How to Size Backup Power for Your Home

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Sizing backup power for a home outage is where most people get it wrong. Too small — you buy a 500-watt power station and it can’t run your fridge for a full day. Too big — you spend $8,000 on a whole-home generator when a $1,800 portable would have covered every actual outage you’ve ever had.

This guide walks through the sizing math for real emergency scenarios: which loads must run, which can go dark, and how to match the right size of backup power to the outages your specific home faces.

The core sizing question

Backup power sizing comes down to two numbers:

1. Continuous wattage: the maximum instantaneous load you need to run. Determined by the largest appliance you’ll operate simultaneously.
2. Total energy (kWh): how much energy you need over the outage duration. Determined by your daily energy consumption × expected outage length.

Both numbers matter. A generator with high wattage output but low fuel capacity will exhaust your fuel quickly. A battery with high capacity but low output can’t start a well pump. Get both dimensions right.

Step 1 — Prioritize your loads

Every home has three tiers of electrical loads:

Critical (must-run)

• Refrigerator + freezer (food preservation).
• Medical equipment (CPAP, oxygen concentrator, home dialysis if applicable).
• Well pump (if on well water).
• Sump pump (if in flood-prone basement).
• Heat source (furnace fan for gas heat, mini-split for electric heat, at minimum in freezing weather).
• Router/modem/networking (increasingly critical for communication).
• Emergency lighting.

Important (should-run)

• Small kitchen appliances (microwave for reheating).
• Phone charging.
• Laptop charging.
• TV/entertainment (mental health matters during long outages).
• Coffee maker (do not underestimate).

Nice-to-have (can go without)

• Central air conditioning (a portable window unit in one room is a compromise).
• Electric oven / stove (use gas grill or camp stove instead).
• Dryer, dishwasher, washing machine.
• Non-critical outlets throughout the house.

Only critical + a subset of important loads need backup power. Trying to run everything wastes budget on generator or battery capacity you’ll never actually use.

Step 2 — Sum critical wattage

Get wattage from each appliance label (or an online database). Sum only what runs simultaneously.

Typical household critical load sum:

• Fridge (running): 150W (idles at 15-30W, cycles up to 500W on compressor start)
• Freezer: 100W (chest) or 200W (upright)
• Furnace fan (gas heat): 500W
• Well pump: 800-1,500W running, 3,000W+ starting surge
• Sump pump: 800-1,200W running, 2,500W starting surge
• CPAP: 30-60W
• Router + modem: 30W
• LED lighting (5 rooms): 50W

Add up the largest simultaneous combination. A gas-heat home with a well: fridge (150) + freezer (100) + furnace (500) + well pump running (1,000) + CPAP (60) + router (30) + lights (50) = 1,890W continuous. Peak surges from well pump start could hit 4,000W momentarily.

Step 3 — Size continuous output

Match your backup power source’s continuous output rating to your summed critical load. Add a 20% safety margin. Add a separate check on your source’s surge rating (usually 1.5-2x continuous) to cover motor start-up.

For the example above (1,890W continuous, 4,000W surge):

• Portable generator: 3,000-4,000W continuous rating, 5,000W+ surge. Common consumer generators like the Honda EU3200i or Champion 4000W inverter fit.
• Solar generator / battery: 2,400W+ continuous inverter rating, 3,600W+ surge. EcoFlow Delta Pro (3,600W continuous, 4,500W surge with X-Boost) or Bluetti AC300 (3,000W continuous, 6,000W surge) work.
• Whole-home standby generator: 8-12 kW natural gas / propane unit. Big enough that continuous rating is not the constraint.

Below-sized units will trip the overload cutoff when the well pump starts. Above-sized units waste fuel or battery through inverter idle losses.

Step 4 — Size total energy for outage duration

Continuous wattage tells you what fits online at once; total energy tells you how long the system runs before refueling or recharging.

Estimate daily energy from critical loads. Use a percentage of continuous wattage rather than assuming 100% duty cycle:

• Fridge: 150W × 8 hours/day (~1/3 duty cycle × 24 hours) = 1,200 Wh/day
• Freezer: 100W × 8 hours = 800 Wh/day
• Furnace fan: 500W × 6 hours (winter cycling) = 3,000 Wh/day
• Well pump: 1,000W × 30 min/day (typical household water use) = 500 Wh/day
• CPAP: 40W × 8 hours = 320 Wh/day
• Router + laptops + phone charging: 500 Wh/day
• Emergency lighting: 50W × 6 hours = 300 Wh/day

Daily total for this example: ~6,600 Wh/day. Multiply by expected outage duration:

• 2-day outage: 13,200 Wh = 13.2 kWh
• 1-week outage: 46 kWh
• 2-week outage: 92 kWh

Step 5 — Choose the backup power source

Match energy requirement to source:

Solar generators / battery-only systems

Best for 1-3 day outages with critical-load-only support. A 3.6 kWh EcoFlow Delta Pro with 400W solar covers roughly one day of essentials, with solar top-up extending indefinitely in reasonable weather. Multiple batteries stack for longer support. Silent, indoor-safe, no fuel logistics.

Break-even: 1-3 day outages a few times per year. See best portable power stations for emergencies for specific unit picks.

Portable fuel generators

Best for 1-7 day outages with medium-to-heavy loads. 3,000-8,000W continuous units. Requires stored fuel (10-30 gallons at minimum, with fuel stabilizer). Loud, outdoor-only operation, carbon monoxide risk if misused.

Break-even: multi-day outages annually, need to run heavier loads (window AC, well pump) than a battery can sustain. See emergency backup power comparison for the fuel-vs-battery decision framework.

Whole-home standby generators

Best for extended outages (1-2+ weeks) with full-house support. 12-22 kW natural gas or propane units, permanently installed, automatic transfer switch. Continuous operation as long as fuel supply lasts (natural gas = indefinite for most municipalities).

Break-even: frequent multi-day outages (2+ per year), home-based work that can’t tolerate downtime, home-based medical needs, or high-value property where insurance discounts materialize.

Hybrid setups

Combine sources for redundancy. Common patterns:

• Solar generator for daily silent essentials + fuel generator kept for extended events.
• Whole-home natural gas generator for baseline + solar generator for indoor-safe daytime use.
• Solar-plus-battery permanent system with generator connection for winter extended outages.

See our related solar + generator hybrid setups guide for detailed configuration options.

Fuel-generator fuel sizing

For gasoline/diesel generators, fuel storage matters as much as generator wattage. Rule of thumb:

• 3,000-5,000W generator at 50% load: ~0.5 gallon/hour = 12 gallons/day.
• 7,000-10,000W generator at 50% load: ~0.75-1.0 gallon/hour = 18-24 gallons/day.
• 1-week outage on a 5,000W generator: 84 gallons — that’s 6 x 14-gallon cans, an unusual quantity to store safely.

Propane and natural-gas generators solve the fuel-storage problem. Propane tanks (100-500 gallon) sit for years without degradation; natural gas comes from municipal supply. See fuel storage safety for emergency power for storage rules and shelf life.

Common sizing mistakes

• Ignoring motor starting surge. Well pumps, sump pumps, and refrigerator compressors can spike 2-3x their running wattage for 1-3 seconds during startup. Undersized inverters will trip.
• Adding up nameplate wattage instead of measured wattage. Nameplates are peak; actual consumption is often 30-70% of nameplate. Kill A Watt meter costs $25 and pays for itself in accurate sizing.
• Sizing for full-house load instead of critical loads. A 20 kW whole-home generator is overkill for a family that only runs fridge + freezer + furnace + lights during outages.
• Forgetting the transfer switch or interlock. Backfeeding through a regular outlet is illegal, unsafe, and kills line workers. Required: manual transfer switch, interlock kit, or automatic transfer switch.
• Only planning for summer scenarios. Winter outages have different load profiles (furnace/heat pump, less AC) and shorter solar-charging windows.

Sizing worksheet — quick version

For fast sizing without spreadsheets:

1. List critical loads and their running wattage. Add.
2. Add 20% margin. This is your minimum continuous wattage rating.
3. Multiply the largest single-motor load by 2x. This is your minimum surge rating.
4. Estimate daily kWh: rough rule = 3-5 kWh/day for essentials-only, 10-15 kWh/day for essentials plus comfort loads.
5. Multiply daily kWh × expected outage duration = total energy required.
6. Match backup power source to both continuous wattage AND total energy.

Realistic sizing for common home profiles

• Apartment renter, gas heat, no medical: 500-1,000 Wh solar generator + 100W panel. Runs router, lights, phone charging. $300-$600.
• Suburban home, gas heat, no well: 2,000-3,600 Wh solar generator + 400W panels OR 3,000-4,000W portable fuel generator. Runs fridge + furnace + electronics. $1,500-$3,000.
• Rural home, well water, medical equipment: 7,000-10,000W portable fuel generator OR whole-home 12 kW standby. Runs well pump + heat + medical continuously. $2,500-$8,000.
• Remote off-grid property: hybrid solar + battery + fuel generator. 8-15 kWh battery, 3-6 kW inverter, 3,000-5,000W backup generator. $8,000-$25,000.

Key takeaways

• Sizing has two dimensions: continuous wattage (what runs at once) and total energy (how long).
• Prioritize loads into critical / important / nice-to-have; only critical must-run all outage.
• Add motor-start surge to your continuous math (2-3x running wattage for pumps and compressors).
• Match the backup power type to duration: solar/battery for 1-3 days, portable fuel for 1-7 days, whole-home standby for 1-2+ weeks.
• Never backfeed through a regular outlet — use a transfer switch or interlock kit.

FAQ

What size generator do I need to run my whole house? For continuous whole-house operation including HVAC, expect 20-24 kW natural gas/propane. For essentials + some comfort loads, 8-12 kW is usually enough. Whole-house electric heat or all-electric homes need higher — often 24 kW+ or a battery-hybrid setup.

Can I run my well pump on a battery/solar generator? Yes if the battery inverter has enough surge capacity — well pumps commonly need 2,500-4,000W surge. A 3,600W continuous / 4,500W surge unit (EcoFlow Delta Pro with X-Boost) handles most residential well pumps. Smaller battery units cannot start a well pump.

Is a bigger generator always better? No. Oversized generators consume more fuel at low load (fuel efficiency curves peak around 50-70% load), cost more upfront, and waste capacity if you never need it. Size to your actual outage-time load, not the theoretical worst case.