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To choose the right battery size, focus on your overnight electricity use. If your home consumes 20 kWh per day, around half may occur after sunset, suggesting a battery near 10 kWh as a starting point. Use your solar monitoring app to measure actual overnight grid imports for greater accuracy. Avoid oversizing for rare extreme days and consider backup needs separately from daily savings.
What Does Battery Size Actually Mean?
Battery size is measured in kilowatt-hours, or kWh. This number represents how much energy the battery can store, not how fast it can deliver it. The easiest way to think about it is as a storage bucket. A 10 kWh battery can store 10 units of electricity to be used later.
The size you choose determines how much of your evening and overnight electricity use can be covered by stored solar energy instead of buying power from the grid.
How Much Electricity Do You Use Overnight?
The single most important number when sizing a battery is your overnight consumption. A battery’s primary role is to store excess solar generated during the day and use it after sunset.
If you do not yet have solar, look at your electricity bill and find your average daily usage. Many households use roughly half of their daily electricity outside of solar production hours. For example, if your home uses 20 kWh per day, a starting estimate would be around a 10 kWh battery.
If you already have solar, your monitoring app provides a more accurate answer. Check how much energy you import from the grid between sunset and sunrise. That overnight grid import is the amount your battery would ideally cover.
Should You Size for Your Worst Day of the Year?
No. Oversizing a battery to cover your single highest usage day is usually not cost-effective. A sensible approach is to size the battery to cover most evenings, accepting that on unusually high-demand days you may still draw some power from the grid.
Seasonal differences also matter. Winter heating and summer cooling can change consumption patterns. The goal is to find a size that performs consistently well across most of the year rather than chasing perfection for extreme scenarios.
Does Backup Power Change the Size You Need?
If blackout protection is important, battery size may need additional consideration. Running essential loads such as lighting, refrigeration and internet equipment requires far less energy than attempting to power the entire house.
Understanding what you truly want to run during an outage helps determine whether your chosen battery size is sufficient. Backup requirements and daily bill savings are related but separate considerations.
