Off-Grid Solar System Size Calculator

Add up every load you run in a day: for each device multiply its power in watts by the hours per day it runs, then total the results. A 60 W fridge averaging 8 hours of compressor run time is about 480 Wh, a 15 W light for 5 hours is 75 Wh, and so on. The grand total in watt-hours per day is the number this calculator starts from. Build in a margin for cloudy days and future loads, and remember that nameplate watts are usually a maximum, not an average.

A solar system never delivers 100 percent of the panel rating to your loads. Panels run hotter than their lab rating, wiring and the charge controller lose a few percent, and round-tripping energy through the battery and inverter costs more. The 0.75 derate factor bundles these typical losses into one number, so the array comes out large enough to actually meet the load. If your run is short, the panels stay cool, and you use an efficient MPPT controller, real systems can do a little better, but designing to 0.75 keeps you off the edge.

Days of autonomy is how long the battery alone can carry your loads with no solar input, such as during a stretch of overcast weather. Two to three days is common for a weekend or RV setup, while a year-round off-grid home often targets three to five days. More autonomy means a bigger, more expensive battery bank, so it is a trade-off against having a backup generator. Lithium tolerates deeper, more frequent discharge than lead-acid, which is why the depth-of-discharge factor differs by chemistry.

No. The solar array wattage depends only on your daily energy and sun hours, not on whether you run a 12, 24 or 48 V bank. Voltage instead affects current: a higher-voltage system carries the same power at lower amperage, which means smaller wire, a smaller charge controller and lower losses. That is why larger systems move to 24 or 48 V. The battery amp-hours also scale inversely with voltage, since amp-hours times voltage is the energy that stays fixed.

No. The results are planning estimates from standard engineering values, not a stamped electrical design. Real installations must follow the National Electrical Code, manufacturer specifications and your local authority having jurisdiction, including correct overcurrent protection, grounding and conductor sizing. Use these numbers to scope a system and compare options, then have the final design and any wiring verified by a licensed electrician or qualified solar installer.