A well-sized 12V system turns an RV into a reliable off-grid home. In practice, success comes from balancing four parts—panels, charge controller, battery bank, and inverter—based on your actual loads. Below is the step-by-step process I use on real builds, with simple math you can verify.
1) Start with a Daily Energy Audit
I inventory every device (DC and AC) and calculate daily watt-hours (Wh = Watts × Hours). Small “phantom” loads count. Here’s a sample day to illustrate the math:
| Appliance | Power (W) | Daily Use (h) | Daily Energy (Wh) |
|---|---|---|---|
| LED Lights (x4) | 12 | 4 | 48 |
| Roof Fan | 35 | 8 | 280 |
| Water Pump | 60 | 0.5 | 30 |
| Laptop | 65 | 3 | 195 |
| 12V Fridge (≈ 33% duty) | 50 | 8 | 400 |
| Total | 953 Wh |
2) Size the LiFePO4 Battery Bank
LiFePO4 offers deep usable capacity and long cycle life. I convert Wh to Ah at system voltage and add a buffer for inverter and wiring losses.
Ah ≈ Daily Wh ÷ 12 V. With 953 Wh: 953 ÷ 12 ≈ 79.4 Ah. Adding ~20–25% headroom, a 100 Ah LiFePO4 pack is a practical starting point for this profile. Confirm your battery’s recommended charge/discharge rates and low-temperature charging limits in the datasheet.
3) Determine Solar Array Wattage
Your array must replace daily Wh and cover charge inefficiencies. I use local peak sun hours (PSH)—not daylight length. In many U.S. locations, annual-average PSH is roughly 4–5, but winter or northern routes can be lower.
Array W ≈ Daily Wh ÷ PSH × derate. Using 953 Wh, PSH = 4, derate 1.25 (to cover heat, angle, wiring): 953 ÷ 4 × 1.25 ≈ 298 W → I round to a 300 W+ array. If you travel in winter or shade-prone forests, oversize further.
PV basics and maps: NREL • U.S. DOE Solar.
4) Select an MPPT Charge Controller
MPPT tracks the panel’s optimal voltage/current and converts surplus voltage into charging amps—most helpful when array Vmp is well above battery voltage, in cold weather, or with variable light. Gains depend on conditions; they are not a flat percentage.
Controller amps ≈ Array W ÷ Battery V. For 300 W at 12 V: ≈ 25 A. I choose the next size up (e.g., 30 A) and ensure the controller’s maximum input voltage (Voc rating) exceeds the string’s worst-case cold Voc (panel Voc × series count × cold multiplier from the datasheet).
More on controllers and off-grid design: NREL Off-Grid Considerations.
5) Pick a Pure Sine Wave Inverter
Pure sine wave avoids issues with sensitive electronics. Size to your simultaneous AC loads and include surge margin.
Example: Microwave 700 W + laptop 65 W → ≥ 765 W continuous. A 1000 W inverter provides comfortable headroom. Check surge for compressors and tools.
6) Wiring, Protection, and Losses (Don’t Skip)
Undersized cables and missing protection are the top performance and safety failures I see.
- Voltage drop target: I aim for ≤ 3% from panel to controller in 12 V systems. Quick check: Drop (%) ≈ (2 × length (m) × current (A) × cable Ω/m) ÷ voltage.
- Fusing/Breakers: Protect panel strings, battery leads, and inverter feeds per device ratings. Put battery-side protection as close to the battery as practical.
- Terminations: Tight, clean, and strain-relieved connections prevent heat and hidden losses.
7) Field Notes: How I Validate a New Build
- Measure at the battery: I place a wattmeter between controller and battery and log charge current through a clear-sky window (10:00–14:00 local).
- Angle and shading passes: I re-aim panels 1–2 times during a long session and eliminate any cable shadows. This alone often yields the biggest gains.
- Cold and heat: In cold, MPPT harvest improves; in heat, expect lower panel voltage. I avoid charging Li-based batteries below the manufacturer’s minimum temp.
Putting It Together
Solar, controller, battery, and inverter must be matched. A large array is wasted on a small battery or a current-limited controller; a big inverter is pointless without the DC supply to feed it. With the audit (Wh), conversions (Ah), PSH-based array sizing, controller current and Voc checks, and basic wiring/protection rules, you’ll have a balanced 12 V system that actually performs on the road.
FAQs
How do I estimate my RV’s daily power usage?
List each device, multiply Watts × Hours for Wh, and sum. The total informs battery Ah (Wh ÷ 12) and array watts (Wh ÷ PSH × derate).
Why choose LiFePO4 over lead-acid?
Higher usable depth of discharge, lower weight, flatter voltage curve, and long cycle life. Still follow the BMS and temperature limits.
Do I always need MPPT?
No. If panel Vmp is close to battery voltage and conditions are stable, PWM can suffice. MPPT helps most when Vmp » battery voltage or conditions vary.
Disclaimer: Follow your battery BMS, controller, and inverter manuals. Electrical work must comply with local regulations and standards.
