How to Size an Off-Grid Solar System From Your Own Loads
A solar panel and battery calculator sizes an off-grid system by starting from the one number that matters most: how much energy your appliances actually use in a day. You list each load with its wattage and how many hours a day it runs, and the calculator adds up the daily energy in watt-hours. From that figure it works out how many watts of solar panels you need to replace that energy each day, how large a battery bank must be to carry you through the night or a cloudy spell, and what size charge controller and inverter the system needs. Off-grid homeowners, van and boat builders, and anyone planning a backup power system use it to buy the right amount of gear.
The panel array is sized as panelWatts = dailyWh ÷ (peakSunHours × systemLossFactor). Peak sun hours is the number of hours per day of full-strength sunshine your location effectively receives, and the loss factor, which defaults to 0.75, accounts for dirt, heat, wiring and controller losses that stop panels ever delivering their full rated output. The battery bank uses batteryAh = dailyWh × autonomyDays ÷ (systemVoltage × depthOfDischarge × inverterEfficiency). Depth of discharge is how far you safely drain the battery — around 0.5 for lead-acid and 0.8 to 0.9 for lithium — and autonomy days is how many days of no sun the bank must survive. The charge controller current is panelWatts ÷ systemVoltage × 1.25, where the 1.25 is a safety margin for bright conditions.
A worked example makes the chain clear. Say your appliances total 2000 watt-hours per day and your site gets 5 peak sun hours. With the default 0.75 loss factor the array needs 2000 ÷ (5 × 0.75) = 533 watts of panels. For one day of autonomy on a 12 volt system with lead-acid batteries at 0.5 depth of discharge and a 0.9 efficient inverter, the bank must be 2000 × 1 ÷ (12 × 0.5 × 0.9) = 370 amp-hours. A 600 watt array on 12 volts needs a charge controller rated at 600 ÷ 12 × 1.25 = 62.5 amps. Divide the array by your chosen panel wattage to get the panel count, and the bank by your chosen battery size to get how many batteries in the string.
The reason this tool exposes every assumption as an editable input is that hiding them is exactly why two solar calculators can disagree by 40 percent for the same house. Someone planning a lithium van build sets depth of discharge to 0.8 and autonomy to one day, and gets a far smaller battery than a cabin owner on lead-acid who sets 0.5 and three days of autonomy for winter storms. A designer in a sunny desert region enters 6 peak sun hours, while one in northern Europe enters 2.5, changing the panel count dramatically. Because the loss factor, depth of discharge, inverter efficiency and autonomy are all visible, you can see precisely why your system is the size it is and adjust each figure to your own hardware.
A few cautions keep the estimate honest. Peak sun hours vary by season, so for a year-round off-grid system you should use a winter figure rather than an annual average, or the battery will run flat on the shortest days. The model covers a stand-alone off-grid system, not a grid-tied one with net metering, and it does not separate MPPT from PWM controller efficiency or model panel-temperature derating beyond the lumped loss factor. The most common mistake is underestimating appliance run hours, which quietly undersizes everything downstream, so be generous and honest with the hours. Treat the result as a planning estimate and confirm the final design with a qualified solar installer under your local electrical code. All of this runs in your browser, so nothing you enter is uploaded.
