Battery Sizing Calculator

How much battery storage do you need? Select your essential appliances and desired backup duration to find the right battery system for your home.

Last updated: February 2026

Select Your Essential Appliances

150W
20W
10W ea
15W ea
65W ea
1,200W
3,500W
1,200W
4,500W
W

Backup Duration

Load: 230W

How Battery Sizing Works

1

Sum Your Loads

Select all appliances you want to power during an outage. The calculator sums their wattage to determine your total power demand (kW).

2

Calculate Energy Needed

Total energy = power demand x backup hours x 1.2 overhead. The 20% overhead accounts for inverter conversion losses and battery management system (BMS) overhead.

3

Match to Batteries

We calculate how many units of each battery you need, checking both capacity (kWh) and power output (kW) to ensure reliable operation.

AC-Coupled vs DC-Coupled Battery Storage

AC-Coupled

  • Connects to your home AC electrical panel
  • Works with any existing solar inverter
  • Ideal for adding storage to existing solar
  • Round-trip efficiency: 85-90%
  • Examples: Enphase IQ Battery, sonnen

DC-Coupled

  • Connects directly to solar panels (DC side)
  • Higher efficiency, fewer conversion steps
  • Best for new solar+battery installations
  • Round-trip efficiency: 90-95%
  • Examples: Tesla Powerwall 3, Franklin WH

Frequently Asked Questions

How do I determine the right battery size for my home?
Start by identifying the essential appliances you want to power during an outage: refrigerator, lights, WiFi, phone chargers, and possibly a window AC or medical equipment. Sum their wattage to get total load. Then multiply by the number of backup hours you need, adding 20% overhead for inverter losses. For example, 500W of essential loads for 24 hours needs 500W x 24h x 1.2 = 14.4 kWh of battery capacity. A single Tesla Powerwall 3 (13.5 kWh) would nearly cover this, while two Enphase IQ Battery 5P units (10 kWh total) would fall short.
What is the difference between battery capacity (kWh) and power output (kW)?
Battery capacity (kWh) is how much total energy the battery stores — think of it as the size of the fuel tank. Power output (kW) is how much power the battery can deliver at any moment — think of it as the size of the engine. A 13.5 kWh battery with 11.5 kW output can power 11,500 watts of appliances simultaneously but will drain faster with higher loads. You need enough kWh for your total energy needs AND enough kW for your peak simultaneous load. Running a central AC (3,500W) plus other loads requires at least 5 kW of continuous power output.
Should I choose AC-coupled or DC-coupled battery storage?
AC-coupled batteries (like Enphase IQ Battery) connect to your home electrical panel and work with any existing solar inverter, making them ideal for retrofitting battery storage to an existing solar system. DC-coupled batteries (like the Tesla Powerwall 3 with integrated inverter) connect directly to the solar panels before conversion, offering slightly higher round-trip efficiency (90-95% vs 85-90%). For new installations, DC-coupled systems are generally more cost-effective. For adding batteries to existing solar, AC-coupled is simpler and avoids replacing your inverter.

Related Resources

Battery Backup Calculator → Solar Payback Calculator → Solar Cost Calculator → All Batteries → AC vs DC Coupled Battery → Best Home Batteries 2026 →