Battery Sizing

Deep dive: Battery Sizing tools in practice

Battery sizing is the foundation of any reliable off-grid solar, RV, or marine power system. Undersize the bank and you run out of energy on cloudy days or during evening peaks. Oversize it and you pay for capacity you rarely use, plus extra weight, space, and charging time. The right size balances daily load, sun hours, depth of discharge, and inverter demand into a bank that survives real-world use—not just a spreadsheet best case.

Start by listing every load you expect to run, its wattage, and how many hours per day it operates. Convert those figures into daily watt-hours, then add a safety margin for inverter losses, battery aging, and seasonal shortfall. In off-grid solar, plan for the worst sun month in your location, not the annual average. Pair that load profile with your charge source—solar array watts, generator hours, or shore power—and estimate how many amp-hours you need at your system voltage.

Depth of discharge matters as much as total capacity. Lead-acid batteries should rarely be drawn below 50% state of charge if you want long cycle life. Lithium iron phosphate (LiFePO₄) tolerates deeper daily cycling, but you still need headroom for cloudy stretches. Size the bank so normal daily use lands around 70–80% state of charge after a typical solar day, leaving reserve for one or two poor generation days before you must shed loads or start a generator.

Inverter sizing runs in parallel with battery sizing. A bank that can supply enough watt-hours still fails if it cannot deliver peak surge for motors, compressors, or pump starts. Match inverter continuous and surge ratings to your largest simultaneous loads, then confirm the battery can supply the DC current those peaks require without excessive voltage sag.

WattQuick's battery sizing calculators help you move from rough estimates to defensible numbers in minutes. Use them to compare bank voltages, estimate inverter requirements, and sanity-check vendor quotes before you buy cells, BMS hardware, or racking. Consistent inputs across tools make it easier to iterate as your load list evolves.