Solar Battery Size Calculator for EV Charging

Why Pair a Home Battery with Solar and an EV?

Solar panels produce electricity during the day, but most EV owners charge at night. Without a battery, you rely on net metering to credit your daytime export against your nighttime draw. A home battery changes that equation by storing the solar energy directly, so you use your own electrons when the sun goes down instead of buying them back from the grid.

This matters most in states with time-of-use (TOU) electricity rates, where the cost per kWh is higher during evening peak hours (typically 4 to 9 PM) when people arrive home and plug in their EVs. Storing solar in a battery and discharging during those peak hours can save significantly more than a flat-rate net metering credit.

Understanding Battery Capacity Specs

Battery manufacturers advertise two numbers: nominal capacity and usable capacity. Nominal capacity is the total amount of energy the battery can hold. Usable capacity is the amount you can actually draw before the battery management system stops discharging to protect the cells.

Depth of discharge (DoD) is the ratio of usable to nominal capacity. A battery with 13.5 kWh nominal at 90% DoD provides 12.15 kWh of usable energy per cycle. Round-trip efficiency accounts for losses during the charge and discharge cycle. At 95% round-trip efficiency, you need to put in roughly 1.05 kWh to get 1 kWh back out. This calculator applies both factors so you get a realistic system size.

Do You Really Need a Battery?

In states with strong full-retail net metering policies, the grid already acts as a free battery. Your solar system exports during the day, the utility credits you at the full retail rate, and you draw that credit back at night for EV charging. Adding a physical battery on top of full-retail net metering is harder to justify on cost alone and typically adds 10 or more years to the payback period.

Batteries make more economic sense when: your utility offers only avoided-cost net metering (paying you wholesale rather than retail for exports), you are on TOU rates with high evening peaks, you want backup power for outages, or your utility is considering eliminating net metering. In California (NEM 3.0) and several other states, the economics now favor battery storage alongside solar for new installations.