What Size Generator or Battery Backup Do You Need for a House?

After 20 years wiring homes and sizing backup systems, here’s the truth: most people overbuy. You don’t need to power your entire house during an outage — you need enough capacity to run critical loads until the grid comes back or the sun rises. For an average home, that’s typically 10-15 kWh of battery storage or a 7,500-10,000 watt generator.

I installed my first whole-home battery in 2019 after a three-day outage convinced me the math made sense. Since then, I’ve helped over 200 homeowners right-size their backup systems. Most spend way more than necessary because they’re designing for worst-case scenarios that’ll never happen. Let me show you how to actually calculate what you need.

Start With Your Critical Loads

Forget “whole home backup” unless you’re willing to spend $30,000+. Instead, identify what actually matters during an outage. I’m talking about keeping food from spoiling, staying warm or cool, and keeping devices charged — not running your pool pump.

Here’s what I run on my 13.5 kWh battery during outages:

• Refrigerator (150W running, 600W startup)
• Freezer (100W running, 400W startup)
• Well pump (750W running, 2200W startup)
• Gas furnace blower (600W)
• LED lights (50W total)
• Internet/router (15W)
• Phone charging (20W)
• Laptop charging (65W)

Total running load: about 1,750W. Peak startup surge: around 3,000W. My battery handles this easily for 24+ hours, and my solar panels recharge it during the day.

Generator vs. Battery Backup: Which One?

I get this question constantly. The answer depends on outage patterns in your area and whether you have solar. Here’s the breakdown I give homeowners:

Factor	Generator	Battery Backup
Upfront Cost	$800-$5,000	$10,000-$20,000 (with solar)
Fuel/Energy	Gasoline, propane, or diesel	Grid or solar (free after install)
Runtime	Unlimited (with fuel supply)	8-24 hours per charge cycle
Noise	60-75 dB (very loud)	Silent operation
Maintenance	Oil changes, fuel stabilizer, annual service	Essentially zero
Startup Time	Manual: minutes Automatic: 10-30 seconds	Instant (seamless transfer)
Best For	Extended outages (3+ days), rural areas, no solar	Daily cycling, solar owners, short outages

My take? If you have solar or you’re planning to go solar, batteries make way more sense. You’re already paying for the inverter — adding storage is incremental. If you live rural where outages last days and you have no solar plans, a propane generator is the practical choice.

How to Calculate Your Battery Backup Size

Here’s the actual formula I use when sizing systems, not the marketing fluff you’ll hear from installers trying to upsell you.

Step 1: Add Up Your Critical Load Wattage

Walk your breaker panel and list every circuit you want backed up. Check the nameplate on each appliance or use a Kill-A-Watt meter to measure actual draw. Don’t guess — I’ve seen people overestimate by 300%.

Step 2: Calculate Daily Energy Consumption

Multiply each appliance’s wattage by the hours you’ll run it daily during an outage. A refrigerator runs about 8 hours out of 24 (cycling on/off), so a 150W fridge uses 1.2 kWh per day (150W × 8h = 1,200 Wh).

Example calculation for critical loads:

• Refrigerator: 150W × 8h = 1.2 kWh
• Freezer: 100W × 8h = 0.8 kWh
• Furnace blower: 600W × 4h = 2.4 kWh
• Lights: 50W × 6h = 0.3 kWh
• Internet/devices: 100W × 24h = 2.4 kWh

Total daily usage: 7.1 kWh

Step 3: Add Surge Capacity Buffer

Motors and compressors need 2-3× their running wattage for 2-3 seconds during startup. Your battery needs enough instantaneous power (measured in kW) to handle these surges. If your largest motor is a well pump at 750W running with a 2200W startup surge, your battery needs at least 3 kW continuous power output.

Step 4: Choose Your Battery Size

Take your daily kWh usage and add 30-50% buffer. If you calculated 7 kWh daily usage, you want 10-12 kWh of usable battery storage minimum. This gives you runway for miscalculations and efficiency losses.

Most home battery backup systems come in these sizes:

• 10-13.5 kWh: Standard size, covers critical loads for 12-24 hours (most popular)
• 20-27 kWh: Extended backup or higher loads, 24-48 hours runtime
• 30+ kWh: Near whole-home backup for larger houses or full energy independence

How to Size a Generator for Your House

Generator sizing follows similar logic but focuses on peak wattage instead of energy storage. You need enough running watts to handle your continuous load plus enough starting watts to kick on your largest motor.

Portable Generator Sizing

For most homes running critical circuits, I recommend 7,000-10,000 watt portable generators. This handles a refrigerator, freezer, furnace, well pump, and lights with headroom for surges.

A quality inverter generator in this range costs $800-$2,000 and you can move it between properties. I keep a 9,000W unit in my garage for extended outages when my battery would drain before solar recharges it.

Standby Generator Sizing

Whole-home automatic standby generators range from 12 kW to 24 kW for most residential installs. These run on natural gas or propane and kick on automatically within 10 seconds of power loss.

Sizing guide based on home size:

• 12-16 kW: 1,500-3,000 sq ft homes, covers essentials plus some comfort loads
• 18-20 kW: 3,000-4,500 sq ft homes, handles most circuits except central AC
• 22-24 kW: 4,500+ sq ft homes or full whole-home backup with AC

But here’s what the installers won’t tell you: most people never need the capacity they pay for. A 20 kW standby generator install costs $8,000-$15,000. Unless you’re regularly facing week-long outages, that money goes further toward solar + battery.

Real-World Sizing Examples

Here’s what I actually spec’d for three recent clients — all different situations, all happy with the results.

Example 1: Suburban Family, Grid-Tied Solar

2,400 sq ft home, 4 occupants, gas heat, electric water heater. Average 2-4 hour outages twice a year.

Solution: Single 13.5 kWh battery with existing solar array. Critical load panel backs up fridge, freezer, furnace blower, garage door, internet, and master bedroom circuits. Cost: $12,000 installed. Handles outages easily and provides daily solar arbitrage savings.

Example 2: Rural Home, No Solar Plans

1,800 sq ft home, well water, propane heat, outages last 8-36 hours several times per year.

Solution: 9,000W dual-fuel portable generator with manual transfer switch. Backs up well pump, refrigerator, freezer, furnace, and essential circuits. Cost: $2,500 installed. Client keeps propane and gasoline on hand. Runs about 12 hours per day during outages, costs $20-30/day in fuel.

Example 3: Small Off-Grid Cabin

850 sq ft, solar array with 48V battery bank, no grid connection.

Solution: 20 kWh LiFePO4 battery bank plus 5,000W backup generator for winter. Battery handles daily loads year-round; generator charges batteries during extended cloudy periods. Cost: $15,000 total system. Zero monthly energy bills.

Common Sizing Mistakes I See Repeatedly

After helping 200+ homeowners, these are the mistakes that cost people the most money:

1. Sizing for worst-case peak load instead of actual average usage. Your AC draws 3,500W but you probably don’t need backup AC — heat and fans work fine for a day or two. Size for realistic outage behavior, not summer peak with every appliance running.

2. Forgetting about surge/startup wattage. Your generator might handle 7,000W continuous but can it start your well pump that surges to 2,200W? Check both running and starting watts on the spec sheet.

3. Not accounting for future loads. Adding an EV? That’s 7-11 kW right there. Plan for the next 10 years, not just today.

4. Ignoring local outage patterns. If your grid only goes down 2 hours per year, a $15,000 battery might not pencil out. A $300 portable power station might be enough.

What About Solar Recharging?

This is where battery backup really shines compared to generators. If you have solar, your batteries recharge during the day — unlimited backup as long as the sun’s out.

I designed my system to recharge a depleted 13.5 kWh battery by 2 PM even in winter. My 8 kW solar array produces 25-35 kWh per day, so even after running critical loads during the day, there’s plenty of excess to refill the battery before sunset.

Without solar, you’re dependent on grid recharging or generator charging. Many modern solar battery systems include generator inputs so you can charge batteries from a small portable unit during extended outages — best of both worlds.

Frequently Asked Questions

How long will a 10 kWh battery backup power my house?

Depends entirely on your load. If you’re drawing 1 kW average (refrigerator, lights, devices), you’ll get 10 hours. Drawing 2 kW? Five hours. Most homes with critical loads only pull 1-2 kW during outages, giving you 12-24 hours of runtime from a 10-13 kWh battery. Add solar and you extend that indefinitely during daylight hours.

Can I run my central AC on a battery backup?

Technically yes, practically no unless you have a huge battery bank. Central AC draws 3-5 kW continuously — that’ll drain a 13.5 kWh battery in under 3 hours. I tell clients to use window fans or a small portable air conditioner during outages instead. If AC backup is non-negotiable (medical needs), you need 25+ kWh of storage or a generator.

What’s better: one large battery or multiple smaller ones?

Multiple smaller batteries give you modularity and redundancy, but cost more per kWh. One large integrated unit (like a Tesla Powerwall or similar) is usually more cost-effective for residential installs. I went with a single 13.5 kWh unit because it was $4,000 cheaper than two 7 kWh batteries with the same capacity. Unless you need 20+ kWh, stick with one unit.

Do I need a transfer switch for a generator?

Absolutely yes, no exceptions. You cannot backfeed your panel without a transfer switch — you’ll energize the utility lines and potentially kill a lineman working to restore power. Manual transfer switches cost $300-800 installed for a 6-10 circuit setup. Automatic transfer switches for standby generators run $1,500-3,000. This is not optional — I won’t even quote a generator install without proper transfer equipment.

How much does it cost to install a whole home battery backup?

For a typical 13-15 kWh battery system with installation, expect $10,000-$15,000 without solar, or $12,000-$18,000 added to a solar install. Larger systems (20-30 kWh) run $18,000-$30,000. Prices have dropped 40% since 2019 and keep falling. With the federal solar tax credit, you get 30% back if installed with solar, making the net cost $7,000-$12,000 for most systems.