Off-Grid Builds: a small boat house battery
A simple 12V house system for a small boat: one LiFePO4 battery, two 100W panels and a PWM controller for nav lights, electronics and a bilge pump.
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For about 400 Wh a day, our sample uses one 12V 100Ah LiFePO4 battery (1,280 Wh), two 100W rigid panels (200W) and a 30A PWM controller with a lithium setting. No inverter: everything runs on 12V DC.
- About 400 Wh/day of 12V loads (example).
- PWM is fine for two small panels on a 12V battery.
- Marine wiring standards (ABYC) apply; use marine-grade parts.
The load list
Example loads; replace them with yours in the Power Planner.
| Device | Watts × hours | Wh/day |
|---|---|---|
| Nav / anchor lights | 10 W × 10 h | 100 |
| Chartplotter / fishfinder | 15 W × 8 h | 120 |
| VHF radio (receive) | 5 W × 10 h | 50 |
| Bilge pump (example duty) | 50 W × 1 h | 50 |
| USB charging | 20 W × 4 h | 80 |
| Total | 400 |
Sizing math
Battery: 400 Wh/day × 2 day(s) of autonomy ÷ 0.92 inverter ÷ 0.80 usable ≈ 1,087 Wh (0.92 from the NREL PVWatts manual, 0.80 from Victron LiFePO4 data). 1 × 12V 100Ah LiFePO4 batteries (1,280 Wh each: 12.8V × 100Ah) give 1,280 Wh as a 12V bank. See how to size a battery bank.
Keep the engine's starting battery separate from the house battery.
Solar: 400 Wh ÷ (4 peak sun hours × 0.77, the PVWatts V1 derate) ≈ 130 W of panels. We use 200W. Check your site's sun hours in NREL PVWatts and size for your worst month. See solar panel sizing.
Two 100W panels cover the 130 W estimate with some margin.
Controller: 200W ÷ 12V ≈ 17A, within a 30A PWM controller that supports lithium.
Inverter: none; all loads are 12V DC.
Parts list
The main parts. Size fuses, breakers and cable from each maker's manual and a wire-gauge table.
LiTime 12V 100Ah LiFePO4 battery (Group 24)
One Group 24 LiFePO4 as the house battery.
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Renogy 100W solar panel
Two rigid 100W panels for a bimini or rail mount.
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Renogy Wanderer Li 30A PWM controller
30A PWM controller with a lithium setting.
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Klein Tools MM400 multimeter
For tracing faults in DC circuits.
Check price on Amazon (opens in a new tab)Wiring outline
Panel → fused PV line → PWM controller → house battery (fused at the terminal) → battery switch → DC distribution panel with a breaker for each circuit. Many boats wire the bilge pump so it stays powered when the house switch is off, with its own fuse; follow ABYC guidance.
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
Is a PWM controller OK on a boat?
For one small panel matched to a 12V battery, yes. MPPT gains more energy with larger or higher-voltage panels.
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.
- LiFePO4 (LFP)
- Lithium iron phosphate battery chemistry.
- Inverter
- Converts DC battery power to AC household power.
- 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.
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.