Solar Battery Backup Runtime Calculator
Estimate how long your battery reserve may support essential household loads during a power outage. The calculator separates energy capacity in kWh from instantaneous power in kW, so the result is useful for early-stage backup planning without pretending to be a full electrical design.
Real battery presets
Start with a battery from the Home Solar Atlas catalog
Selecting a product updates usable capacity per battery only. Battery count, reserve, average critical load and efficiency remain independent screening inputs.
Save or share this outage scenarioThe link stores your battery, units, reserve, average load, peak load, runtime target and efficiency assumptions.
113.5 kWhinstalled usable storage
→22.7 kWh20% held in reserve
→32.4 kWhafter 90% efficiency
→41.6 hoursat 1.5 kW average load
Continuous-power screening
Can the selected battery support your peak outage load?
Runtime uses average kW. This separate check compares a higher simultaneous-load target with published continuous battery power.
Combined sizing recommendation
How many batteries meet both your runtime and power targets?
This screening takes the larger of the unit count required for delivered energy and the unit count required for continuous power.
This recommendation uses full modeled usable battery capacity for the runtime target. The backup-reserve percentage above is a separate operating-reserve scenario, not a requirement that the selected runtime target fit inside reserve-only energy.
Battery target comparison
Compare all catalog batteries against the same outage target
Every row uses 1.5 kW average load, 8 hours of target runtime, 5.0 kW peak load and 90% delivered-energy efficiency.
| Battery | Per-unit capacity | Continuous power | Units by runtime | Units by power | Minimum units | Use |
|---|---|---|---|---|---|---|
| Tesla Powerwall 3See Tesla current technical documentation | 13.5 kWh | 11.50 kW | 1 | 1 | 1 | |
| Enphase IQ Battery 5PSee current Enphase datasheet | 5.0 kWh | 3.84 kW | 3 | 2 | 3 | |
| FranklinWH aPower 2Lithium iron phosphate (LFP) | 15.0 kWh | 10.00 kW | 1 | 1 | 1 |
A lower unit count does not by itself mean a better battery. Compare installed cost, warranty, inverter compatibility, backup architecture, usable energy, continuous power and installer support before choosing equipment.
Load sensitivity
How long the same battery lasts at different household loads
Runtime changes almost inversely with average load. Use this table to see why reducing nonessential loads during an outage can matter as much as adding storage.
| Backup scenario | Average load | Reserve-only runtime | Full-battery runtime |
|---|---|---|---|
| Light essentialsFridge, networking, a few lights and electronics | 0.4 kW | 6.1 hours | 30.4 hours |
| Typical essentialsMore lighting, refrigeration and small household loads | 0.8 kW | 3.0 hours | 15.2 hours |
| Essentials + cycling HVACModerate average load with heating or cooling cycling | 1.8 kW | 1.4 hours | 6.8 hours |
| High household backupSeveral larger loads operating during the outage | 3.0 kW | 0.8 hours | 4.0 hours |
| Heavy simultaneous loadHigh average demand; verify inverter and battery kW limits | 5.0 kW | 0.5 hours | 2.4 hours |
Screening estimate only. Actual outage runtime depends on battery state of charge when the outage starts, inverter and battery power limits, temperature, battery age, HVAC duty cycle, motor-start loads, solar production during the outage, manufacturer reserve behavior and which circuits are backed up. The continuous-power check uses the first published continuous-kW value stored for the selected catalog battery and does not model short-duration surge capability.
How to use this calculator
Start with the usable capacity published for one battery and the number of units in the system. Set the reserve percentage you expect to keep for outages, then enter the average load of the circuits you actually want to keep running. A refrigerator, lights and networking equipment may average well under 1 kW, while HVAC, well pumps, electric cooking or other large loads can increase both average demand and short-term power requirements.
The basic screening relationship is delivered reserve energy divided by average critical load. For example, 5.4 kWh of delivered reserve supporting an average 1.5 kW load is roughly 3.6 hours. Real systems vary because loads cycle on and off, batteries may not begin the outage fully charged, and solar can recharge storage during daylight.
kWh runtime is not the same as kW capability
A battery may contain enough energy to run a home for many hours but still be unable to start or continuously supply a large appliance. Backup design needs both adequate stored energy and adequate continuous and surge power. Always compare your supported-load target with the manufacturer's inverter and battery power ratings.
What this screening does not model
This calculator does not model an hourly load profile, weather sequence, solar recharge during an outage, generator interaction, transfer equipment, battery degradation, cold-weather derating, motor-start surge, load-shedding controls or manufacturer-specific reserve logic. Use it to compare scenarios and understand scale, then verify final backup design with the battery and inverter specifications and a qualified installer.