Battery Sizing Calculator
≈ 27 kWh of storage
Estimate based on typical household loads as of 2026, your utility data gives the exact answer. State and utility programs, where available, apply to the installed cost.
How battery sizing actually works
Home battery sizing is two questions, not one: how much energy you need per day (capacity, in kWh), and how much power you need at once (output, in kW). A Tesla Powerwall stores 13.5 kWh and delivers 11.5 kW, so the calculator translates your backup goals into a number of units.
Essentials-only backup (refrigerator, lights, internet) idles around 10 kWh/day. Most-of-home living runs nearer 18. Whole-home with air conditioning is the big ask, cooling alone can add 12 kWh/day, and its starting surge is why whole-home configurations start at two units.
The solar multiplier
Solar changes the math more than any battery count: an array refills the bank every morning, so even a modest system turns a one-day battery into an outage outlaster. If extended outages are your concern, pairing solar usually beats buying a third Powerwall. See how many Powerwalls do I need for the full reasoning.
From estimate to exact
This tool uses typical household budgets; your utility's interval data gives the precise answer. During a free assessment we pull your actual usage, check your panel, and size the system to your home, not a template. State and utility storage rebates still apply; see Powerwall rebates.
Translating the result into hardware
The calculator's unit count assumes Powerwall-class hardware (13.5kWh, 11.5kW output). Translating elsewhere: modular systems (Enphase and peers) reach the same kWh in smaller increments, useful when your number lands between whole Powerwalls. Output matters as much as storage: verify any alternative delivers the kW your surge loads demand, since capacity without output is a battery that stores energy it can't serve fast enough.
The count also assumes healthy placement, shaded or conditioned space, short wire runs to the panel. A hot-garage install in Phoenix quietly costs capacity over years; siting is part of sizing.
The two mistakes this calculator prevents
Under-buying happens when sizing ignores surge loads: a system that stores plenty but stalls starting the well pump serves poorly at the worst moment. Over-buying happens when whole-home ambitions ignore load priorities: ranking circuits (refrigeration always, EV charging paused) routinely shrinks a three-unit design to two with nearly identical outage experience.
Both errors trace to sizing from square footage instead of behavior. Your utility's interval data, which the free assessment pulls, replaces assumptions with your household's actual electrical fingerprint.
Sizing for tomorrow without buying it today
Battery needs grow, an EV arrives, a heat pump replaces gas, work-from-home raises the daytime stakes. The expansion-ready design answers growth without upfront overbuying: gateway and wiring sized for a future unit, placement that leaves the second battery's wall space, conduit stubbed for the run.
That preparation costs almost nothing at first install and converts the future addition into a mounting-and-commissioning visit instead of a redesign. Size for today's verified loads; engineer for the household you're becoming, the calculator handles the first, the assessment the second.
Common sizing questions, answered by the data
Does square footage predict battery needs? Poorly, behavior dominates. Two identical floor plans differ threefold in consumption depending on occupancy, appliances, and habits. Does climate? Strongly, through cooling and heating loads. Does an EV? Only if you plan to charge during outages, most owners pause charging and shrink their requirement by a third.
The pattern across every question: utility interval data answers what assumptions guess. Fifteen minutes of authorization during the assessment replaces the entire rule-of-thumb genre. Until then, treat this calculator's output as your planning envelope: firm enough to budget against, flexible enough that the data-driven final design lands within it rather than beside it.
Frequently asked questions
One for essentials, two for most homes including limited A/C, three for whole-home backup with cooling and EV charging. Your consumption data, which we review in a free assessment, gives the exact number.
Yes, the A/C option adds roughly 12 kWh/day of cooling load and enforces the output floor A/C demands. Central air is the single biggest variable in backup sizing.
For long outages, no, batteries alone only buy hours, while solar refills the bank daily. If outage duration worries you, solar-plus-storage beats stacking batteries.
Systems balance across units automatically, so the practical difference is output ceilings and redundancy, two units keep partial backup if one ever faults. Beyond that, buy the kWh and kW your loads need in whatever increments price best.
The app makes it a slider: a common setting reserves 20–30% for backup while the rest arbitrages rates daily. Storm-watch features auto-fill ahead of forecasted weather, set it once and forget it.
Yes, a heat pump can dominate winter consumption, and backing it up meaningfully raises the kWh requirement. The design choice: full HVAC backup (more units) or essential-circuits-plus-space-heating-strategy (fewer). Your climate and tolerance decide; the assessment models both.