How to Size a Home Battery Backup System: The Right Way (2026)

The biggest mistake homeowners make when buying a battery backup system is guessing the size they need. Buy too small, and you run out of power 6 hours into a 12-hour outage. Buy too large, and you've spent $10,000 more than necessary.

This guide walks through the math step by step so you know exactly what you need before you talk to a single installer.

Step 1: Define Your Backup Goals

Before calculating anything, answer these three questions:

1. What loads do you need to back up?

• Essential only (fridge, lights, phone, router, sump pump) — most efficient, lowest cost
• Essential + comfort (+ window AC or space heater) — moderate cost
• Whole home (everything including central HVAC, EV charger, electric range) — maximum cost

2. How long do you need to last?

• 4–8 hours — covers most common utility outages
• 12–24 hours — covers overnight outages and severe weather
• 48–72 hours — covers extended storm events (requires solar to recharge or large battery bank)

3. Do you have or plan to add solar? With solar, you can recharge during the day and dramatically extend backup duration. Without solar, you're limited to what the battery holds when the outage starts.

Step 2: List Your Critical Loads

Here are typical wattages for common household loads:

| Appliance | Running Watts | Startup Surge | |-----------|--------------|---------------| | Refrigerator | 100–200W | 400–800W | | Freezer | 100–150W | 400–600W | | LED lighting (per room) | 20–40W | None | | Wi-Fi router | 10–20W | None | | CPAP machine | 30–60W | None | | Smartphone charger | 5–20W | None | | Laptop | 45–90W | None | | Sump pump | 400–800W | 1,200–2,400W | | Window AC (8,000 BTU) | 700–900W | 2,100–2,700W | | Window AC (12,000 BTU) | 900–1,200W | 2,700–3,600W | | Mini-split AC (1-ton) | 700–1,200W | 1,200–1,800W | | Central AC (2-ton) | 1,500–2,000W | 4,500–6,000W | | Central AC (3-ton) | 2,500–3,500W | 7,500–10,500W | | Gas furnace (fan only) | 400–700W | 800–1,400W | | Electric water heater | 3,500–5,500W | None | | Microwave | 900–1,200W | None | | Electric range (one burner) | 1,200–2,500W | None | | EV charger (Level 2) | 3,800–7,200W | None |

Step 3: Calculate Your Daily kWh Needs

For each load you want to back up: Watts × Hours per day ÷ 1,000 = kWh per day

Example: Critical Loads Only

| Load | Watts | Hours/Day | kWh/Day | |------|-------|-----------|---------| | Refrigerator | 150W | 24 (cycles ~30%) | 1.1 kWh | | LED lighting (4 rooms) | 120W | 6 | 0.72 kWh | | Wi-Fi router | 15W | 24 | 0.36 kWh | | Phone chargers (×4) | 40W | 4 | 0.16 kWh | | Sump pump | 600W | 0.5 (intermittent) | 0.30 kWh | | CPAP (if applicable) | 50W | 8 | 0.40 kWh | | Total | | | 3.04 kWh/day |

Add 15% for battery round-trip efficiency losses: 3.5 kWh/day actual draw

For 12 hours: ~1.75 kWh For 24 hours: ~3.5 kWh For 48 hours: ~7 kWh

Even a 5 kWh battery (Enphase IQ Battery 5P) covers critical loads for 24+ hours.

Example: Critical Loads + Window AC

| Load | Watts | Hours/Day | kWh/Day | |------|-------|-----------|---------| | Critical loads (from above) | — | — | 3.04 kWh | | 10,000 BTU window AC | 1,000W | 8 (50% duty) | 4.0 kWh | | Total | | | 7.0 kWh |

With 15% efficiency buffer: 8.1 kWh/day

For 12 hours: ~4 kWh (one Enphase IQ Battery 5P just covers it) For 24 hours: ~8 kWh (two Enphase IQ Battery 5Ps or one Powerwall 3)

Example: Whole Home (Including Central AC)

| Load | Watts | Hours/Day | kWh/Day | |------|-------|-----------|---------| | Critical loads | — | — | 3.04 kWh | | 3-ton central AC | 3,000W | 8 (50% duty) | 12.0 kWh | | Refrigerator (already counted) | — | — | — | | Lights/misc | 500W | 6 | 3.0 kWh | | Total | | | 18 kWh/day |

With 15% efficiency buffer: 20.7 kWh/day

For 8 hours: ~7 kWh (one 13.5 kWh Powerwall handles it, barely) For 12 hours: ~10.3 kWh (tight for one Powerwall) For 24 hours: ~20.7 kWh (two Powerwalls)

Step 4: Check the Power Output (Not Just Capacity)

This step is where most sizing guides stop too early. Capacity (kWh) tells you how long it lasts. Power output (kW) tells you what it can run simultaneously.

Key power output specs to check:

| System | Continuous Power | Can It Run Central AC? | |--------|-----------------|------------------------| | Enphase IQ Battery 5P (1 unit) | 3.84 kW | No (3-ton AC needs 3.5+ kW) | | Generac PWRcell 9 | 4.5 kW | Borderline (2-ton AC only) | | Tesla Powerwall 3 | 11.5 kW | Yes (up to 3-ton) | | FranklinWH aPower + aGate | 12.0 kW | Yes (up to 3.5-ton) |

If your central AC draws 3,500W and your battery only outputs 3,840W continuous, you'll have no headroom for anything else when AC is running.

Rule of thumb: Your battery's continuous power output should be at least 20% higher than your largest single load.

Step 5: Account for Startup Surges

Compressor motors (AC units, refrigerators, well pumps) surge to 3–5× their running wattage for 1–3 seconds on startup. Your battery's inverter must handle this peak or the circuit trips.

Example: A 3-ton central AC running at 3,500W has a startup surge of up to 10,500W. Only inverters with a high surge rating (the Tesla Powerwall 3 handles 185A / ~22kW surge) will reliably start this load.

If you're sizing for central AC backup, always confirm the battery's peak/surge rating with your installer.

Quick Sizing Reference

| Goal | Recommended Minimum | Example System | |------|--------------------|--------------------| | Critical loads, 12 hours | 5 kWh | Enphase IQ Battery 5P | | Critical loads, 24 hours | 8–10 kWh | 2× Enphase IQ Battery 5P | | Critical loads + window AC, 12 hours | 8–10 kWh | Generac PWRcell 9 | | Whole home (no AC), 12 hours | 10–15 kWh | Tesla Powerwall 3 | | Whole home (with central AC), 12 hours | 20–30 kWh | 2× Tesla Powerwall 3 | | Whole home (with central AC), 24 hours | 40+ kWh | 3× Powerwall 3 + solar |

Common Sizing Mistakes

1. Forgetting startup surge. Calculating only running watts and buying a system that can't start the AC.

2. Sizing for average use, not peak simultaneous load. During an outage everyone is home, appliances run at the same time, and it's usually extreme weather.

3. Not accounting for efficiency losses. Every battery has round-trip losses of 10–13%. A 13.5 kWh battery delivers about 11.7–12.1 kWh of usable energy.

4. Ignoring the recharge source. Without solar, when the battery runs out in a multi-day outage, you have no backup. Plan for how you'll recharge if the grid stays down for 3+ days.

Now that you know what size you need, see our home battery backup without solar guide to compare specific systems, and our complete cost breakdown to budget your project.