How to Size a Solar Panel System for Your Home or Van

Why Solar Sizing Matters

An undersized solar system leaves you short on power when you need it most. An oversized system wastes money on panels and batteries you'll never use. Getting the math right saves you hundreds or thousands of dollars and ensures your system actually works the way you expect it to.

Whether you're powering a home, a van build, or an off-grid cabin, the sizing process follows the same fundamental steps. The numbers change, but the logic doesn't.

Step 1: Calculate Your Daily Energy Usage

Everything starts here. You need to know how many watt-hours (Wh) you consume per day.

For a home, the easiest method is checking your electric bill. Find your monthly kWh usage, divide by 30, and you have your daily number. A typical US home uses about 30 kWh per day, but this varies enormously based on climate, home size, and efficiency.

For a van or RV, list every device you plan to run. Multiply each device's wattage by the hours per day you'll use it. A 45-watt fridge running 12 hours is 540 Wh. A 65-watt laptop charging for 4 hours is 260 Wh. Add them all up for your total daily demand.

Step 2: Account for System Losses

Solar panels, charge controllers, inverters, and wiring all lose energy in the conversion process. A typical system loses 15-25% of its theoretical output to these inefficiencies. Use a loss factor of 0.80 (20% loss) for conservative estimates.

This means if you need 1,000 Wh per day, your panels actually need to produce 1,250 Wh to deliver 1,000 Wh to your appliances.

Step 3: Factor in Your Sun Hours

Peak sun hours represent how many hours per day your location receives the equivalent of full-intensity sunlight. This isn't the same as daylight hours. A location might have 14 hours of daylight but only 5 peak sun hours because morning, evening, and cloudy periods produce less than full intensity.

Arizona gets 6.5-7.5 peak sun hours. Arkansas gets 4-5. The Pacific Northwest gets 3-4. This single number has the biggest impact on how many panels you need.

Step 4: Size Your Panels

Divide your loss-adjusted daily Wh by your peak sun hours. The result is the total panel wattage you need.

Example: 1,250 Wh needed divided by 5 sun hours = 250 watts of panels. Two 200W panels (400W total) would give you comfortable headroom.

For home systems, panels typically come in 400W sizes. For vans and RVs, 100W and 200W panels are standard due to roof space constraints.

Step 5: Size Your Battery Bank

Batteries store energy for use when the sun isn't shining. Size them based on how many days of autonomy you want (ability to run without sun).

For LiFePO4 batteries (recommended), you can safely use 80% of rated capacity. For AGM lead-acid, only 50%. This means a 100Ah LiFePO4 battery at 12V gives you 960 usable Wh, while a 100Ah AGM gives you only 600 Wh.

For 2 days of autonomy with 1,000 Wh daily usage on LiFePO4: 2,000 Wh / 0.80 = 2,500 Wh needed. At 12V, that's about 208 Ah, so a 200Ah battery bank is close and a 300Ah bank gives comfortable margin.

Step 6: Size Your Charge Controller and Inverter

The charge controller must handle the amperage from your panels. Divide total panel wattage by battery bank voltage, then add 25% headroom. For 400W of panels on a 12V system: 400/12 = 33A, times 1.25 = 41A. A 40A or 50A MPPT controller works.

The inverter must handle your largest single load plus some margin. If your biggest appliance is a 1,000W microwave, a 1,500W inverter gives you safe headroom.

Common Mistakes

The most common sizing mistake is using lab-rated panel output instead of real-world output. A 400W panel produces 400W only under perfect test conditions. Real-world output is typically 75-85% of rated capacity due to temperature, angle, shading, and dirt.

The second most common mistake is undersizing batteries. Running batteries below their recommended depth of discharge dramatically shortens their lifespan. A $1,000 LiFePO4 battery used at 50% depth of discharge will last 5,000+ cycles. The same battery used at 100% depth might last only 2,000 cycles.

Third, people forget about winter. If you're in a location with significant seasonal variation, size your system for the worst month (usually December or January), not the annual average.