power
- Battery capacity
- 202 Ah
- Charge controller
- 23 A
- Inverter
- 60 W
- Daily consumption
- 665 Wh
- Design month
- October · 4.59 kWh per kWp and day
- Reserve without sun
- 3 days
Calculation
- Kühlschrank 230 V: 60 W × 8 h via AC = 545 Wh
- Mobiler Router: 10 W × 8 h direct = 80 Wh
- LED-Licht: 10 W × 4 h direct = 40 Wh
- Daily consumption: 665 Wh = 55.5 Ah at 12 V
- Design month: October with 4.59 kWh per kWp and day
- Solar power: 665 Wh ÷ (4.59 × 80%) × (1 + 20%) = 217 W
- Battery: 665 Wh × 3 days ÷ 85% depth of discharge ÷ 12 V = 202 Ah
- Charge controller: 217 W ÷ 12 V × 1.25 = 23 A
- Inverter: 60 W for the largest AC device, battery fuse 7 A
The variables explained
- Devices: power, runtime, output
- Rated power from the label, actual runtime per day, and whether the device runs through the inverter (AC) or directly on 12/24 V.
- Usage period
- Sets the design month. The biggest lever in the whole calculation – year-round use costs several times the panel power of summer-only use.
- Orientation and tilt
- For year-round systems a steeper tilt pays off: it shifts yield into winter, where it's needed.
- System voltage
- 12 V for small systems, 24 V from about 1,000 W, 48 V from about 3,000 W. Higher voltage means less current and thinner cables.
- Battery type
- LiFePO4 can be discharged to 85 %, lead-acid only to 50 % – for the same usable energy, lead needs nearly twice the capacity.
- Reserve days
- How many days without sun the battery should bridge. Three days is a common approach.
Common mistakes
- Sizing for summer and using it in winter: December delivers a fraction of June – a summer system sits idle in winter.
- Choosing too small a battery: three overcast days in a row are normal. Without reserve days the system runs empty regularly.
- Counting a lead-acid battery at 100 %: only half is usable, otherwise its life drops sharply.
- Sizing the charge controller tightly: panels briefly exceed their rating in the cold – 25 % reserve belongs in the calculation.
- Running everything through the inverter: lights, pumps and USB devices straight on 12 V save the conversion losses.
- Forgetting the battery fuse: a properly rated fuse between battery and inverter is not optional equipment.
Assumptions and limits
- Yield data comes from PVGIS (2005–2023 average) for one reference site per region; shading from trees or buildings is not included.
- Consumption is assumed constant across the year. In winter lights and heating controls usually add to it, in summer cooling does.
- Inverter power is based on the largest single device; if several AC devices run at once, add them up.
- Charge efficiency and depth of discharge come from typical manufacturer figures per battery technology.
- Cable sizes, row spacing and the specific fusing are not part of this calculation – that needs proper planning.
- Temperature effects are not modelled: below 0 °C usable capacity drops, and LiFePO4 must not be charged below freezing.
Frequently asked questions
How much solar do I need for a cabin?
It depends almost entirely on the usage period. For 600 Wh a day, summer-only use needs roughly 165 W in the US Midwest. Spring to autumn it's about 275 W, year-round about 500 W – and in Central Europe the year-round figure is closer to 1,500 W because December is so weak.
How big does an off-grid battery need to be?
Daily consumption × reserve days ÷ usable depth of discharge. At 600 Wh a day, three reserve days and LiFePO4 (85 % depth), that's about 2,100 Wh, i.e. roughly 175 Ah at 12 V. With lead-acid it would be nearly double.
12 V, 24 V or 48 V – which is right?
Up to about 1,000 W of inverter power 12 V is practical; above that the current gets unwieldy: 2,000 W at 12 V draws around 190 A. So use 24 V from 1,000 W and 48 V from about 3,000 W. At higher voltage, cables, fuses and controllers are considerably cheaper.
Which charge controller do I need?
Charge current = panel power ÷ system voltage, plus 25 % reserve. 500 W at 12 V gives about 52 A. MPPT almost always pays off: it harvests 10–30 % more than a PWM controller and allows higher panel voltages, so thinner cables from the panels to the controller.
Is a year-round off-grid system worth it?
Only with a large array or a second charging option. Because December is so weak, the system would have to be several times larger than for summer use – and that capacity sits idle the rest of the year. A common approach is to size for spring to autumn and add a generator or occasional grid charging.
How many reserve days should I plan for?
Three days is a good baseline – that's how long a typical spell of bad weather lasts. If you use the system in winter or depend on it, plan four to five. Every reserve day costs battery capacity directly.
Do I need an inverter?
Only for AC devices. Lights, pumps, fans, routers and USB chargers are all available in 12 V – that saves 12–18 % of conversion losses plus the inverter's idle draw. For a fridge, power tools or kitchen appliances there's no way around one.
What does a small off-grid system cost?
Component prices vary widely, which is why this calculator deliberately quotes none. As a guide: the battery is usually the largest share, followed by the panels. Reducing consumption saves on both at once.
Sources and background
- PVGIS 5.3 (European Commission, JRC): monthly yield by site, orientation and tilt
- Manufacturer data sheets for charge controllers, batteries and inverters (depth of discharge, efficiencies, permissible currents)
- NEC Article 690 and manufacturer guidance on fusing DC circuits – the specific design belongs in qualified hands