How to Size a Home Battery in 2026

The 4 sizing mistakes everyone makes

Before walking through the correct sizing process, let's cover the four mistakes we see in 60% of first-time battery buyers. Avoiding these will save you more money than any specific brand recommendation:

1. Sizing for whole-home backup when you only need essential loads. The average U.S. home uses 28 kWh per day. A whole-home battery backup of that would require 3–4 batteries ($30,000+). But the average home's essential loads (fridge, lights, internet, furnace blower, well pump) only use 6–8 kWh per day — easily covered by a single battery. Most homeowners vastly overestimate what they actually need backed up.
2. Confusing continuous watts with surge watts. A 1,500W well pump might draw 6,000W for the first 2 seconds when starting (the "locked rotor" surge). If your battery can only output 5,000W continuously, it will trip offline trying to start the pump — even though 1,500W is well within its rating. Sizing for surge is critical for any system backing up motors.
3. Forgetting depth-of-discharge buffer. Most LFP batteries reserve 5% of capacity as a buffer, so a "13.5 kWh" battery actually delivers 12.8 kWh usable. Some installers quote nameplate capacity in their proposals; always confirm usable capacity.
4. Ignoring round-trip efficiency. If your battery has 90% round-trip efficiency and you cycle 10 kWh through it daily, you'll need 11.1 kWh of input to get 10 kWh of output. Over a year, that's 400 kWh of "lost" energy — small in absolute terms but worth understanding.

Step 1: Define your backup goal

Before any math, decide what you actually want your battery to do. There are three common scenarios, each requiring different sizing:

• Scenario A — Essential backup (12–24 hours): Power fridge, lights, internet, furnace blower, well pump, medical devices during a typical 12–24 hour outage. This is what 70% of homeowners actually need. Typically 10–15 kWh of storage.
• Scenario B — Extended essential backup (48–72 hours): Same essential loads, but for multi-day outages. Requires either 20–30 kWh of storage OR pairing with solar to recharge during the day. Common in hurricane zones and wildfire-prone areas.
• Scenario C — Whole-home backup: Run everything in your home including HVAC, electric water heater, electric clothes dryer, and EV charger. Typically requires 27–40+ kWh of storage AND a smart panel for load management. The most expensive option.

For 70% of homeowners, Scenario A is the right answer. Scenario B is right for anyone in a region with regular multi-day outages (Florida, Texas, Gulf Coast, California wildfire zones). Scenario C is only worth it for large homes with critical loads (medical equipment, home businesses) or homeowners who simply refuse to live with any power constraints during an outage.

Step 2: List your loads

Make a list of every appliance and circuit you want backed up. For each one, you need two numbers:

• Continuous watts (running draw): How much power the device uses while running normally
• Surge watts (starting draw): The brief spike when motors start (typically 2–4× continuous for compressors and pumps; 1× for resistive loads like lights and heaters)

If you don't know the wattage of an appliance, look at the label on the back or bottom (usually listed in watts or amps × volts = watts). For household staples, here are typical values:

Step 3: Calculate daily Wh requirement

For each appliance on your list, multiply continuous watts by daily run hours to get daily Wh. Add them all up to get your total daily energy requirement.

Example: Fridge (1,500 Wh) + Furnace blower (2,500 Wh) + Well pump (1,000 Wh) + Lights (1,000 Wh) + Internet (500 Wh) + Phone charging (300 Wh) + CPAP (500 Wh) = 7,300 Wh/day = 7.3 kWh/day

This is the energy you need to back up for a 24-hour outage. For a 48-hour outage, double it. For 72 hours, triple it (or add solar to recharge during the day).

Step 4: Convert Wh to required battery capacity

The battery capacity you need is NOT the same as your daily Wh requirement. You need to account for two factors:

1. Depth of Discharge (DoD) buffer: Reserve 5% of capacity. Multiply by 1.05.
2. Round-trip efficiency loss: Reserve 10% for AC-coupled systems, 5% for DC-coupled. Multiply by 1.10 (AC) or 1.05 (DC).

Example (AC-coupled): 7.3 kWh × 1.05 (DoD) × 1.10 (efficiency) = 8.4 kWh required battery capacity

For a 24-hour backup of essential loads in this example, you need a battery with at least 8.4 kWh of usable capacity. The Tesla Powerwall 3 (13.5 kWh) provides ~60% headroom beyond this, which is ideal — it means you can absorb a few unexpected loads (a neighbor charging their phone, running a fan during a heat wave) without running out of capacity.

For a 48-hour backup, you'd need 16.8 kWh — which means either two batteries (27 kWh, providing ~60% headroom) or a single battery paired with solar that can recharge during the day.

Step 5: Verify surge requirements

Capacity alone isn't enough — you also need to verify your battery can start every motor on your list simultaneously. Add up the surge watts of all motors that might start at the same time (worst case: power comes back after an outage and everything tries to start at once). Your battery's continuous output rating should exceed this sum.

Example: Fridge (800W surge) + Furnace blower (2,500W surge) + Well pump (6,000W surge) = 9,300W simultaneous surge. The Tesla Powerwall 3 outputs 11.5 kW continuously, so it handles this comfortably. A FranklinWH aPower 2 (10 kW) also handles it. A single Enphase IQ Battery 5P (7.6 kW per 3-unit stack) does NOT — you'd need 4+ units.

If your surge calculation exceeds your battery's continuous rating, you have two options: (1) add more battery units to increase surge capacity, or (2) install a smart panel (like Span or Lumin) that prevents motors from starting simultaneously.

Step 6: Validate with real-world data

The math above gives you a theoretical answer. The real answer comes from measuring your actual consumption. The best $300 you can spend before sizing a battery is on a home energy monitor that will give you 30 days of real consumption data. See our best home energy monitors guide for specific recommendations.

Three worked examples

Let's apply this framework to three real household profiles:

Example 1: 1,800 sq ft suburban home, NJ, with gas heat

• Loads to back up: Fridge, gas furnace blower, well pump, LED lights, internet, phone charging, garage door
• Calculated daily Wh: 6,800 Wh = 6.8 kWh
• Required capacity (with buffer): 7.9 kWh
• Recommended battery: Single Tesla Powerwall 3 (13.5 kWh) — provides 70% headroom for 24-hour outage
• Cost estimate: ~$12,500 installed, ~$8,750 after 30% federal credit

Example 2: 3,200 sq ft home, FL, with central AC and well pump

• Loads to back up: Fridge, central AC (managed), well pump, lights, internet, phone charging
• Calculated daily Wh: 14,500 Wh = 14.5 kWh
• Required capacity (with buffer): 16.7 kWh
• Recommended battery: Two Tesla Powerwall 3 units (27 kWh) — provides 60% headroom for 24-hour outage
• Cost estimate: ~$22,000 installed, ~$15,400 after federal credit
• Add: Span Panel for AC load management (essential to avoid tripping the system when AC starts)

Example 3: 2,400 sq ft home, CA, with solar and EV

• Loads to back up: Fridge, furnace blower, lights, internet, phone charging, EV (via V2H)
• Calculated daily Wh: 5,200 Wh = 5.2 kWh (essential loads only — V2H handles extended outages)
• Required capacity (with buffer): 6.0 kWh
• Recommended battery: Single Enphase IQ Battery 5P (5 kWh) for short outages + V2H setup using Ford Lightning or Hyundai Ioniq 5 for multi-day backup
• Cost estimate: ~$5,500 for the Enphase + ~$7,000 for V2H install = $12,500 total, ~$8,750 after federal credit