Battery backup scenario
Battery Backup for Essential Loads (4–8 Hours)
This page models a short outage with essential loads only. It is a planning label, not a promise of runtime or automatic payback improvement.
Content updated 2026-11-02; source checks retain their individual dates.
Authored examples · updated November 2, 2026 · not measured households
Power, energy and surge answer different questions
This example starts with 10 kWh of usable battery capacity, keeps a 10% reserve and assumes 90% inverter efficiency: 10 × 0.9 × 0.9 = 8.1 kWh delivered to loads. These are editable planning assumptions, not a battery specification. Confirm whether a quoted usable capacity is DC or already delivered AC before applying an efficiency factor.
| AC running load | Energy for 8 hours | Static runtime |
|---|---|---|
| 0.5 kW | 4.0 kWh | 16.2 hours |
| 0.75 kW | 6.0 kWh | 10.8 hours |
| 1.2 kW | 9.6 kWh | 6.8 hours |
A refrigerator cycles; its nameplate watts are not its daily average. Motor starting power can exceed running power. Adequate kWh does not establish that an inverter can start a pump, supply the required voltage or power every selected circuit. This worksheet does not size surge or protection equipment.
Enter your own essential loadsIn the worksheet, open the constant-load examples to load these exact inputs, then compare 4, 8, 12 and 24 hours. If the quoted capacity is already delivered AC energy, set efficiency to 100% rather than deducting the same inverter loss twice.
Download all 12 example calculations (CSV)Complete inputs, formulas and limits (JSON)
Labeled planning assumptions
- Critical load average: about 0.6–1.2 kW for fridge, LED lights, router, and device charging — not HVAC or well pumps.
- Outage target: 4–8 hours of backup for essential loads only.
- Usable battery capacity: plan with 80–90% of nameplate kWh unless the quote states a lower usable rating.
- No surge modeling: motor startup (garage door, sump pump) can exceed average load and shorten runtime.
- Daytime solar recharge during an outage depends on inverter backup design and weather — not guaranteed here.
Example load list
- Refrigerator (120V, cycling compressor)
- LED lighting in main living areas
- Internet modem and router
- Phone and device charging
Usually excluded in this scenario
- Central HVAC or heat strips (240V, high draw)
- Well pump or sump pump without surge review
- Electric dryer, oven, or EV charging
- Whole-home backup without transfer equipment review
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
Is 4–8 hours enough for most outages?
It can cover many brief grid events if you limit backed-up loads to essentials. Longer outages or heat waves usually need a larger battery, load shedding, or generator backup.
Can I use this instead of an installer load study?
No. This is a labeled planning scenario. An installer or electrician should confirm circuits, surge, transfer equipment, and code before you buy.
Which loads belong in an essential-load scenario?
Common starters: refrigerator, lights, internet modem/router, phone charging, and maybe a garage door opener. Add HVAC, well pumps, or cooking loads only after voltage and surge review.
What to do next
Compare battery and no-battery quotes with the same solar design, then verify backed-up loads and voltage before signing.