How Much Battery Storage Do I Need for My Home?

How Do I Know How Much Backup Storage I Need? 

Home batteries store electricity from your solar system or the grid for use during outages, when the grid is most expensive, or at night when it is dark. A well-sized system can keep essential appliances running, lower your utility bill and protect you from grid disruptions. Here is how to estimate the right amount of backup battery storage for your home.

Step 1: Know Your Energy Baseline

Energy use is measured in kilowatt-hours (kWh)—the total amount of electricity your home consumes. To estimate your daily usage, take a recent utility bill and divide the total kWh by the number of days in the billing cycle. For example, if you used 900 kWh in 30 days, that’s 30 kWh per day. 

Energy usage is often seasonal. Summer air-conditioning or pool pumps push usage sharply higher, while winter electric heating can do the same. It is common for peak-season consumption to exceed your spring or fall baseline. According to the U.S. Energy Information Administration, the median American home used about 10,500 kWh in 2023—approximately 29 kWh per day¹. Your actual usage will vary based on your region, home size, and level of electrification (e.g., EVs, heat pumps, induction cooking).

Understanding your historical energy usage gives you a starting point for estimating how long a battery can power your home during an outage. 

Step 2: Plan For Outage Length

Battery storage is your energy safety net. Size your system based on how long you want backup power. The average U.S. outage in 2022 lasted 5.5 hours², but extreme cases like hurricanes can stretch for more than 2 weeks³.

• Short outage (6 hours): Multiply your consumption by the fraction of a day an outage may last.
  Example: 12 kWh/day x (6hrs/24hrs in a day = 0.25) = 3 kWh needed.
• Multi-day outages (2 days): Multiply daily consumption by number of days. 
  Example: 12 kWh/day x 2 = 24 kWh needed.

If your solar system can generate electricity during the outage (e.g., fire safety shutoffs in California¹²), you may need less storage. But during storms or with snow on the roof, assume minimal solar generation.

Step 3: Decide What Is Essential 

Not everything needs to run during a power outage. The U.S. Department of Energy (DOE) recommends identifying a limited set of your most critical loads⁴ – the essentials you want to keep powered:

Not running large loads like HVAC, pool pumps, or laundry appliances during an outage means 13.5 kWh of stored energy can run for over 2 days without recharge. However, if during an outage you continue to use large loads, you may need significantly more capacity: typically, 40-90 kWh for 2-3 days of autonomy⁴.

Step 4: Understand Your Power Needs

While energy (kWh) tells you how long a battery can run, power (kW) determines how many things it can run at the same time. Some batteries offer just 3–5 kW of power—enough for lights, a fridge, and a few other essentials.

Example: A Modular Battery System that Scales

Quality home battery systems are modular, which means that you can scale both energy storage capacity and output power based on your needs.

A Sizing Example

If your goal is to cover 5 kWh of most critical loads per day for 2 days of backup autonomy, without any recharge from solar:

• Energy required: 5 kWh × 2 days = 10 kWh
• Power required: 8.22kW
• Solution: A single Powerwall 3 (13.5 kWh, 11.5kW) is enough to cover
   

Step 5: Pair With Solar Power 

Pairing your battery with solar panels does more than provide backup – it can keep your home powered without relying on the grid. When the grid goes down, your solar system can continue generating electricity during the day, powering your home, and recharging your battery simultaneously. This extends backup runtime and moves you closer to full energy independence.

Energy production varies by geographic location. For example, a 5 kW solar system might generate around 20 kWh per day in sunny California, but closer to 12 kWh per day in cloudier regions like Maine. To understand the impact, compare this output to your home’s daily energy use.

Step 6: Think Long-Term  

Your energy needs might change over time. Adding an electric vehicle, upgrading to an electric water heater, expanding your household, or installing a pool can all increase your future electricity usage. Planning ahead by sizing your battery slightly larger now can help avoid costly or complex upgrades.

Key Takeaway 

Investing in backup storage is a practical step toward a self-sufficient, cleaner energy future. By designing your system thoughtfully and pairing it with solar, you can save on energy costs, reduce your reliance on the grid, and improve the grid’s resilience.