How to Size a Solar System for Your Needs

The most expensive solar system is the one that doesn't produce enough power. The second most expensive is the one that's massively oversized for what you actually need. Proper sizing eliminates both problems — and it starts with a simple exercise: listing what you want to power and doing the math.

This guide walks through the complete sizing process step by step, with real-world examples for the most common use cases: RV/van life, off-grid cabin, and home backup.

Why Sizing Matters

Every component in a solar system has a capacity limit. Panels can only produce so much, controllers can only process so much, batteries can only store so much, and inverters can only convert so much. The art of system design is matching all these limits to your actual energy consumption so nothing is wasted and nothing falls short.

Undersized systems mean running out of power on cloudy days. Oversized systems mean money wasted on capacity you never use. A well-sized system covers your daily needs with enough reserve for 1.5–2 cloudy days, and panel capacity that recharges your battery bank fully during a typical sunny day.

Step 1: Calculate Your Daily Loads

Grab a notebook and list every electrical device you plan to run. For each device, record its wattage (check the label or manual) and estimate how many hours per day you'll use it. Multiply watts × hours = watt-hours (Wh) per day for that device.

Step 2: Determine Your Sun Hours

"Peak sun hours" refers to the number of hours per day when sunlight intensity averages 1,000 W/m² — the standard test condition for panel ratings. This isn't the same as total daylight hours. A location with 12 hours of daylight might only have 5 peak sun hours because morning, evening, and cloudy periods produce lower intensity.

US average peak sun hours by region:

NREL's PVWatts Calculator is the gold standard for location-specific estimates — it factors in your exact coordinates, tilt angle, and historical weather data.

Step 3: Size Your Panels

Take your daily Wh total from Step 1, add a 25% buffer for system losses (inverter efficiency, wire losses, controller overhead, dust on panels), then divide by your peak sun hours.

Formula: (Daily Wh × 1.25) ÷ Peak Sun Hours = Required Panel Watts

Using our example: (1,668 × 1.25) ÷ 5 = 417W minimum. Rounding up and oversizing slightly for cloudy-day resilience, a 500W panel array is the right target. That's five 100W panels or two to three modern 200W panels.

Step 4: Size Your Battery Bank

Your battery bank needs to store enough energy to cover your loads when the sun isn't shining. The standard recommendation for off-grid systems is 1.5–2 days of autonomy.

Formula: Daily Wh × Days of Autonomy ÷ Battery Voltage ÷ Usable DoD = Required Ah

Using our example with LiFePO4 (90% DoD) and 2 days of autonomy at 12V:

1,668 Wh × 2 ÷ 12V ÷ 0.9 = 309 Ah

A 300Ah LiFePO4 bank (three 100Ah batteries in parallel) or two 200Ah batteries covers this.

With AGM at 50% DoD, the same calculation yields 556 Ah — nearly double the battery capacity needed, at greater weight and cost over time.

Step 5: Size Your Charge Controller

Your charge controller must handle the full output of your panel array. Two numbers matter:

Maximum PV Input Voltage: Add up the Voc (open circuit voltage) of all panels wired in series, then multiply by 1.15 to account for cold-weather voltage rise. This number must not exceed your controller's maximum PV voltage rating.

Maximum Charge Current: Total panel watts ÷ battery voltage = minimum controller amp rating. Add 25% headroom. For our 500W array on 12V: 500 ÷ 12 = 41.7A, so a 50A or 60A MPPT controller is appropriate. The Renogy Rover 40A would be at its limit; a Victron 100/50 would have comfortable headroom.

Step 6: Size Your Inverter

Your inverter must handle the peak simultaneous load of all AC devices running at the same time, plus startup surges from motors (fridges, pumps, power tools).

List the wattage of everything that could run at the same time, add them up, then add 20–25% for headroom. Most off-grid cabin builds land on a 2,000W–3,000W pure sine wave inverter. Larger homes may need 5,000W or more.

Don't oversize unnecessarily — larger inverters have higher idle power consumption (typically 15–40W just to stay on), which drains your battery when loads are light.

Real-World Sizing Examples

Weekend RV / Van Life

Loads: 12V fridge (600 Wh/day), LED lights (100 Wh), phone/laptop charging (200 Wh), water pump (50 Wh) = ~950 Wh/day

Panel array: 300W (three 100W panels)

Battery bank: 200Ah LiFePO4 at 12V (one or two batteries)

Charge controller: 30A MPPT (Victron 100/30 or Renogy Rover 30A)

Inverter: 1,000W pure sine wave (if running AC devices)

For the complete RV power picture — including shore power, alternator charging, and generator backup — visit RVGear.co.

Off-Grid Cabin (Full-Time)

Loads: 1,500–2,500 Wh/day (lighting, fridge, laptop, fans, water pump, small appliances)

Panel array: 800W–1,200W

Battery bank: 400Ah–600Ah LiFePO4 at 12V (or 200–300Ah at 24V)

Charge controller: 60A–80A MPPT (Victron 150/60 or similar)

Inverter: 3,000W pure sine wave

Emergency Home Backup

Loads: Fridge, lights, phone chargers, router, sump pump = ~1,500 Wh for a 24-hour outage

Solution: A portable solar generator (EcoFlow DELTA series, Bluetti AC200 series, or Jackery Explorer 2000 Plus) paired with 200W–400W of portable panels. These all-in-one units provide instant backup without permanent installation, and can be recharged from solar, wall power, or a vehicle.

These examples represent starting points — your actual loads may differ significantly. The sizing formulas above work for any scenario. Run the numbers with your real devices, verify with a tool like NREL's PVWatts, and build from there.