Off-Grid Builds: a weekend cabin system
A 12V sample system for a weekend cabin: lights, water pump, a small fridge and charging, with sizing math and parts.
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For about 1,200 Wh a day of cabin loads, our sample uses three 12V 100Ah LiFePO4 batteries (3,840 Wh), 400W of solar, a 40A MPPT controller and a 1,000W inverter.
- About 1,200 Wh/day of example loads.
- 3,840 Wh of 12V LiFePO4 covers the 3,261 Wh two-day target without sun.
- 400W of panels refills a day's use in good sun (estimate).
The load list
Example loads; replace them with yours in the Power Planner.
| Device | Watts × hours | Wh/day |
|---|---|---|
| LED lights, 6 × 8 W | 48 W × 5 h | 240 |
| 12V water pump | 60 W × 0.5 h | 30 |
| Small fridge (example) | ≈30 W average × 24 h | 720 |
| Phones and laptop | 70 W × 3 h | 210 |
| Total | 1,200 |
Sizing math
Battery: 1,200 Wh/day × 2 day(s) of autonomy ÷ 0.92 inverter ÷ 0.80 usable ≈ 3,261 Wh (0.92 from the NREL PVWatts manual, 0.80 from Victron LiFePO4 data). 3 × 12V 100Ah LiFePO4 batteries (1,280 Wh each: 12.8V × 100Ah) give 3,840 Wh as a 12V bank. See how to size a battery bank.
Solar: 1,200 Wh ÷ (4 peak sun hours × 0.77, the PVWatts V1 derate) ≈ 390 W of panels. We use 400W. Check your site's sun hours in NREL PVWatts and size for your worst month. See solar panel sizing.
Controller: 400W ÷ 12V ≈ 33A of charge current, so a 40A MPPT controller has headroom. Keep the array's open-circuit voltage under the controller's input limit in the coldest weather. See charge controllers.
Inverter: the largest AC load here is a laptop charger, so a 1,000W pure sine inverter is plenty. Its idle draw counts against the battery; switch it off when unused.
Parts list
The main parts. Size fuses, breakers and cable from each maker's manual and a wire-gauge table.
Renogy 12V 100Ah LiFePO4 battery (mini)
Three of these, in parallel, for a 3,840Wh 12V bank.
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Renogy Rover 40A MPPT charge controller
40A MPPT controller for the 400W array.
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Renogy P2 1000W pure sine inverter (12V)
1,000W pure sine inverter for AC loads.
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Victron SmartShunt 500A battery monitor
Shows state of charge so you know what's left.
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Klein Tools MM400 multimeter
For checking voltages during install.
Check price on Amazon (opens in a new tab)Wiring outline
Panels → fused combiner → PV disconnect → MPPT controller → fused battery bus → batteries in parallel. The inverter connects to the battery bus through its own fuse and switch. The shunt sits on the battery negative so it sees all current. DC loads (pump, lights) run from a fused DC panel off the bus.
This is a planning outline, not a wiring diagram. Follow each manufacturer's manual, fuse every positive conductor close to the battery, and have a qualified installer check the work. Local codes apply to buildings and vehicles.
FAQ
Can I use lead-acid batteries instead?
Yes, but you'd need about twice the rated capacity for the same usable energy, because lead-acid is usually planned at 50% depth of discharge. See LiFePO4 vs lead-acid.
Where do I go next?
Try the power planner to put numbers to your own situation, or see the product picks on the deals page.
Terms used here
- Watt (W)
- Rate of power use or production.
- Watt-hour (Wh)
- Energy: one watt for one hour.
- Depth of discharge (DoD)
- Share of a battery's capacity that is used.
- State of charge (SoC)
- How full a battery is, as a percentage.
- LiFePO4 (LFP)
- Lithium iron phosphate battery chemistry.
- Inverter
- Converts DC battery power to AC household power.
- Charge controller
- Regulates charging from panels to battery.
- MPPT
- Maximum power point tracking controller.
- PWM
- Pulse-width modulation controller.
- Peak sun hours
- Hours of equivalent 1,000 W/m² sunlight per day.
- Derate
- Factor for real-world losses in solar output.
- Open-circuit voltage (Voc)
- Panel voltage with no load; rises in cold.
Sources
This guide is informational and based on published specifications and general references, not hands-on testing. Check the manufacturer's current specs before you buy.