Battery Bank Sizing Calculator

Depth of discharge (DOD) is the fraction of a battery's nameplate capacity you're allowed to safely use before recharging. If you limit yourself to only 50% DOD instead of 80%, each battery contributes only 50% of its rated capacity toward your actual usable energy, so you need roughly 60% more nameplate capacity -- and therefore more batteries -- to deliver the same usable energy. This is a genuine trade-off: a lower DOD extends battery cycle life (especially for lead-acid chemistries) at the cost of needing a larger, more expensive bank.

Lead-acid batteries (flooded and VRLA/AGM alike) suffer accelerated degradation from deep, repeated discharge cycles, which is why IEEE 1115 and IEEE 1561 generally recommend limiting routine discharge to around 50% for meaningfully longer service life, even though these batteries can technically discharge deeper occasionally. LiFePO4 (lithium iron phosphate) cells tolerate much deeper routine discharge, commonly 80-90% per manufacturer specification, without the same accelerated wear -- which is a major reason LiFePO4 banks can be smaller and lighter than a lead-acid bank sized for the same usable energy.

Both the series count (system voltage divided by battery voltage) and the parallel string count (required capacity divided by energy per string) are rounded UP to the next whole battery or whole string, since you can't install a fractional battery. This rounding means the actual installed bank almost always delivers somewhat more usable capacity than your bare minimum requirement -- which is a reasonable safety margin in practice, not a sizing error.

No -- it only estimates battery count and usable energy capacity. A real battery bank installation also requires correctly rated interconnecting cables, overcurrent protection (fuses or breakers) sized to the battery chemistry's short-circuit current capability, disconnects, and often a battery management system, none of which this calculator addresses. Battery banks carry genuine fire and thermal-runaway risk if these protections are undersized or omitted -- always follow the battery manufacturer's installation instructions and applicable electrical code, and have the actual system design and wiring reviewed by a qualified installer.

Days of autonomy is a direct multiplier on total energy required -- two days of autonomy means the bank must store roughly twice the energy that one day requires, before depth of discharge is even factored in. Because this multiplies straight through to required capacity and therefore to battery count, autonomy is one of the most consequential inputs in this calculator: even a modest increase from 1 to 3 days of autonomy roughly triples the size (and cost) of the required battery bank.