How Many kWh for Whole-Home Battery Backup? A Sizing Guide for Every House

Quick Answer: How Many kWh Do You Need for Whole-Home Backup?

For essential backup only (lights, refrigerator, outlets, WiFi — no HVAC), plan for 10–15 kWh. This gives most homes 24–36 hours of coverage on a single charge. For whole-home backup including central HVAC, plan for 27–40 kWh or more — that's two to three Tesla Powerwalls or equivalent. If you have an all-electric home with an electric water heater and EV charging, you may need 40–60 kWh for full coverage over a 48-hour outage.^[1]

The right answer depends on your home's size, climate, appliances, and what you're willing to run (or not run) during an outage. This guide walks through the math.

Step 1: Understand Your Daily Energy Use

Before sizing a battery, know your baseline. The average US home uses about 29 kWh per day (886 kWh per month according to EIA data).^[2] But that average hides significant variation:

• Small home (under 1,500 sq ft), mild climate: 15–22 kWh/day
• Medium home (1,500–2,500 sq ft), moderate climate: 25–38 kWh/day
• Large home (2,500–4,000 sq ft), hot or cold climate: 40–65 kWh/day
• All-electric home with EV charging: Add 10–40 kWh/day for the vehicle alone

Your utility bill shows your actual monthly kWh. Divide by 30 for your daily average, then by 24 for your hourly average load. This is the number you'll use to size a battery for 24 hours of backup coverage.

Step 2: Essential vs. Whole-Home Backup — Know the Difference

This is the most important decision in battery sizing, and most homeowners don't think about it clearly until they're mid-quote. There are two fundamentally different backup strategies:

Essential Load Backup

An essential load panel powers only your most critical circuits: refrigerator, lights, outlets, internet router, phone chargers, and any medical equipment. HVAC, electric range, and EV charging are typically excluded. A 10–15 kWh battery (one Tesla Powerwall 3 or two Enphase IQ Battery 5P units) handles essential loads for most homes for 24–36 hours.^[3]

Pros: Lower cost, simpler install, smaller battery footprint.
Cons: No heating or cooling during extended outages, no cooking on electric range.

Whole-Home Backup

Whole-home backup means the battery powers everything — HVAC included. This requires a full-panel transfer switch and a much larger battery bank. The math changes dramatically because central air conditioning and heat pumps are the two largest residential loads:

• Central AC (2-ton / 24,000 BTU): 2–3 kWh per hour of operation
• Heat pump (heating mode): 1.5–3 kWh per hour
• Electric furnace (resistance heat): 10–15 kWh per hour — enormously inefficient for backup
• Electric water heater: 4–5 kWh per heating cycle

Running a 2-ton AC for 8 hours uses 16–24 kWh by itself. Add overnight refrigerator, lights, and other loads, and you're easily at 35–50 kWh per day — which is why whole-home backup typically requires at least two Powerwalls or equivalent.^[4]

Step 3: Sizing by House Size and Backup Goal

Use this table as a starting point. These are sizing recommendations for 24 hours of backup coverage in a typical US climate. Extreme heat or cold climates require more capacity:

Home Size	Essential Backup (24 hrs)	Whole-Home w/ AC (24 hrs)	Whole-Home (48 hrs, solar-topped)
Under 1,500 sq ft	10–13 kWh (1 battery)	20–27 kWh (2 batteries)	27–40 kWh (2–3 batteries)
1,500–2,500 sq ft	13–15 kWh (1–2 batteries)	27–40 kWh (2–3 batteries)	40–60 kWh (3–4 batteries)
2,500–4,000 sq ft	15–20 kWh (1–2 batteries)	40–60 kWh (3–4 batteries)	60+ kWh (4+ batteries)

Tip: If you have solar panels, they continuously top up your battery during daylight hours during an outage — dramatically extending your backup duration. A 6 kW solar array on a sunny day generates 24–36 kWh of recharge, which can extend a 13.5 kWh battery to cover several days with essential loads.^[5]

Step 4: What Appliances Draw the Most Power?

Not all appliances are created equal. Here are typical loads for common household appliances to help you decide what to include in your backup plan:

High-Draw Appliances (Think Before Including)

• Electric furnace / resistance heat strips: 10,000–15,000W
• Electric clothes dryer: 5,000–7,500W
• Electric range / oven: 2,000–5,000W
• EV charger (Level 2): 3,800–11,500W
• Central air conditioner (3-ton): 3,500–5,000W
• Electric water heater: 4,000–5,500W

Moderate-Draw Appliances (Usually Manageable)

• Heat pump (heating or cooling): 1,500–3,000W
• Sump pump: 800–1,500W
• Microwave: 900–1,200W
• Window AC unit (5,000 BTU): 500–700W
• Dishwasher: 1,200–1,500W

Essential Appliances (Low Draw, Always Include)

• Refrigerator: 100–200W average (compressor cycles)
• LED lighting (10 bulbs): 50–100W
• WiFi router + modem: 15–25W
• Phone and laptop chargers: 20–65W each
• Medical equipment (CPAP): 30–60W
• Sump pump starter (surge): plan for 3x running watts at startup

The most common mistake in backup sizing is forgetting about surge watts. Motors (AC compressors, well pumps, sump pumps) draw 2–3x their running wattage on startup. Your battery and inverter must handle the surge, not just the steady-state load. Always verify your battery's peak output spec.^[6]

Step 5: Battery Comparison for Whole-Home Backup

The three most popular batteries for whole-home backup in 2026:

Tesla Powerwall 3

13.5 kWh capacity, 11.5 kW continuous output (30 kW peak), built-in solar inverter. The Powerwall 3's high continuous output is what makes it suitable for whole-home backup — it can actually run a 3-ton AC compressor at startup without flinching. Two Powerwalls (27 kWh) handle most homes for 24 hours with AC. Installed cost: approximately $11,500–$15,000 per unit in California.^[7]

Enphase IQ Battery 5P

5 kWh capacity, 3.84 kW continuous output per unit. Stacks in increments of 5 kWh up to a large bank. Each unit has its own microinverter, making the system highly resilient — one failed unit doesn't take down the whole system. For whole-home backup requiring 27 kWh, you'd need five or six units. Pairs best with existing Enphase solar systems. Installed cost: approximately $5,000–$7,500 per unit.^[8]

Franklin WH aPower5.0

5 kWh capacity, 5 kW continuous output per unit (10 kW peak). Franklin WH stands out for its high per-unit output relative to capacity — useful for loads with high surge requirements. Compatible with third-party solar inverters, making it a flexible retrofit option. Installed cost: approximately $5,500–$8,000 per unit.

What Does Whole-Home Backup Actually Cost?

Battery storage is a significant investment. Rough installed costs for whole-home backup systems in California in 2026:

• Essential backup only (1 Powerwall 3): $12,000–$16,000 installed
• Medium whole-home backup (2 Powerwall 3): $22,000–$30,000 installed
• Large whole-home backup (3 Powerwall 3 or 15 kWh Enphase bank): $30,000–$45,000 installed

California homeowners may be eligible for the SGIP rebate (up to $1,000/kWh for Equity Resiliency households), which can dramatically reduce these costs. Use the EnergyScout incentive finder to check programs available in your zip code, or read our California SGIP rebate guide for step-by-step application instructions.

Additionally, California's property tax exclusion (AB 2872) means adding batteries does not increase your assessed home value — lowering the long-term cost of ownership.

Frequently Asked Questions

How many Tesla Powerwalls do I need for whole-home backup?

For most homes under 2,500 sq ft with central air conditioning, two Tesla Powerwall 3 units (27 kWh total) provide 18–24 hours of whole-home backup. Larger homes or all-electric homes with high HVAC loads may need three or four. If you pair with solar, two Powerwalls can sustain indefinitely in good sun. Use the EnergyScout battery advisor for a personalized recommendation.

Can a single home battery power an entire house?

A single 13–14 kWh battery (Tesla Powerwall, Enphase IQ 5P ×3, etc.) can power an entire house for 24 hours on essential loads — refrigerator, lights, Wi-Fi, phone charging, and small appliances. However, it cannot sustain central AC, electric water heater, or electric stove simultaneously for that duration. For true whole-home backup including HVAC, two or more batteries are typically required.

Does battery backup work without solar panels?

Yes. Standalone batteries charge from the grid and discharge during outages or peak-rate periods. Without solar, however, the battery cannot self-replenish during a multi-day outage. Pairing with solar allows the battery to recharge each sunny day, extending backup coverage indefinitely. Learn more in our guide to home batteries without solar.

How do I size a battery for a heat pump vs. a gas furnace?

Heat pumps are far more efficient to back up than resistance electric heat. A heat pump at moderate temperatures draws 1,500–3,000W — roughly 2–4 kWh per hour of operation. A gas furnace only draws power for the blower motor (300–900W), making it ideal to pair with battery backup — the gas still heats even if the grid is down, as long as you back up the electronics. If you have a gas furnace and want to avoid sizing for HVAC, a single Powerwall handles most other loads with ease.

What is the minimum battery size for outage protection?

For a bare-minimum outage kit — keep the fridge cold, lights on, and phones charged — a 5–7.5 kWh battery provides roughly 12–18 hours of coverage for most homes. Products like the Enphase IQ Battery 5P (one unit), EcoFlow DELTA Pro Ultra, or Bluetti AC500 can fill this role. For multi-day coverage or medical equipment dependency, a dedicated home battery system (10 kWh+) is strongly recommended over a portable power station.