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Whole-home or critical-loads battery backup?
Updated August 7, 2026 · using typical appliance wattages and 2026 residential battery ratings · by Cali Energy
Whole-home or critical-loads backup? Add up the appliances you want to run in an outage. A fridge, lights and internet draw well under 4 kW — a compact battery handles them. Add central AC or a well pump and the surge pushes you toward a high-power battery (like an 11.5 kW Powerwall 3) or two. It’s a power question first, energy second.
✓ Sizes the power your loads need against real 2026 battery classes. See every figure it uses ↓
What do you want to keep on during an outage?
Method: peak = total continuous watts + the largest single motor’s starting surge (motors pull roughly 2–3× running watts to start). Appliance watts are typical values — your exact draw varies by model and efficiency. Battery specs are manufacturer-reported. The runtime figure is an idealized full-load estimate, not a guarantee. A load calculation by a licensed electrician confirms the real design.
The rates and figures this calculator usestap to verify
The tool sums the running and surge watts of the loads you tick, then compares them to typical residential battery power classes. The appliance figures are typical estimates; the battery classes reflect 2026 products.
| Compact | ~3.8 kW continuous / 7.6 kW peak |
| High-power | ~10–11.5 kW continuous (Powerwall 3 = 11.5 kW) |
| Beyond that | two batteries or a load-managing smart panel |
| Running vs surge | each load carries a run watt and a startup surge |
| Usable energy | nameplate kWh × ~0.90 |
| Continuous power | sum of running watts |
| Peak power | continuous + the largest single surge |
| Runtime energy | continuous kW × hours vs usable kWh |
The appliance running and surge watts are typical estimates for common household equipment — your exact models vary, especially motor-driven loads (AC, well pump, pool pump) whose startup surge can be several times the running draw. The battery power classes reflect 2026 residential products (a Tesla Powerwall 3 delivers 13.5 kWh at 11.5 kW continuous). Usable energy is estimated at ~90% of nameplate after reserve and losses. This is an idealized full-load estimate; real runtime shifts with load cycling, inverter limits, reserve settings, temperature, battery age and any daytime solar recharge.
Example results you can reproduce
Tick the same loads above to reproduce these — the class depends on continuous and surge power.
| Loads kept on | Roughly needs |
|---|---|
| Fridge + lights + internet | A compact battery (< ~3.8 kW continuous) |
| + Central AC or a well pump | A high-power battery — the motor surge dominates |
| Whole home incl. EV charging | Two batteries or a load-managing smart panel |
How this calculator works
It adds the running watts of every load you tick to get continuous power, then adds the single largest startup surge on top for peak power — because motors don’t all start at once, but one starting while the rest run is the realistic worst case. It compares both to typical battery classes. If you enter a battery capacity and target hours, it also checks the energy side: continuous kW × hours against ~90% of the battery’s usable kWh. Power first, energy second.
How to read your result
Two numbers matter: continuous kW (can the battery run everything at once?) and peak kW (can it handle the biggest surge?). A set can fit on energy but fail on power — a well pump that runs 30 seconds an hour barely moves the kWh but can spike several kilowatts starting. That’s why the tool leads with power and treats the runtime energy as a secondary check.
What moves the result the most
Motor surge — central AC, well pumps and pool pumps — is by far the biggest driver, often deciding compact vs high-power on its own. After that, the number of loads you keep on (continuous power) and, for runtime, the hours you want and the battery’s usable capacity. EV charging is a special case: at 7–11 kW it can exceed a single battery’s output by itself.
Common questions
What’s the difference between whole-home and critical-loads backup?
Critical-loads backup keeps a few essential circuits alive (fridge, lights, internet) on a compact battery; whole-home backup runs everything, which needs far more power — often a high-power battery or two plus a smart panel.
Why does surge matter so much?
Motors draw several times their running watts for a split second when they start. A battery that easily runs a load can still be unable to start it, so peak (surge) power, not just continuous, sets the requirement.
What’s a compact vs high-power battery?
Compact units deliver roughly 3.8 kW continuous; high-power units like the Powerwall 3 deliver about 11.5 kW, enough to start and run larger loads including many central ACs.
Can one battery back up a whole home?
Sometimes — a high-power battery can run a modest all-electric home, but big or simultaneous motor loads (AC + well pump + EV) usually need two batteries or a smart panel that sheds loads.
How long will the battery last in an outage?
That’s the energy side. Enter your battery capacity and target hours here for a rough check, or size the kWh from scratch with our battery sizing calculator.
Does solar help during an outage?
Yes — a solar-plus-battery system recharges by day, so you can run more loads for longer than the battery alone would allow. A battery-only setup is limited to its stored energy.
Sources
- EnergySage — Critical load panels: what to know
- Tesla — Powerwall 3 datasheet (11.5 kW, 185 LRA)
- Enphase — IQ Battery 5P datasheet (3.84 kW continuous, 7.68 kW peak)
- FranklinWH — aPower 2 datasheet (10 kW continuous, 15 kW peak)
- SPAN — Smart electrical panel & backup load management
- HVAC Base — Appliance wattage & generator sizing chart (running vs starting watts)
Estimates only, not a quote. Assumptions, rates, incentives and program rules can change — confirm current details with your utility, the program administrator or a licensed professional.
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