Whole-Home vs Essential-Loads Backup: How Many Batteries?
Short answer: Scope first, brand second. Essential loads: 1 example unit in the worked example below. Whole home: 2 units — and large loads may still need load management. That one choice moves the price more than anything else on this page.
Planning estimate only — not a quote, not tax advice, not installation advice. Get a site-specific quote from a licensed installer, check your utility's rules, and speak to a tax professional before buying.
Forget brand for a minute. The first sizing decision — the one that moves the price most — is simpler and less glamorous: scope. Do you want the battery to run the whole house during an outage, or only the loads you genuinely cannot lose? Answer that, and the size almost answers itself.
| Scope | What it covers | What it costs in size |
|---|---|---|
| Essential loads | Fridge, lights, internet/router, phones, medical loads, and a few chosen circuits | Smaller — often 1 example unit in the worked example below |
| Whole home | Everything, including large loads (air conditioning, electric cooking, laundry, EV charging if included) | Larger — often 2+ example units, and large loads may still need load management |
The same 12 hours, two different answers
Whole home: 30 kWh/day × 12 hours ÷ 24 = 15.0 kWh → 15.0 ÷ 13.5 = 1.11 → 2 example units (27.0 kWh nameplate).
Essential loads: 12 kWh/day × 12 hours ÷ 24 = 6.0 kWh → 6.0 ÷ 13.5 = 0.44 → 1 example unit.
Try your own numbers in the size calculator. The formula leaves out usable capacity, inverter limits, surge/startup loads, solar recharging during an outage, degradation, and panel/utility rules — confirm all of those with a licensed installer and the spec sheet.
Choose in this order
- List the loads you truly need during an outage, and their daily kWh — from your bill and appliance ratings, not from a guess.
- Price both scopes with the cost calculator (roughly $12,000–$16,000 per 13.5 kWh unit in the research range, before any confirmed programme payment).
- Ask the installer, in writing: what happens when a large load starts, what is backed up if the battery runs low, and what panel work is needed for each scope.
Frequently asked questions
Can one battery run a whole house?
Sometimes for a short outage with small loads, but not generally for a full day of whole-home use including large loads. Run the size calculator for your actual daily use and outage hours, then ask an installer about inverter limits and load management.
Is essential-loads backup just a smaller battery?
Mostly yes in size, but also in panel work: essential-loads backup usually needs a critical-loads panel. That panel work is part of the quote — ask for it itemised.
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