Quick answer
Backing up a whole Illinois home usually takes two or more batteries, mainly to run central air, an electric range or a well pump; backing up essentials usually takes one. ComEd and Ameren pay about $300 per kWh of battery capacity, around $8,100 for two 13.5 kWh batteries, if you enroll in the utility's virtual power plant program.
What's the difference between whole-home and essential-loads backup?
| Essential loads | Whole home | |
|---|---|---|
| What stays on | Chosen circuits: refrigerator, lights, internet, sump pump, furnace fan | Most or all circuits, including large 240-volt appliances |
| Typical battery count | One battery | Two or more, depending on the home |
| Wiring | A separate backup or critical-loads panel | The battery system backs up the main panel |
| Best for | Riding out short outages affordably | Homes with central air, electric cooking, well pumps or medical equipment, or long outages |
What decides it is power as much as storage. A battery has to deliver enough power at once to start motors like an air-conditioner compressor or a well pump, and enough stored energy to last the outage. NREL recommends sizing from the loads you actually need during an outage, which can be quite different from your everyday usage.
How many batteries does a whole home need?
| Battery | Usable energy | Continuous power | Stackable |
|---|---|---|---|
| Tesla Powerwall 3 | 13.5 kWh | 11.5 kW | Up to 4 units |
| FranklinWH aPower 2 | 15 kWh | 10 kW | Up to 15 units per aGate |
| Enphase IQ Battery 5P | 5 kWh | 3.84 kW | Multiple units combine |
Most homes that add storage install about one battery's worth: Berkeley Lab found the median home battery installed in 2024 was 13.5 kWh. Whole-home backup usually means more. Berkeley Lab's modeling shows that counting heating and cooling as must-run loads sharply raises the battery size needed to get through multi-day outages, so central air is usually what pushes a home to two or three batteries.
Ask installers about load management or a soft starter for the air conditioner, which can reduce how much battery power it takes to start. An installer should size the system from your panel and appliances, not a rule of thumb.
How much is the Illinois battery rebate?
| Battery setup | Capacity | Rebate |
|---|---|---|
| 1 × 13.5 kWh battery | 13.5 kWh | $4,050 |
| 2 × 13.5 kWh battery | 27.0 kWh | $8,100 |
| 3 × 13.5 kWh battery | 40.5 kWh | $12,150 |
Illinois law sets the storage payment at $300 per kWh of nameplate capacity through December 31, 2029. Since June 1, 2026, both utilities require you to enroll the battery in their Scheduled Dispatch Virtual Power Plant programs, which let the utility draw on it at set summer peak times; Ameren expects to dispatch on summer weekdays from 4 to 6 p.m. starting in 2027. Your installer should leave enough charge in reserve for backup.
Program details are still rolling out under the 2026 energy law, so confirm current terms before you buy. See the Illinois battery rebate guide.
Where can a home battery be installed?
National fire code rules adopted by many towns limit how much battery capacity can go in different parts of a home: about 40 kWh in total in utility closets and storage spaces, and 80 kWh in a garage, on an exterior wall or outdoors. Your city or village decides which code applies and permits the installation, so your installer should confirm the local rules.
Garages and exterior walls are the most common spots in Illinois. Batteries have operating temperature limits, so ask how the system handles Illinois winters and summer heat in the location you choose.
What should I ask installers about whole-home backup?
- 1
Which loads will run in an outage?
Get it in writing, including whether central air, an electric range or a well pump can start.
- 2
How long will it last?
Ask for an estimate of hours or days of backup with and without solar recharging.
- 3
How does the virtual power plant affect backup?
Ask how much of the battery the utility may use and how much stays in reserve.
- 4
What's the itemized battery price?
Get the battery priced separately from the solar, then subtract the storage rebate. Check the solar part with our quote checker.
Sources
- ComEd: Distributed generation and storage rebates
- Ameren Illinois: DER Bulletin 2026-04 (storage rebate and virtual power plant)
- Illinois Public Utilities Act, 220 ILCS 5/16-107.6: Distributed generation and storage rebates
- Citizens Utility Board: Upcoming programs from the 2026 energy law (Sept 2026)
- Tesla: Powerwall 3 installation manual, specifications
- FranklinWH: aPower 2 datasheet
- Enphase: IQ Battery 5P
- NREL: Battery storage for resilience (fact sheet)
- Berkeley Lab: Backup power from residential solar and storage (2024)
- Berkeley Lab: U.S. distributed solar and storage, 2025 data update
- NFPA: Residential energy storage system regulations
This guide is general information, not tax, legal or financial advice. Incentive programs change, so confirm current terms with your installer, your utility and a tax professional.