Battery Size Calculator

Battery sizing begins with energy, not merely the wattage printed on an appliance. Power describes the rate at which a load uses energy, while watt-hours describe how much energy is consumed over time. A 100-watt load operating for five hours needs 500 watt-hours at the load. A real battery must store more because wiring, inverters, charge controllers, and the battery itself lose some energy during conversion and delivery. This calculator divides load energy by the entered system efficiency so those expected losses are included. Depth of discharge is equally important. Most battery chemistries should not deliver every stored watt-hour during normal use. If an installation permits 80 percent depth of discharge, only four fifths of nominal capacity is treated as usable. The calculator therefore divides again by the usable discharge fraction. Combining both adjustments gives nominal watt-hours equal to power multiplied by runtime, divided by efficiency, then divided by depth of discharge. Nominal amp-hours are found by dividing those watt-hours by system voltage. Choose inputs that reflect the actual installation. Use continuous or average power for a steady load, but separately confirm that the battery and inverter can support short startup surges from motors, pumps, compressors, and power supplies. Efficiency should represent the whole delivery path rather than an optimistic component rating. Lead-acid systems often use a shallower routine discharge than lithium systems, while manufacturer limits, temperature, discharge rate, and desired cycle life can change the appropriate value. The result is an engineering estimate rather than a product recommendation. Available batteries come in standard sizes, so round up rather than down. For series-connected batteries, voltage increases while amp-hour capacity stays the same; for parallel strings, amp-hours increase while voltage stays the same. Also allow margin for battery aging, unusually cold conditions, load growth, and days when charging is unavailable. Critical backup, medical, marine, and off-grid systems should be reviewed against applicable standards and the battery manufacturer’s data. This battery capacity calculator is useful for solar storage, uninterruptible power supplies, camping systems, communications equipment, emergency lighting, and other direct-current or inverter-backed loads. By exposing every assumption, it makes alternatives easy to compare and provides a practical starting point for selecting a battery bank with enough usable energy.