Adding battery storage to a solar system changes what solar can do for you. Instead of exporting excess power to the grid and buying it back at night, a battery lets you store that energy at home and use it when the sun is not shining — or when the grid goes down. The challenge most homeowners face is figuring out how much battery capacity they actually need.

The answer is not always intuitive. A single 13.5 kWh Powerwall sounds like a lot, but if you run a window AC unit, a refrigerator, lights, and phone chargers during a two-day summer outage, you could easily need 30 to 40 kWh of storage. This calculator helps you size your battery system before you talk to installers.

How Battery Sizing Works

Battery capacity is measured in kilowatt-hours (kWh). But usable capacity is not the same as rated capacity. Lithium iron phosphate (LFP) and lithium nickel manganese cobalt (NMC) batteries are typically rated at 80 to 90% depth of discharge (DoD) — meaning you should not drain them completely if you want a long battery life.

The sizing formula is straightforward:

Total storage needed = Daily load (kWh) × Backup days ÷ Depth of discharge

If you need 15 kWh per day for two days at 80% DoD, you need 37.5 kWh of installed capacity. At 13.5 kWh per Powerwall, that is three units. Rounding up is always the right call for critical backup applications.

What Counts as a Critical Load?

When we talk about critical loads in a battery context, we mean the appliances you genuinely cannot do without for 24 to 72 hours. Refrigerators are almost always on the list — a fridge running 24 hours uses roughly 1.5 kWh per day. Lighting adds another 0.3 to 0.5 kWh per day for LED bulbs. Medical equipment such as CPAP machines or home oxygen concentrators may add 0.5 to 2 kWh per day.

The biggest wildcard is air conditioning. A window AC unit running four to six hours uses 4 to 8 kWh per day. A central AC system can use 20 to 40 kWh per day. Many homeowners choose to run ceiling fans and open windows during outages rather than include AC in their battery sizing, which significantly reduces the number of batteries required.

The load reference table in the calculator below lists common appliances with typical daily consumption so you can build your own load profile.

Choosing the Right Battery

The calculator defaults to 13.5 kWh per unit, matching the Tesla Powerwall 3’s usable capacity. But other batteries are worth comparing. The LG RESU10H offers 9.3 kWh of usable capacity and is often priced lower per kWh. The Enphase IQ Battery 5P provides 5 kWh and works especially well in Enphase solar systems. Franklin Electric’s apower series offers 13.6 kWh and has been gaining market share in 2024 and 2025.

Battery chemistry matters too. LFP (lithium iron phosphate) batteries tolerate more charge cycles before degrading and have a better safety profile. NMC batteries can achieve higher energy density but degrade faster at high temperatures. For homes in hot climates, LFP is generally the safer long-term choice.

Battery Cost Expectations

The installed cost estimate in the calculator uses a rough figure of $12,000 per Powerwall-class unit. This includes the battery hardware, inverter or gateway if required, electrical work, and labor. In practice, quotes range from $9,000 to $15,000 per unit depending on your location, installer, and the complexity of your electrical panel setup.

Battery-only installations (without adding solar panels) are possible and sometimes make sense if you already have a solar system. However, most utility incentives and some state programs require a paired solar system for battery rebates.

If you have not yet calculated your solar panel sizing or estimated your savings from solar, start with our solar savings and payback calculator before deciding on battery storage. Understanding your solar production first helps you size both the panels and the batteries together rather than treating them as separate decisions.

For more background on how residential solar systems are designed and installed, the best roof type for solar panels guide covers structural and orientation considerations that affect both panel placement and where battery systems can be wall-mounted.