Solar System Sizing Guide
Calculate your daily watt-hour solar budget to size panels, batteries, and inverters.
The Four Steps of Solar Sizing
Designing a solar power system, whether for an RV, an off-grid cabin, or a grid-tied home, involves four main steps: load calculation, battery sizing, solar array sizing, and inverter selection.
1. Load Calculation (Watt-Hours)
First, determine how much energy you need per day. List every electrical device, its wattage, and how many hours it runs daily.
Sum the daily energy for all devices. Add a 20% safety margin for system losses.
2. Battery Bank Sizing
Batteries store your solar energy. You need enough capacity to run your loads at night and during cloudy days (days of autonomy). Battery capacity is often measured in Amp-Hours (Ah).
Battery Capacity (Ah) = Battery Capacity (Wh) ÷ Battery Voltage (e.g., 12V)
3. Solar Array Sizing
Your solar panels need to replenish your daily energy usage during the available peak sun hours. Peak sun hours vary by location and season (e.g., 4-6 hours).
Add 20-30% to account for panel inefficiencies, heat, and charge controller losses. Divide the total array watts by the wattage of individual panels to find the number of panels needed.
4. Inverter Sizing
The inverter converts DC battery power to AC household power. The inverter must handle the simultaneous continuous wattage of all devices running at once, plus any surge watts from motors (like refrigerators or pumps).
Worked Example: Off-Grid Cabin
Loads: LED lights (50W x 4h = 200Wh), Laptop (60W x 3h = 180Wh), Small Fridge (100W x 10h = 1000Wh). Total = 1,380Wh.
Total + Margin (20%): 1,656 Wh / day.
Batteries (2 days autonomy, 12V Lithium): 1,656 Wh × 2 = 3,312 Wh. 3,312 Wh ÷ 12V = 276 Ah.
Solar Array (5 peak sun hours): 1,656 Wh ÷ 5h = 331 W. Add 30% inefficiency = 430 Watts. (e.g., two 250W panels).
Inverter: Peak simultaneous load is 50+60+100 = 210W continuous. Fridge surge might be 600W. A 1000W inverter is sufficient.