Solar Panel System Sizing: A Complete Guide for Residential & RV Installations (2026)
Mar 21, 2026
Sizing a solar system correctly is the difference between energy independence and an expensive disappointment. Undersized systems cannot keep up with demand; oversized systems waste money on unused capacity. This guide walks through the complete sizing process for residential rooftop and RV/off-grid installations.
Step 1: Calculate Your Daily Load
List every device that will run on solar power with its wattage and daily hours of use. Multiply watts by hours to get watt-hours per day (Wh/day). Add 20% for system losses (inverter efficiency, wiring losses, battery charge/discharge losses).
| Device | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lights (10) | 100 | 5 | 500 |
| Refrigerator | 150 | 8 | 1,200 |
| Laptop | 60 | 6 | 360 |
| TV | 80 | 4 | 320 |
| Phone charging (4) | 40 | 3 | 120 |
| Subtotal | 2,500 | ||
| + 20% losses | 3,000 Wh/day |
Step 2: Size the Solar Array
Divide your daily Wh requirement by the peak sun hours for your location. For most of the continental US, this ranges from 4 to 6 hours. Using 5 peak sun hours: 3,000 Wh / 5 hours = 600W of solar panels. Round up to the nearest standard panel size. A system of two 320W panels (640W total) would cover this load with margin.
Step 3: Size the Battery Bank
For off-grid systems, size the battery bank for 2 days of autonomy (no sun). 3,000 Wh x 2 = 6,000 Wh storage needed. For LiFePO4 batteries at 80% depth of discharge: 6,000 / 0.80 = 7,500 Wh = a 48V 156Ah bank or a 12V 625Ah bank. In practice, two 12V 200Ah LiFePO4 batteries in series (24V) with a 24V inverter is a common and cost-effective configuration.
Step 4: Select the Charge Controller
MPPT (Maximum Power Point Tracking) controllers are 15-30% more efficient than PWM and are recommended for all systems over 200W. Size the controller for the array's maximum voltage (Voc in cold conditions) and maximum current (Isc). For a 640W array at 24V nominal: 640W / 24V = 26.7A minimum controller rating. A 30A or 40A MPPT controller provides adequate headroom.
RV-Specific Considerations
RV roof space limits panel size. Typical RV setups use 2-4 panels of 100-200W each. Mount panels with at least 2 inches of clearance for airflow to prevent heat-related efficiency loss. Use flexible panels only on curved surfaces; rigid panels deliver better long-term performance. All wiring must comply with NEC Article 551 for RVs and ABYC E-11 for marine vessels.
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