How to Size a Battery Bank for Home Backup Power

Battery bank sizing is the step where most home backup systems go wrong in both directions. Too small and the system runs out of power before the outage ends. Too large and you have spent several thousand dollars on capacity you will never use. Getting it right requires three things: an honest accounting of what you actually need to run, a clear decision on how many days of backup you want, and an understanding of how depth of discharge affects what you can actually use from the battery you buy.

This guide walks through the sizing process step by step, with a real appliance wattage table, worked examples for three different household types, and the most common mistakes that lead to undersized or oversized systems. Browse our generator batteries collection once you have your target capacity.

Why Sizing Matters: Undersized vs. Oversized

An undersized battery bank runs out before the outage ends. In a multi-day storm scenario, that means reverting to the exact situation you were trying to avoid. An oversized bank means you have paid for capacity that sits unused through every outage you actually experience, which for most households means most of the time. Both outcomes are avoidable if the sizing is done correctly upfront.

The goal is a system that handles your real-world worst-case scenario with a reasonable margin, not a system sized for the absolute worst winter storm imaginable, and not one sized for a sunny afternoon test run.

Step 1: List Your Critical Loads

Write down every appliance you genuinely need to run during a power outage. Be honest and be selective. Most households do not need to run their electric oven, clothes dryer, or electric vehicle charger during an outage. What most households do need:

Starting wattage vs. running wattage matters for motors (pumps, compressors, AC units). Motors draw 2 to 3 times their running wattage for a brief moment at startup. Your inverter needs to handle the starting surge; your battery sizing is based on running wattage and duration.

Step 2: Calculate Your Daily Wh Consumption

For each appliance on your list, multiply running watts by the number of hours per day you expect to run it. Add the results together.

Formula: Appliance Wh = Running Watts x Hours per Day

Example for a typical essential-loads setup:

Note: The refrigerator figure above (1,500 Wh) is an estimate for an older full-size unit. Modern Energy Star models use 400 to 800 Wh/day. Check your model's energy label (annual kWh / 365) for an accurate figure rather than multiplying the compressor's running watts by 24 hours.

Step 3: Choose Your Autonomy Window

Autonomy is how many days you want the battery to run your essential loads with no solar recharge and no generator input. This is the worst-case scenario your system needs to handle.

• 1 day (24 hours): Covers short outages and overnight gaps between solar recharge periods. Appropriate for areas with reliable utility restoration and a solar system that recharges the bank daily.
• 2 to 3 days: The practical target for most home backup systems in storm-prone areas. Covers most weather-related outages without requiring a generator as a backup.
• 5 to 7 days: Extended off-grid autonomy, appropriate for remote properties or areas with historically long outage durations. Requires a significantly larger battery bank and typically a solar system or generator for recharge.

Step 4: Account for Depth of Discharge

Depth of discharge (DoD) is the percentage of a battery's rated capacity that can safely be used before recharging. This is where most sizing calculations go wrong. The Ah or Wh number printed on a battery label is the total capacity, not the usable capacity.

LiFePO4 DoD: 80%. A 100Ah LiFePO4 battery gives you 80Ah of usable energy per cycle. Divide your energy requirement by 0.8.

AGM DoD: 50%. A 100Ah AGM battery gives you 50Ah of usable energy per cycle. Divide your energy requirement by 0.5.

The sizing formula:

Battery Bank (Wh) = Daily Load (Wh) x Days of Autonomy / DoD

Using the example from Step 2 (2,750 Wh/day), for a 2-day LiFePO4 system:

2,750 Wh x 2 days / 0.8 = 6,875 Wh required bank capacity (6.9 kWh)

For the same load with a 2-day AGM system:

2,750 Wh x 2 days / 0.5 = 11,000 Wh required bank capacity (11 kWh)

The DoD difference means you need 60 percent more AGM capacity to deliver the same usable energy as LiFePO4, which largely explains why LiFePO4 is more cost-effective at scale despite its higher unit price.

Step 5: Adjust for Solar Input

If your battery bank is paired with solar panels, you can subtract the expected daily solar generation from your daily load before calculating autonomy. A 400W solar array in a location averaging 4 peak sun hours per day produces approximately 1,600 Wh per day. Applied to the 2,750 Wh/day example load, that reduces the net daily draw to 1,150 Wh, which reduces the required bank size proportionally for multi-day scenarios.

In winter, reduce your solar production estimate significantly: shorter days, lower sun angles, and potential snow coverage can cut effective production by 50 to 70 percent compared to summer figures in most northern US locations.

Sample Sizing by Household Type

Common Sizing Mistakes

Sizing for the average day instead of the worst day. Battery backup is not for the typical Thursday; it is for the ice storm in February when the power stays out for three days. Size for the worst realistic scenario, not the median outage duration in your area.

Forgetting starting surge for motors. A well pump, sump pump, or refrigerator compressor draws 2 to 3 times its running wattage for a brief moment at startup. Your battery bank handles sustained load fine, but your inverter must handle the startup surge. Confirm the inverter's surge rating covers the largest motor on your list.

Not accounting for DoD. Treating a 10 kWh battery bank as 10 kWh of usable energy is a sizing error. A 10 kWh LiFePO4 bank gives you 8 kWh of usable energy. A 10 kWh AGM bank gives you 5 kWh. The difference is large enough to fundamentally change which products you need to buy.

Ignoring cold weather derating. If your battery bank is in an unheated space in a cold climate, plan for 80 to 90 percent of rated capacity at 32°F and less in colder conditions. A system that barely meets your needs in summer may fall short in the outage that happens during a February ice storm.

Adding 20 to 30 percent buffer. Sizing exactly to the calculated minimum leaves no margin. Build in a buffer that covers the uncertainty in your load estimates, cold weather derating, and the possibility of one more appliance being added later.

Frequently Asked Questions