How to size a solar battery for backup

A backup battery is sized by two separate questions: how much energy you need over time (kWh) and how much power you need at once (kW). Get both right and the system carries your home through an outage; get either wrong and it either dies early or trips on startup. Here is the method.

Step 1 — Pick your critical loads

Backup rarely means running the whole house. List the circuits that must stay on during an outage:

• Refrigerator and freezer
• A few lighting and outlet circuits
• Internet and phone charging
• Well pump or sump pump, if applicable
• Furnace blower or a mini-split for heating and cooling

Leave out large resistive and high-draw loads — electric ranges, dryers, and central AC — unless you deliberately plan for them, because they dominate both energy and power.

Step 2 — Estimate daily energy (kWh)

For each critical load, multiply its wattage by the hours per day it actually runs, then sum:

• Refrigerator: cycles, so estimate about 1 to 2 kWh/day.
• Lights: 400 W for 5 hours = 2 kWh/day.
• Furnace blower: 500 W for 6 hours = 3 kWh/day.

Add everything up. A modest critical-loads panel commonly lands around 8 to 12 kWh per day. Use your utility bill as a sanity check on whole-home usage.

Step 3 — Choose days of autonomy

Days of autonomy is how long the battery alone carries the critical loads with little or no solar input:

• One day is typical when the battery recharges from the array each day.
• Two or more days suits cloudy climates or homes without daytime solar recharge.

Multiply daily energy by days of autonomy. At 10 kWh/day and one day, you need 10 kWh delivered.

Step 4 — Adjust for usable depth of discharge

Batteries should not be fully drained. Divide the energy you need by the usable depth of discharge (often 80 to 95% for lithium):

• 10 kWh needed / 0.90 usable = about 11.1 kWh nameplate.

Round up to the next real product size. Round-trip efficiency losses (roughly 90%) further nudge you toward a slightly larger pack.

Step 5 — Check the power rating (kW)

Now confirm the battery inverter can supply the simultaneous draw. Add the running watts of loads that overlap, and account for motor surge on pumps, compressors, and the furnace blower, which can spike to several times running current at startup. If the continuous kW rating is too low, the system trips even when plenty of kWh remain.

A quick worked example

Say your critical loads total 9 kWh/day, you want one day of autonomy, your battery has 90% usable depth of discharge, and your largest simultaneous draw (fridge plus furnace blower plus lights, with motor surge) peaks around 4 kW:

• Energy: 9 kWh x 1 day / 0.90 = 10 kWh nameplate, rounded up to the next product size.
• Power: choose a battery inverter with a continuous rating above 4 kW and enough surge headroom for the blower and fridge compressor starting together.

If you later want two days of autonomy, the energy figure simply doubles to about 20 kWh while the power rating stays the same.

Battery interconnection also touches the 120% busbar rule and, depending on chemistry, DC- or AC-coupled wiring — plan those early with our reference library. Confirm requirements with your AHJ, and have a licensed electrician or PE review and stamp the installation. For array sizing, see how to size a residential solar system.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.