Storage
Whole-home vs partial-home battery backup: what each covers and costs
Short answer
Partial-home backup moves the essential circuits — refrigerator, lights, internet, outlets, a gas furnace fan, a well pump — onto a separate backup panel, and usually needs one battery of 10 to 15 kWh. Whole-home backup keeps every circuit in the house running, including central air and large electric appliances, which typically takes two or three batteries, more power to start motors, and transfer equipment at the service. Partial backup costs roughly half as much and lasts longer per kWh; whole-home backup is the one that feels like nothing happened.
The difference comes down to where the switch is
In an outage, a battery system disconnects the house from the grid and powers whatever is on the protected side of that disconnect. What changes between the two approaches is how much of the house is on that side.
- Partial-home backup. The essential circuits are moved out of the main panel into a smaller backup loads panel fed by the battery system. In an outage those circuits stay on; everything left in the main panel goes dark.
- Whole-home backup. The transfer equipment — a backup gateway or automatic transfer switch — sits between the utility meter and the main panel, so the entire panel stays powered. Large loads are usually managed by the system so they can't all run at once.
Both use the same batteries. The choice is about how many of them, how much power they have to deliver at once, and how the house is wired around them.
What a home actually uses in an outage
Two numbers matter for every load: the power it draws while running, in watts, which decides whether the battery can carry it at all, and the energy it uses over a day, in kWh, which decides how long the battery lasts. Typical figures:
| Load | Running power | Energy per day in an outage |
|---|---|---|
| Refrigerator | 100–200 W, cycling | 1–2 kWh |
| LED lighting, 10–15 fixtures | 100–150 W | 0.5–1 kWh |
| Internet, phones, a laptop | 50–100 W | 0.5–1 kWh |
| Television | about 100 W | 0.3–0.5 kWh |
| Gas furnace fan | 400–800 W | 2–4 kWh in cold weather |
| Well pump, 1 hp | about 1,000 W, three times that to start | 1–2 kWh |
| Microwave | 1,000–1,500 W | 0.2–0.4 kWh |
| Central air, 3 ton | 3,000–4,000 W, with a large starting surge | 15–30 kWh in hot weather |
| Electric water heater | 4,500 W | 6–12 kWh |
| Electric range and oven | 2,000–5,000 W | 1–3 kWh |
| Electric dryer | about 5,000 W | 2–3 kWh per load |
| EV charger, Level 2 | 7,000–11,500 W | 20–40 kWh per full charge |
The essentials in the top half of the table add up to roughly 6 to 12 kWh a day. Central air, an electric water heater and an EV can each use more than all of the essentials together — which is the whole story of whole-home backup in one line.
Whole-home vs partial-home at a glance
| Partial-home (essential loads) | Whole-home | |
|---|---|---|
| What stays on | Essential circuits moved to a backup loads panel | Every circuit in the main panel, with large loads managed |
| Typical use per day in an outage | 6–12 kWh | 25–60 kWh, most of it air conditioning |
| Typical battery capacity | 10–15 kWh, one battery | 27–45 kWh, two or three batteries |
| Continuous power needed | 4–8 kW | 11–20 kW or more, plus motor starting |
| Runtime on a full battery, no sun | About 1 to 2 days | About half a day to a day with the AC running |
| Runtime with solar recharging | Indefinitely in most weather | Several days if AC use is managed |
| Extra equipment | Backup loads panel; circuits moved into it | Backup gateway or transfer switch at the service; load controllers; often a soft starter for the AC |
| Rough installed cost, before incentives | $12,000–$18,000 | $25,000–$45,000 or more |
| Best for | Short and occasional outages, tighter budgets | Long outages, hot climates, medical equipment, all-electric homes |
Costs vary widely by region, battery brand and how much electrical work the house needs. Treat the ranges as a starting point for a conversation, not a quote, and check which federal, state and utility incentives apply at the time of purchase — they change often.
A worked example
Take a 2,000-square-foot home in Florida with a 3-ton central air conditioner, gas cooking and an 8 kW solar array, during a summer outage.
| Partial-home | Whole-home | |
|---|---|---|
| Batteries | One 13.5 kWh | Two 13.5 kWh (27 kWh) |
| Loads backed up | Fridge, lights, internet, outlets, ceiling fans | Everything, with the AC set to 78°F |
| Use per day | About 8 kWh | About 40 kWh |
| Hours on battery alone | About 36 hours | About 15 hours |
| Solar on a sunny day | About 32 kWh — far more than needed | About 32 kWh — close to break-even |
The partial system rides out a multi-day outage with room to spare, because the sun refills it every day. The whole-home system keeps the house comfortable, but on a cloudy stretch the owner would be managing the thermostat to make the batteries last.
Power, not just storage: starting the air conditioner
Capacity in kWh says how long a battery lasts. Its power rating in kW — and its motor starting rating — says whether it can carry the loads at all. A central air compressor draws several times its running current for a fraction of a second when it starts. A battery that can't supply that surge trips, no matter how full it is.
That's why whole-home designs check the air conditioner's locked-rotor amps against the battery system's starting capability, often add a soft starter to the compressor, and frequently need two batteries for power as much as for energy. Partial-home designs simply leave the air conditioner off the backup panel.
What changes on the permit set
- Partial-home: the single-line diagram shows the backup loads panel, the circuits moved into it, and how it's fed from the battery system. The panel schedule lists what's backed up.
- Whole-home: the transfer equipment is drawn at the service entrance, between the meter and the main panel. Some jurisdictions and utilities review that work more closely, and an older meter-main combination may need replacing.
- Both: an AC-coupled battery adds backfeed to the main panel, so it counts in the 120% busbar rule together with the solar inverter. Batteries also have location limits: under the residential code (IRC R328), an individual unit is limited to 20 kWh, and the total is capped at 40 kWh in utility closets, basements and storage rooms and 80 kWh in garages and outdoors — which matters most for whole-home systems. Local fire departments often add their own rules.
Which one to choose
- Choose partial-home if outages are occasional, the budget matters, or the house has gas heat and cooking. It covers what people actually miss in an outage for about half the cost.
- Choose whole-home for long outages, hot or very cold climates, medical equipment that can't be on a separate circuit, or an all-electric home — and plan the batteries around the air conditioner, not the refrigerator.
- Either way, list the loads first. The battery count falls out of the table above.
To see how a battery changes the main panel, try it in the free solar line diagram builder, or check the backfeed on its own in the busbar 120% calculator. When it's time to permit, we draw the battery sheet — location, clearances, the backup panel or transfer equipment and the line diagram — as part of our battery storage design.