How this tool works
The recommendation engine multiplies each load's typical wattage by hours-per-day used during an outage, then adds a 25% safety margin and an off-grid days multiplier. The result is your daily kWh need. We size:
- Battery kWh capacity: daily kWh × backup-duration multiplier (1 day for “essentials”, 2–3 for multi-day, 5–7 for off-grid).
- Continuous kW output: sum of all selected loads' continuous wattage.
- Surge kW: largest motor-start surge from selected loads (well pump, AC compressor, sump pump).
- Chemistry: LFP recommended for all 2026 residential applications (safer, longer cycle life). NMC is older.
- Climate adjustment: winter-snow regions get a +20% size bump because solar can't recharge fast during snow cover.
What the tool can't tell you
- Whether your electrical panel can support whole-home backup — that requires an electrician and the inverter's pass-through current rating (typically 200A for whole-home).
- Whether your specific utility allows certain battery dispatch programs (e.g. ConnectedSolutions in MA/CT/RI, Tesla VPP in TX/CA, Xcel Battery Bonus in MN).
- Whether your specific home loads have unusually high inductive surge (older AC compressors, deep-well pumps in hardpan soil, etc).
This tool gives a starting recommendation. Confirm with your installer's site walk before signing a contract.
Frequently asked questions
Why does the tool ask about snow cover?
Battery + solar systems "recharge" during the day from solar production. If snow covers your panels for 30 consecutive days in January, your battery has to last that whole stretch on stored capacity alone (minus whatever the grid provides if you're not in an outage). Snow-prone regions need 20-40% more capacity than equivalent loads in mild climates.
What's "surge" and why does it matter?
Motor-driven appliances (AC compressors, well pumps, sump pumps, refrigerator compressors) draw 3-7x their running wattage for ~10 seconds during startup. If your battery / inverter can't handle that surge, the appliance won't start. Tesla Powerwall 3, Enphase IQ Battery 10C, and Sol-Ark hybrids all handle most residential surges; older Powerwall 2 + IQ Battery 5P cannot start a 3-ton AC.
Does this assume I have solar?
The tool works with or without solar. If you don't have solar, the battery is sized for stand-alone storage capacity only (no daytime recharge). If you do have solar, the tool factors in expected daytime recharge to recommend a smaller, more cost-effective battery.