Off-Grid Solar & Battery Calculator
Add up what you run in a day and get the solar array, the battery storage, and the inverter an off-grid setup needs. A planning estimate from NREL and DOE methods, not an engineered system design. Size for your worst month, not the yearly average.
What you run in a day
Peak sun hours vary by place and season. Look up your worst-month value at NREL’s PVWatts and use that, since an off-grid system has to survive winter.
How it is sized
Add up the watt-hours you use in a day. The array is that daily energy divided by your peak sun hours and an off-grid loss factor of about seventy-five percent, lower than a grid-tied system’s because batteries and a charge controller lose energy too. Battery storage is your daily use times the days of backup you want, divided by how deeply the battery can be drawn down, about half for lead-acid and eighty percent for lithium. The inverter must cover everything that might run at once, with headroom for the surge a motor draws when it starts.
Size for the worst month
A system sized to the yearly-average sun runs out of power in December. Off-grid designers size the array to the worst month and often add a generator for the darkest stretch. Appliance figures here are typical values; a fridge or freezer is best measured in watt-hours a day, not nameplate watts, because the compressor cycles on and off.
Sources
Array and loss method (the PVWatts model and its default losses): NREL. Peak-sun-hours and solar-resource data: NREL. Battery depth-of-discharge by chemistry: Battery University. Appliance energy method and figures: US DOE Energy Saver and the US EIA. Days of backup and the off-grid loss factor are engineering rules of thumb, not single published constants.