The most honest answer is also the least satisfying: it depends on what “back up my house” means. A refrigerator and Wi-Fi for one night is not the same job as central air, an electric range, laundry, and a well pump for three cloudy days.
Battery sizing starts with loads, not square footage. A 2,000-square-foot home with efficient appliances may need less backup capacity than a smaller home with electric resistance heat.
Start With the Loads That Matter
A home battery stores electricity in kilowatt-hours, or kWh. One kWh is enough to run a 1,000-watt load for one hour. If essential loads use 1.5 kWh per hour, a 13.5 kWh battery will not last 13.5 hours once inverter losses and usable reserve are considered, but the math gives a starting point.
EnergySage notes that roughly 13.5 kWh is a common battery size for keeping essential devices running during outages. That does not mean it is enough for every home. It means a single-battery design is often aimed at essentials, not unlimited whole-home comfort.
A homeowner can make a simple list:
– Refrigerator and freezer
– Wi-Fi, router, and phone charging
– A few lighting circuits
– Garage door opener
– Medical equipment, if needed
– Heating or cooling equipment, if realistic for the battery size
This is where a home load backup calculator can be more useful than a generic rule of thumb.
Solar Recharge Changes the Math
A battery does not sit alone if it is connected to solar panels. During daylight, the panels may power the home and recharge the battery. On a sunny day, that can extend backup time significantly. On a stormy winter day, it may not.
NREL’s PVWatts calculator is commonly used to estimate solar production by location, roof angle, system size, and weather patterns. It is not a backup guarantee, but it helps show why a system in Arizona and a system in Michigan should not be sized the same way.
The backup plan should also account for power output. Capacity is how long the battery can supply energy. Output is how much power it can deliver at once. Starting a well pump or running air conditioning may require more power than keeping lights on.
Essential Loads Beat Guesswork
Many households get a better result by designing for tiers. Tier one is essential: food, communications, lights, and safety. Tier two is comfort: some heating or cooling, selected outlets, and maybe a microwave. Tier three is whole-home living, which requires more capacity and cost.
A battery storage sizing tool can help turn those tiers into a realistic conversation with an installer. It also makes it easier to avoid paying for backup capacity that will rarely be used.
The right number of batteries is not the biggest number a roof-and-garage layout can fit. It is the amount that covers the loads that matter, for the outage duration that is realistic, with enough solar recharge to make the system useful when the grid stays down longer than expected.