The number of batteries you need is determined by two different things: how much power your home may demand at one time and how much energy you want available over the course of an outage. Square footage alone is not enough. A smaller all-electric home with central air, an electric water heater and an EV charger can be harder to back up than a larger home with mostly gas appliances.
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Start With the Loads, Not the Battery Brand
List the circuits you want available during an outage and sort them into three groups:
- Must have: refrigerator, internet, medical/accessibility equipment where applicable, security, selected lighting and outlets.
- Comfort: furnace blower, garage door, microwave, selected cooking, one air-conditioning system or heat pump.
- High-demand / optional during outage: EV charging, electric water heating, pool/spa equipment, multiple HVAC systems, electric resistance heat and large cooking loads.
The Simple Runtime Formula
A useful first-pass calculation is: theoretical runtime ≈ usable battery energy (kWh) ÷ average backed-up load (kW). This is only an estimate because real loads cycle on and off, batteries maintain reserve settings and the system has conversion losses.
| Average backed-up load | 10 kWh theoretical runtime | 15 kW theoretical runtime/th> |
|---|---|---|
| 0.5 kW | 20 hours | 30 hours |
| 1.0 kW | 10 hours | 15 hours |
| 2.0 kW | 5 hours | 7.5 hours |
| 4.0 kW | 2.5 hours | 3.75 hours |
These examples are theoretical and intentionally exclude reserve settings, conversion losses, battery power limits and solar recharge. They are for understanding the math—not a runtime guarantee.
Why HVAC Changes the Answer
Air conditioners, heat pumps, well pumps and other motors have two demands: the power they use while running and the short burst of power needed to start. A battery may have enough kWh for several hours of HVAC runtime but still need sufficient output and motor-start capability to start the compressor safely. This is why battery model selection matters as much as battery count.
Solar Recharge Can Change Multi-Day Backup
If compatible solar remains available during a grid outage, daytime production may supply home loads and recharge the battery. A household that conserves energy during the evening and recharges well during the day can stretch backup much farther than the same battery used without solar. Cloud cover, array size, season and home consumption all affect the result.
A Practical Sizing Questionnaire
- Do you want essential-load or whole-home backup?
- How many central air conditioners or heat pumps do you have?
- Is the water heater gas, heat pump or electric resistance?
- Do you have a pool, spa, well or booster pump?
- Do you cook with electricity?
- Do you want EV charging available during an outage?
- How many hours or days of backup are you trying to plan for?
- Do you have solar now? If yes, what inverter or microinverter system?
- Do you expect new loads in the next few years?
Example Design Paths
- Essentials-focused home: prioritize runtime and selected circuits; one battery may be enough depending on load and outage goal.
- Comfort-focused home: essentials plus one major comfort load such as HVAC; battery output and motor-start capability become important.
- Whole-home / all-electric home: may require multiple batteries, higher-output equipment and/or smart load controls.
- Large home with solar: size storage and load controls around overnight demand and expected daytime solar recharge.
Frequently Asked Questions
It can be enough for a modest essential-load plan, but it may be undersized for whole-home backup or large electric loads. The answer depends on both kW and kWh.
Not automatically. Oversizing can increase cost without improving the homeowner’s real goals. The better approach is to model the loads, outage duration and expansion plan.
The battery system must meet both the air conditioner’s starting demand and running power while also supporting other active loads. We verify the HVAC nameplate and system configuration rather than using a universal count.
Potentially. Solar recharge can reduce the storage needed for a multi-day outage, but only if the solar array and backup architecture can produce enough daytime energy relative to household use.
Bring us your utility usage, solar information and the list of loads you want available during an outage. We will compare practical battery configurations and show you what each one is designed to support.
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©2026 Infinium Solar, Roofing, and Electric
License: #1031953
All Rights Reserved
Privacy Policy
Web Design and Digital Marketing by RYNO Strategic Solutions.
Cookie Preferences