Understanding your panel count
The core formula behind this calculator
Total panel wattage needed equals daily watt-hour load divided by the product of peak sun hours and your system loss factor. A system using 2,000Wh a day, with 5 peak sun hours and a typical 80% loss factor, needs 500 watts of panel. Using 200-watt panels, that rounds up to three panels for 600 watts installed, giving you working margin above the bare minimum. Once you have that number, the panel lifespan estimator shows what it means for the rest of the system. Remote systems are easy to neglect. The annual maintenance cost estimator in our Maintenance & Lifespan section can help you budget for the maintenance that keeps output from slipping.
Why winter sizing changes everything for U.S. and Canadian sites
Peak sun hours swing dramatically by season and latitude. A site in Arizona might average 6.5 hours in summer and 4.5 in winter, while a site in Minnesota or Ontario can see winter averages drop to 2.5 to 3 hours. Sizing for the annual average risks a real shortfall every winter; sizing for the worst month means summer production goes mostly unused but the system never comes up short. Most full-time off-grid builds size for the worst month and treat summer surplus as a bonus, not a plan. The DOE's solar photovoltaic technology basics publishes the current rules, so confirm the details there before you file anything. Even off-grid, the money side matters. The payback period calculator in our Savings & Payback section can help you compare the system cost with the cost of extending the grid.
System losses are not optional to ignore
A panel's rated wattage is measured under lab conditions that rarely match real-world installs. Wiring resistance, MPPT or PWM controller conversion loss, imperfect panel angle relative to the sun, and dust or pollen buildup routinely cut 15% to 25% off a panel's rated output. Skipping this derate is the single most common reason a DIY off-grid system produces less than expected. The MPPT vs PWM comparison covers how controller choice affects this specific loss.
Bigger panels aren't always the answer
Higher-wattage panels reduce your total panel count but increase weight and footprint per panel, which matters for roof-mounted systems with limited space. A build with tight roof space, like an RV or van, often benefits from fewer, higher-wattage panels, while a ground-mount cabin array has more flexibility to use whichever wattage is cheapest per watt. Production estimates throughout draw on NREL's PVWatts solar production calculator.
What to check once you have a panel count
Once you know your panel count and total wattage, run the charge controller sizing tool to match a controller to your array's voltage and current, and the battery bank sizing calculator to size storage that pairs with it. If you started from a target kW figure rather than a daily watt-hour load, the kW to panel count converter in our system sizing tools gets there directly, and the panel lifespan estimator in our maintenance and lifespan tools projects how this array's output will hold up over its warranty period. DOE's solar photovoltaic technology basics is a solid starting point if any of the terminology above needs more background.