Solar Battery Bank Sizing Calculator

How We Calculate Your Battery Bank Size

Sizing a home battery bank comes down to four numbers: how much energy you use each day, how long you want the battery to carry you through an outage, how deeply the battery can be discharged without damage, and how much energy is lost to the inverter and wiring on the way to your appliances. This calculator turns those four inputs into a concrete battery count, a real-world runtime, and a cost adjusted for the incentives available where you live.

The core sizing math

First we convert your daily usage and desired backup hours into the energy the bank must deliver: backup energy = daily usage × (backup hours / 24). A 30 kWh/day household wanting 24 hours of backup needs to supply roughly 30 kWh from the bank. Next we reduce each battery's nameplate capacity by its depth-of-discharge limit (DoD) and your system efficiency — a 10 kWh LFP battery at 90% DoD and 90% efficiency delivers about 8.1 kWh of usable energy. We divide total backup energy by usable energy per unit and round up, because you cannot buy a fraction of a battery. The result is the minimum bank size that meets your target.

LFP vs NMC vs lead-acid vs Powerwall

Lithium iron phosphate (LFP) has become the default recommendation for residential storage because it is the safest lithium chemistry — it will not enter thermal runaway — and it tolerates 6,000+ charge cycles, giving it the longest service life of the options here. Nickel Manganese Cobalt (NMC) packs more energy into a smaller footprint and is lighter, but it carries a shorter cycle life and a higher fire risk. Flooded lead-acid is the cheapest upfront but only tolerates 50% discharge, so a lead-acid bank typically needs roughly twice as many units to deliver the same usable energy — and it requires regular maintenance. The Tesla Powerwall 3 is an integrated LFP unit with its own inverter, a 10-year warranty, and a larger 13.5 kWh capacity per box, which is why it shows up as a single-product option rather than a DIY chemistry.

How incentives reduce your net cost

The federal Investment Tax Credit returns 30% of installed battery storage cost as a non-refundable credit, available for systems placed in service through 2032 as long as the bank is charged by solar at least once during the year. On top of that, a handful of states run their own storage programs — California's SGIP, Maryland's Residential Clean Energy Rebate, Massachusetts' ConnectedSolutions, Oregon's Energy Trust, Nevada's NV Energy rebate, and New York's NYSERDA Retail Energy Storage program among them. This calculator applies both layers and never lets the net cost fall below zero. A state with no storage program still qualifies for the full 30% federal credit, which is why the math works even where the state line shows $0.

What this estimate cannot tell you

The figures here assume a flat average daily load and continuous discharge during an outage, which is rarely how a real blackout unfolds. Your actual runtime depends on which loads you back up (a well pump or electric heater dramatically shortens runtime), surge currents from motors, ambient temperature, and battery age. A professional installer will run a proper load audit, size the inverter to your largest surge, and confirm that the bank is configured correctly for your panel. Treat the output of this tool as a planning baseline — a strong starting number for the conversation — rather than a final install spec.