Solar Battery Sizing Calculator (kWh)

How the calculation works

Solar Battery Size: How Many kWh Does Your Home Need?

A solar array produces most of its energy during daylight, but a home uses electricity at night as well. A battery stores the surplus produced during the day and supplies it when the panels are not generating, after sunset or during a cloudy spell. Sizing it correctly avoids two opposite errors. A battery that is too small leaves surplus solar energy unused and runs out early in the evening. A battery that is too large costs more than the home can use and adds capital that returns little.

How the calculation works

The calculator starts from two inputs: the home's daily electricity use in kWh and the number of days of autonomy, meaning how many consecutive days without sun the system should cover from the battery alone. For a home using 20 kWh per day with one day of autonomy, the energy the battery must deliver is 20 kWh.

A battery cannot deliver its full nominal capacity. Two factors reduce the usable energy. The first is depth of discharge (DoD), the share of capacity that can be withdrawn without accelerating wear. The second is round-trip efficiency, the share of energy that comes back out after a full charge and discharge cycle, because some is lost as heat and in the electronics. The calculator divides the energy need by the product of these two factors:

20 × 1 / (0.90 × 0.95) = 23.4 kWh nominal

The result is then multiplied by 1.15 to add a 15% margin for degradation over the life of the battery, which gives a recommended capacity of about 26.9 kWh. The margin is there so the system still covers the winter nights years after installation, not only on the day it is commissioned.

The formula is written in metric units. The calculator's unit toggle converts automatically between metric and imperial, so any imperial value you enter is converted before the calculation runs.

Battery chemistries compared

The calculator supports three chemistries, and each carries different planning values.

LiFePO4 (lithium iron phosphate) is a common choice for new residential systems. The calculator's planning values for it are a depth of discharge of 0.90 and a round-trip efficiency of 0.95. These are assumptions built into the calculator, not measured figures for any product. Its thermal stability and tolerance of frequent full cycles are the properties that make it suitable for daily use. The datasheet of the chosen battery gives its cycle rating and capacity warranty, and those figures should replace the planning values.

NMC (nickel manganese cobalt) stores more energy in a smaller and lighter package, which matters when space is limited. The calculator's planning values are 0.80 for depth of discharge and 0.93 for efficiency, both assumptions rather than measured figures. Its lower usable depth means more nominal capacity is required for the same usable energy.

Lead-acid (VRLA or AGM) is the oldest of the three and is still used in some off-grid installations. The calculator's planning values are 0.50 for usable depth of discharge and 0.85 for efficiency. Because half the nominal capacity is unavailable, the battery must be roughly twice the size of a LiFePO4 system to deliver the same usable energy. Lead-acid batteries are also heavier and generally need more maintenance. They suit applications where the purchase price matters more than space and service life.

Choosing days of autonomy

Days of autonomy determine how much storage the home needs, and they are the input that changes the result most. A home on a grid-tied system with reliable utility service usually sizes the battery for one day, because the grid covers any shortfall. Homes in areas with frequent outages, and off-grid systems with no utility connection, normally size for two to three days.

Location matters too. Two homes with the same consumption can need different battery capacity depending on local sun. A home in Phoenix, Arizona receives strong sun for most of the year, so one day of autonomy is often enough for a grid-tied system. A home in Seattle, Washington has longer cloudy periods in winter, so its owner may prefer more autonomy for the months of lowest production. The National Renewable Energy Laboratory (NREL) publishes PVWatts, a public estimator of solar production by location, at https://pvwatts.nrel.gov. Its results, together with a production estimate from a local installer, can help make this decision.

Common sizing mistakes

The most frequent mistake is sizing the battery for total daily consumption without considering the energy used directly during daylight. A household that uses 20 kWh per day but draws 8 kWh straight from the panels in the afternoon, for laundry, cooking and a computer, only needs to store the remaining 12 kWh for evening use. Installing a battery for the full 20 kWh in that case leaves much of the capacity unused. Use the hourly profile, not only the daily total, to size the battery.

The second mistake is ignoring degradation. Battery capacity declines gradually with use, and a system sized exactly for today's needs may fall short several years later, especially in winter when the nights are longest. The 15% margin in this calculator is intended to absorb that gradual loss. A larger margin may be justified where the home is expected to grow its consumption, for example after adding an electric vehicle or a heat pump.

A third mistake is overlooking the inverter. The battery must be compatible with the inverter in terms of voltage, maximum charge and discharge power, and total capacity. Many inverters limit the size of the battery bank they support, so check that limit before choosing a capacity above the inverter's rated range.

When to use this calculator, and when not to

The calculator gives a pre-sizing estimate, which is useful for comparing options and for understanding how the inputs interact. It does not replace a site assessment, and it is not suitable for the following cases:

• Critical loads such as medical equipment, where backup power needs a load analysis and a dedicated design.
• Homes with large or irregular loads such as electric vehicle chargers, well pumps or electric heating, whose hourly profile differs sharply from the daily average.
• Commercial systems and multi-unit buildings, which follow different design rules.

For these cases the daily total is not enough. The hourly load profile determines both the battery size and the inverter rating.

Installation, permits and utility rules

Battery storage in the United States is governed by the National Electrical Code (NEC). Article 706 covers energy storage systems, and where the battery is interconnected with the utility grid through a solar system, the rules for interconnected power production sources in Article 705 also apply. Equipment is typically certified to UL 9540, the standard for energy storage systems, and NFPA 855 addresses the placement and fire safety of stationary storage installations. The local authority having jurisdiction, usually the city or county building department, decides which code editions and amendments apply. Utility interconnection requirements are set by the local utility and are separate from the building permit.

A licensed electrical contractor usually handles the installation and the permit application. The design should be confirmed on site by that installer, who can measure the real consumption profile and check the existing electrical panel before any battery is ordered.