Home Backup Power Calculator
Pick what you need to keep running during an outage. Get the generator size, battery storage, fuel and estimated cost in real time.
1Choose what to power
Edit any value to match your appliance labels2Outage & system settings
Load management (common with standby generators) lets water heaters, dryers, ranges and EV chargers take turns. Only the largest one counts toward peak.
Average daily solar production during an outage. Use 0 if you have no panels. A 7 kW array makes about 20–30 kWh on a sunny day.
Battery specs (default: 13.5 kWh class)
🏠 Generator
🔋 Home battery
🧳 Portable power station
📊 Where your energy goes
kWh per day, top loads⚙️ Capacity check
The bar shows how much of the system’s rated output your loads use. Running a generator at 30–80% of its rating keeps fuel use efficient.
Estimates for planning only. Appliance draw, startup surge, fuel use and installed prices vary by model and region. Fuel use is modeled from rated size and average load. Check nameplate ratings and get quotes from licensed installers. The 30% federal 25D battery credit ended for installs after Dec 31, 2025.
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Home Backup Power Calculator: How Much Generator or Battery Do You Really Need?
Add up your essential appliances, set your outage length, and get an instant estimate of the watts, kilowatt-hours, generator size, battery capacity, and fuel your home needs to ride out a blackout.
Most homes need 3,000–7,500 running watts to cover essentials (fridge, freezer, sump pump, furnace fan, lights, Wi-Fi) and 15–30 kWh per day of stored energy to run them through an outage. Whole-home backup with central AC typically needs a 14–24 kW standby generator or two to three 13.5 kWh batteries. To size yours, list your appliances, total their running watts, add the single largest startup surge, then multiply watts × hours of use to get kWh.
Home Backup Power Calculator
Check the appliances you want to keep running, adjust quantities, choose how long your outage might last, and hit Calculate. Energy estimates use typical duty cycles (a fridge compressor, for example, runs only part of each hour).
Assumptions: surge = largest single startup demand; generator fuel ≈ 0.11 gal gasoline, 0.16 gal propane, or 14 cu ft natural gas per kWh produced at moderate load (light loads burn more per kWh). Battery count is based on 13.5 kWh usable / 11.5 kW continuous per unit.
Watts vs. kWh: The Two Numbers That Size Every Backup System
Every backup power decision comes down to two separate questions, and mixing them up is the number-one sizing mistake homeowners make.
Power (watts / kW)
How much electricity your appliances pull at the same moment. This sets the size of a generator or a battery's inverter output. Too little and breakers trip.
Energy (kWh)
How much electricity you use over time. This sets battery capacity or how much fuel you burn. Too little and you run dry at 3 a.m.
Surge (starting watts)
Motors (fridges, pumps, AC compressors) briefly draw 2–3× their running watts when they kick on. Your system must handle that spike.
Generator size (W) = (Total running watts + Largest surge extra) × 1.2
Appliance Wattage Chart for Backup Power Planning
Use these typical values when your appliance label isn't handy. Always check the nameplate sticker (usually inside the door or on the back) for exact ratings — older and larger units can run much higher.
| Appliance | Running watts | Starting watts | Typical daily kWh | Relative load |
|---|---|---|---|---|
| Central AC (3-ton) | 3,500 | ~8,000 | 28–42 | |
| Electric water heater | 4,500 | 4,500 | 10–16 | |
| Space heater | 1,500 | 1,500 | 12–18 | |
| Window AC unit | 1,200 | 1,800 | 10–14 | |
| Well pump (½ HP) | 1,000 | 2,100 | 4–6 | |
| Sump pump (⅓ HP) | 800 | 1,300 | 2–6 | |
| Furnace blower (⅓ HP) | 700 | 1,400 | 5–8 | |
| Refrigerator | 150–200 | 800–1,200 | 1.5–2 | |
| Chest freezer | 100 | 500 | 1–1.2 | |
| Oxygen concentrator | 350 | 350 | 8.4 | |
| LED lights (10 bulbs) | 100 | 100 | 0.6–1 | |
| TV + streaming box | 120 | 120 | 0.6 | |
| CPAP machine | 60 | 60 | 0.5 | |
| Wi-Fi router + modem | 20 | 20 | 0.5 |
Notice the pattern: anything that makes heat or cold electrically (AC, water heaters, space heaters, electric ranges, dryers) dominates both power and energy. Leaving those off your critical-loads list is the fastest way to shrink the system you need — and its price.
How to Size Home Backup Power in 5 Steps
Essentials keeps food cold, water flowing, the basement dry, and phones charged. Whole-home means life feels normal, AC included. The gap between them is often 3–4× in cost.
Walk room by room. Don't forget hidden loads: furnace blower, sump or well pump, garage door, medical devices, and the gas water heater's igniter.
Motors rarely start at exactly the same moment, so you only add the biggest one's extra surge (starting minus running watts) to your total — not all of them.
A fridge rated 150 W isn't drawing 150 W all day; its compressor cycles. Multiply watts by hours actually running, then divide by 1,000.
Generators run best at 50–80% of rated output, and batteries need a reserve. Round up to the next available size, then compare quotes.
Worked Example: Sizing Backup for a Typical Suburban Home
A family wants to cover essentials through a 48-hour winter storm outage. Their gas furnace needs electricity for its blower, and they have a sump pump.
| Load | Running W | Surge W | Hours/day running | kWh/day |
|---|---|---|---|---|
| Refrigerator | 180 | 1,000 | 10 | 1.8 |
| Chest freezer | 100 | 500 | 10 | 1.0 |
| Furnace blower | 700 | 1,400 | 10 | 7.0 |
| Sump pump | 800 | 1,300 | 4 | 3.2 |
| Lights, Wi-Fi, TV, phones | 340 | 340 | 8 | 2.7 |
| Totals | 2,120 | +700 largest extra | — | 15.7 |
- Peak demand: 2,120 + 700 = 2,820 W → with 20% headroom, a 3.5–4 kW generator or one battery handles it easily.
- Energy for 48 hours: 15.7 × 2 = 31.4 kWh → with a 20% reserve, about 38 kWh of storage (three 13.5 kWh batteries) — or one battery paired with solar to recharge daily.
- Fuel for 48 hours on propane: roughly 31.4 × 0.16 ≈ 5 gallons at a good load factor. In reality, a large generator idling at light load burns much more, which is why right-sizing matters.
Generator vs. Home Battery vs. Portable Power Station
Standby Generator
Permanently installed, runs on natural gas or propane, starts automatically. Best for long, multi-day outages and whole-home loads.
Unlimited runtime on NGHandles ACNoise & exhaustAnnual serviceHome Battery
Wall-mounted lithium storage that switches over instantly and silently. Shines when paired with solar, which recharges it every sunny day.
Silent, no fumesInstant switchoverFinite kWhHigh upfront costPortable Gen / Power Station
Lowest-cost entry point. Portable gas generators need a transfer switch or interlock; power stations run fridges and electronics indoors.
AffordableFlexibleManual setupLimited capacity| Factor | Standby generator | Home battery (13.5 kWh) | Portable generator | Power station |
|---|---|---|---|---|
| Typical output | 10–26 kW | ~11.5 kW per unit | 3–12 kW | 1–4 kW |
| Runtime | Days (fuel supply) | Hours–1 day per unit; days with solar | 8–12 hrs per tank | Hours |
| Switchover | Automatic, ~10–30 sec | Automatic, near-instant | Manual | Manual / plug-in |
| Noise | Moderate | Silent | Loud | Silent |
| Indoor safe? | No (outdoor only) | Yes (per code) | Never | Yes |
| Best for | Long outages, whole home | Frequent short outages, solar homes | Budget essentials | Apartments, electronics, CPAP |
How Much Does Home Backup Power Cost in 2026?
Prices vary widely by region, electrical panel condition, permits, and fuel line or trenching work. These are typical installed ranges to help you budget before getting quotes.
| Option | Typical installed cost | Cost scale |
|---|---|---|
| Portable power station (1–3 kWh) | $500 – $3,000 | |
| Portable generator + interlock/inlet | $1,500 – $4,000 | |
| Air-cooled standby generator (14–24 kW) | $9,000 – $20,000 | |
| Single home battery, e.g. Powerwall 3 (13.5 kWh) | $11,500 – $16,500 | |
| Two-battery whole-home system (27 kWh) | $20,000 – $30,000 |
The Section 25D Residential Clean Energy Credit, which covered 30% of qualifying home battery (3 kWh+) and solar costs, was terminated by Public Law 119-21 for expenditures made after December 31, 2025. Systems placed in service by that date could still be claimed on 2025 returns, and unused credit can carry forward. For new 2026 installs, check state programs, utility rebates, and virtual power plant (VPP) payments, which can still offset part of a battery's cost.
Expert Tips for Sizing Backup Power Right
Your utility bill or smart-meter portal shows real daily kWh. Divide your lowest-usage month's total by 30 for a realistic essentials baseline.
A soft-start kit can cut a central AC's startup surge by half or more, often letting you run cooling from a smaller generator or a single battery.
Moving essential circuits to a dedicated subpanel lets a smaller system cover what matters and keeps big loads from draining it.
Size for summer heat waves if you depend on AC, or winter storms if you rely on a furnace blower or well pump. Outage type drives load type.
A 22 kW unit powering a 3 kW load wastes fuel and can suffer "wet stacking." Matching size to load saves money every hour it runs.
A battery alone is a fixed tank; solar turns it into a refillable one, extending backup from hours to days when the sun cooperates.
Backup Power Safety Essentials
Never operate a fuel generator inside a home, garage, basement, or crawlspace, even with doors open. Place it well away from windows, doors, and vents, and install battery-backed CO alarms on every level of your home.
- Never backfeed through a dryer outlet. A "suicide cord" can energize utility lines and electrocute line workers. Use a transfer switch or a code-approved interlock installed by a licensed electrician.
- Get permits. Standby generators and home batteries typically require electrical (and sometimes gas) permits and inspection.
- Store fuel safely. Keep gasoline in approved containers away from living spaces and rotate it with stabilizer every few months.
- Exercise your system. Standby generators self-test weekly; portable units should be started monthly so they work when you need them.
Frequently Asked Questions
What size generator do I need to run my whole house?
Most average homes need a 14–22 kW standby generator for whole-home backup including central AC; larger homes with electric heat, multiple AC units, or EV chargers may need 24 kW or more. Use the calculator above with everything checked to estimate your peak load, then add 20% headroom.
How many kWh do I need for backup power?
Essentials-only backup usually needs 10–20 kWh per day; whole-home backup with AC can exceed 50 kWh per day. Multiply your daily kWh by the number of outage days, then add a 20% reserve to find the battery capacity you'd need without solar recharging.
How long will a 13.5 kWh home battery last during an outage?
Divide usable capacity by your average load. At a 1 kW essentials load, a 13.5 kWh battery lasts about 13 hours; at 500 W, roughly a full day. Running central AC can drain it in 4–6 hours. Solar recharging can extend backup indefinitely in good weather.
Can a home battery run central air conditioning?
Newer high-output batteries (around 11.5 kW continuous) can start and run many central AC systems, especially with a soft starter. The limiting factor is usually energy, not power — AC drains stored kWh quickly, so plan for multiple batteries or solar if cooling is essential.
Is a generator or battery better for home backup?
Generators win for long, multi-day outages and lower cost per kW, especially on natural gas. Batteries win for silent, fume-free operation, instant switchover, daily bill savings with solar, and frequent short outages. Many homeowners combine a battery for short events with a portable generator for long ones.
How much fuel does a home generator use per day?
At moderate load, a generator burns roughly 0.1 gallon of gasoline or about 0.16 gallon of propane per kWh produced. An essentials load of 15 kWh/day works out to about 1.5 gallons of gas or 2.4 gallons of propane per day, but light loads on oversized generators can double that.
Is there still a tax credit for home backup batteries in 2026?
No new federal credit applies. The 30% Section 25D credit ended for expenditures after December 31, 2025. Standby generators never qualified. Look for state incentives, utility rebates, and virtual power plant programs instead — and talk to a tax professional about any carryforward from a 2025 install.
Do I need a transfer switch for a portable generator?
If you want to power hardwired circuits like a furnace or well pump, yes — you need a transfer switch or a code-approved panel interlock with a power inlet, installed by a licensed electrician. Extension cords are only safe for plug-in appliances.
Know your numbers before you get quotes
Run the calculator with your real appliance list, save the CSV, and bring it to installers. Quotes are faster and more accurate when you already know your watts and kWh.
Calculate My Backup Power ⚡Disclaimer: This calculator and guide provide general estimates for planning purposes only. Actual appliance draw, surge, fuel use, and installed prices vary. Consult a licensed electrician for installation and a tax professional for incentive eligibility. Incentive information reflects federal law as of October 2026.
