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Off-grid 12 V power
A first sizing pass for a small Texas travel trailer. Buy only after the floor plan and wire runs are settled.
The parts and what each does
- Solar panel
- Turns sunlight into direct current (DC) electricity.
- MPPT charge controller
- Maximum power point tracking (MPPT) controller takes panel power and charges the battery efficiently.
- LiFePO4 battery
- Lithium iron phosphate (LiFePO4) battery stores 12 V power with less weight than lead-acid.
- Battery monitor and shunt
- A shunt is a precision resistor in the battery negative cable; the monitor uses it to show amps and state of charge.
- Pure-sine-wave inverter
- Changes 12 V DC into smooth 120 V alternating current (AC) for a few ordinary outlets.
- DC-DC charger
- Charges the trailer battery from the truck alternator at a controlled voltage and current.
- Shore-power charger/converter
- Changes campground 120 V AC into battery charging power and 12 V DC.
- Fuse block and main fuse
- The fuse block protects small branch circuits; a Class T or MRBF (marine-rated battery fuse) main fuse protects the large battery cable.
- Battery disconnect switch
- Lets you isolate the battery for service or storage.
Energy budget
Watt-hours (Wh) are watts multiplied by hours. These are planning loads, not promises; hot weather makes the fridge run longer.
| 12 V load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lights | 10 W source | 4 | 40 |
| Compressor fridge | 20 W average source | 24 | 480 |
| Roof vent fan | 48 W source | 4 | 192 |
| Water pump | 96 W (8 A × 12 V) source | 0.25 | 24 |
| Phone/laptop charging | 65 W source | 3 | 195 |
| Small TV | 40 W source | 2 | 80 |
| CPAP | 18 W (1.52 A × 12 V) source | 8 | 144 |
| Diesel or propane furnace fan | 61 W (5.1 A × 12 V) source | 4 | 245 |
| Light / typical / heavy day | 400 / 1,000 / 1,700 Wh; light omits TV, CPAP and furnace, while heavy adds longer fan and furnace use. | ||
Battery
Plan to use about 90% of a LiFePO4 battery but only about 50% of an absorbed glass mat (AGM) lead-acid battery if you want decent life. A 100 Ah LiFePO4 battery is about 24 lb; comparable AGM examples are about 60 lb. LiFePO4 listings commonly claim 4,000 cycles, while AGM listings commonly claim 300–500 cycles. Check the exact warranty and test method. LiFePO4 listing; AGM listing.
A 100 Ah LiFePO4 listing is about $258–$344; compare total delivered price before buying. current listing; current listing. Do not charge ordinary LiFePO4 cells below 32 °F (0 °C) unless the battery heats itself or has a verified low-temperature charging system. battery manual
Solar
Use 5 peak-sun-hours as a Texas planning figure until a city and roof angle are known: 100 W × 5 hours = 500 Wh before heat, dirt and controller losses. Run the location through NREL's PVWatts calculator before purchase. NREL PVWatts. Renogy's rigid 100 W panel is 41.8 × 20.9 in and 14.1 lb; a 200 W rigid panel is roughly twice the roof area and weight. panel listing
Rigid framed panels last longer and leave an air gap to cool. Flexible panels save height but can trap heat and are harder to replace. Bolt roof brackets into structure, seal every fastener, and leave walking and vent space.
Inverter
Keep the inverter small because its idle draw, cables, fuse and battery current all grow with its rating. Use 12 V loads where possible. A 13,500 British thermal unit (BTU) RV air conditioner draws about 12.8 A at 120 V (about 1,536 W) while running, before its startup surge; that is a poor battery load for this trailer. Coleman Mach 3 Plus
Charging from the truck
The 7-pin charge line is a small, long factory wire and loses voltage; treat it as battery maintenance, not a fast charger. A DC-DC charger takes a dedicated, fused truck feed and delivers the correct charging voltage and a chosen limit, such as 20–40 A. It also prevents the trailer battery from draining the parked truck. Victron charger; 7-pin field discussion
Safety
Put the main fuse as close to the battery positive terminal as practical. Choose wire gauge from the maximum amps, the whole round-trip length and permitted voltage drop; do not copy a wire size from another trailer. Blue Sea guide
Do not put a flooded lead-acid battery inside the living space without a vented battery box. Hydrogen made during charging can ignite. OSHA battery ventilation rule
Three systems
Prices are rough current-listing totals for panels, battery, controller, inverter, DC-DC charger, shore charger, fuses, disconnect, monitor and basic cable. They exclude roof fabrication and installation. Component price anchors: battery, panel, DC-DC charger.
| Item | Small | Medium | Large |
|---|---|---|---|
| Solar watts | 200 W | 400 W | 600 W |
| Battery | 100 Ah LiFePO4 | 200 Ah LiFePO4 | 300 Ah LiFePO4 |
| Inverter | 1,000 W | 2,000 W | 3,000 W |
| DC-DC charger | 20 A | 30 A | 40 A |
| Days of typical 1,000 Wh load, battery only | 1.2 | 2.3 | 3.5 |
| Total installed weight | About 70 lb | About 140 lb | About 205 lb |
| Current-listing price range | $1,100–$1,600 | $2,000–$2,900 | $3,000–$4,300 |
What Joe built
Joe used one Relion RB100 lithium battery (100 Ah, about 30 lb), a Victron solar charge controller, a fuse panel and a battery switch. See Part 10.
On the roof he put two Sunflare flexible panels of 180 W each, 360 W in total, stuck down with adhesive tape. See Part 11.
On his first trip, this system ran his 12 V Dometic cooler, LED lights, a fan and a charging station for over three days. See First trip.
Design numbers for my trailer
| Item | Number | Source |
|---|---|---|
| Small roof panel footprint / power-system weight | 12.2 sq ft / about 70 lb | Two 41.8 × 20.9 in, 14.1 lb panels plus battery and hardware source |
| Medium roof panel footprint / weight | 24.4 sq ft / about 140 lb | Four-panel planning area; see tier assumptions above. |
| Large roof panel footprint / weight | 36.6 sq ft / about 205 lb | Six-panel planning area; see tier assumptions above. |
| Battery placement | Low, near axle, in a secured serviceable compartment | Put 24 lb per 100 Ah low to limit a high center of gravity. battery weight |
| Wire runs | Measure battery-to-inverter and truck-to-charger round trips before sizing | Gauge depends on amps and length. wire chart |