Battery backup scenario
Battery Backup for Extended Outages (12–24 Hours)
Longer backup usually means bigger batteries, fewer loads, or accepting partial comfort — not a single default battery size for every home.
Updated 2026-07-03
Labeled planning assumptions
- Outage target: 12–24 hours — often requires larger usable kWh, load management, or partial HVAC strategy.
- Critical load average: 1.5–3.0 kW if you include partial HVAC, a freezer, or a small well pump — verify surge separately.
- Usable capacity: extended scenarios usually need multiple batteries or a very large single unit; nameplate kWh is not runtime.
- Solar recharge during outage: depends on backup inverter mode, sunshine, and whether loads exceed daytime production.
- Multi-day storms: this scenario does not model consecutive cloudy days, generator backup, or utility restoration timing.
Questions for installer quotes
- What usable kWh supports the listed backup hours at the assumed load kW?
- Which loads drop off first if capacity runs low?
- Can solar recharge the battery while backing up loads during the day?
- Is generator integration or load shedding part of the design?
Common planning mistakes
- Treating nameplate kWh as guaranteed runtime hours
- Adding 240V loads without inverter and transfer review
- Assuming whole-home backup from a single small battery
- Mixing resilience goals with simple payback expectations
SolarCalcNow provides planning estimates only. Results are not installer quotes, engineering designs, tax advice, financing recommendations, or utility interconnection approval.
FAQ
Common solar calculator questions
Does a 13.5 kWh battery automatically cover 24 hours?
No. Runtime depends on backed-up load kW, usable capacity, surge events, and whether solar can recharge during the day. A labeled scenario is not a guarantee.
Should I include HVAC in a 12–24 hour plan?
Only after voltage, surge, and transfer equipment review. HVAC often drives the largest load and may require 240V support or a separate critical-load strategy.
How does this relate to payback?
Extended backup can increase battery cost without improving simple bill-savings payback. Treat resilience value separately from annual savings math.
What to do next
Use the battery quote checklist and backup voltage guide before accepting runtime claims on a proposal.