What size backup power does your house actually need?
Vendor-neutral sizing in about two minutes. Real NEC-style load math, then three honest options side by side: standby generator, stacked batteries, or a portable with an interlock — with verified costs and runtime for your outages.
Surge is set by: Central AC (3 ton) · running load is NEC 220.83-style demand-factored
Our recommendation
Standby generator
Your outage profile and loads favor an automatic standby unit (with an LP tank).
Your three options, side by side
Standby generator
Recommended18 kW air-cooled standby generator
$9,000–$16,000installed*
- Class example: Generac Guardian 18 kW (7226/7228)
- Covers 10.9 kW running at ≤80% of rating; 17.6 kW surge within rating
Runtime for your outages
- No natural gas at the meter and no LP tank selected — a standby unit would need an LP tank installed (commonly 250-500 gal).
Budget for an LP tank install — no fuel source selected.
- Starts automatically — works when you're away
- Multi-day runtime from an LP tank
- Carries HVAC and heavy 240 V loads
- Highest installed cost of the three paths
- Needs annual service ($250-600/yr contract or DIY)
- Audible while running (60-67 dB(A) class)
Battery backup
4× Tesla Powerwall 3 + 12 expansion packs
No verified price at this scale
- 216.0 kWh usable · 46.0 kW continuous
- Panel connection: manual transfer switch/inlet ($400-1500 installed) or the vendor's home panel
Runtime for your outages
- ~216.0 kWh usable ÷ your duty-cycled average draw of ~5.7 kW ≈ 38 hours of runtime.
4-unit installed systems are installer-quote territory — no verified price range at this scale.
- Silent and emission-free — runs indoors, no HOA issues
- Automatic switchover in milliseconds
- No fuel logistics; solar-rechargeable
- Energy-bounded: long outages need recharge or load shedding
- Cost scales steeply with stored energy
- Largest 240 V loads may need multiple units
Portable + interlock
9.5-12.5 kW portable + interlock
$1,100–$2,550installed*
- Example unit: Westinghouse WGen9500DF (example unit) — 9.5 kW running / 12.5 kW peak on gasoline
- Interlock kit + inlet on your main panel lets it feed the house safely (no suicide cords)
Runtime for your outages
- At your load (~100% of a 9.5 kW unit): roughly 1.65 gal of gasoline per hour — a 5-gal can lasts about 3 hours.
- Running around the clock that is ~40 gal/day: plan on storing fuel and refueling several times a day, in any weather.
- Manual operation: you start it, plug in the cord, and flip the interlock — no automatic transfer.
Your load (~10.9 kW running / 17.6 kW surge) is beyond a single open-frame portable — this path stops making sense here.
- Cheapest whole-path cost by a wide margin
- Dual-fuel units run on gasoline or LP cylinders
- Doubles as jobsite/camping power
- Manual everything: wheel out, start, cord, interlock, refuel
- Fuel storage and mid-storm refueling are on you
- No automatic protection while you're away
Disclosure: some links below are affiliate or referral links — if you buy or request a quote through them we may earn a commission, at no cost to you. It never changes the math. How we make money.
Every assumption we made for you (5)
- Whole-house running load uses the NEC 220.83(A)-style optional method: first 8 kW at 100%, remainder at 40% (simplification — see methodology).
- Lighting/plugs counted at 3 VA per sq ft (NEC 220.12), 25% assumed in use for runtime math.
- Runtime energy uses per-appliance duty-cycle factors (documented on the methodology page).
- Plug-in battery capacity counted at 85% of nameplate (inverter losses/reserve) — vendors publish nameplate only.
- Generator sizing uses nameplate (LP) ratings; the natural-gas derate is shown, not silently applied.
Educational estimate using a simplified NEC 220.83 method — final sizing must be confirmed by a licensed electrician.
How this calculator works (and why it exists)
Every sizing tool I could find sells something. Generac's tool sizes you into a Generac. Lowe's sizes you into their SKUs. Half the "generator sizing calculator" results are industrial kVA tools for job sites. Nothing compared the three ways homeowners actually solve this — so I built the math myself. BackupSizer recommends a category and a size, not a brand.
Running load. In whole-house mode the calculator sums your connected loads — lighting and plugs at 3 VA per square foot per NEC 220.12, plus each appliance at its verified running wattage — then applies the NEC 220.83(A)-style optional-method demand factors: the first 8 kW at 100%, the remainder at 40%. That factoring is why a house with 40 kW of connected load really needs about 21 kW of generator, not 40. In essentials mode there is no factoring: loads that run continuously are summed straight, and short-cycle loads like the microwave feed the peak check instead.
Starting surge. Motors draw several times their running watts for the first fraction of a second. The calculator applies the largest-motor rule: running load plus the biggest single motor-start delta. Air-conditioner starting draw comes from manufacturer locked-rotor amp (LRA) tables using the 30% voltage-dip method — a 3-ton condenser at 91.9 LRA computes to roughly 10.8 kW of starting demand. A soft starter cuts that 50-70% (Micro-Air's published spec), which is frequently the difference between an 18 kW and a 14 kW unit.
Three paths, honestly compared. Standby sizing applies the 80% continuous-loading rule against the air-cooled ladder (10-26 kW) and won't hide the natural-gas derate. Battery sizing solves both constraints — continuous kilowatts and stored kilowatt-hours for your outage length — against current spec sheets (Powerwall 3, EcoFlow DELTA Pro Ultra, Anker SOLIX F3800 Plus, Bluetti Apex 300). The portable path checks your load against a real 9.5 kW open-frame unit and prices the interlock install. When a path can't honestly cover you — batteries against three days of strip heat, say — the tool says so instead of rounding up the order.
Every wattage, spec, and price range traces to a source captured on 2026-08-02 — manufacturer spec sheets first, retailer listings and cost guides where manufacturers don't publish. The full formula set, duty-cycle table, and source list live on the methodology page. Built by The BackupSizer Team — background on the about page.
Common questions
What size generator do I need for a 2,000 sq ft house?
Square footage alone can't answer it — the loads decide. A 2,000 sq ft house with gas heat and a 3-ton AC typically computes to a 9-11 kW demand-factored running load and lands on a 14-18 kW standby unit (the AC's starting surge decides which). The same house with electric heat can need 24 kW or more. Run the calculator above with your actual loads.
Will a 22-24 kW generator run a whole house?
Usually, yes — with the NEC 220.83 demand-factored method, most gas-heated homes under 4,000 sq ft compute below 17.6 kW of running load (80% of 22 kW). All-electric homes are the exception: strip heat, water heating, and range can push past even 26 kW, which is when installers add load-managing transfer switches.
Can a portable generator power a whole house?
A 9.5-12.5 kW open-frame portable feeding the panel through an interlock can run essentials plus a fair amount more — but not an all-electric home, and you operate it manually: wheel out, start, plug in, refuel every few hours. At roughly $1,100-2,600 all-in it is the budget path, not the convenient one.
Can a battery like the Powerwall 3 replace a generator?
For short outages, often yes: one Powerwall 3 delivers 11.5 kW continuously and stores 13.5 kWh. The limit is energy, not power — multi-day outages with electric heat drain any residential stack. Our calculator does that math honestly: it will tell you when batteries fit and when they don't.
How much does a whole-house generator cost installed?
As of August 2026, installed totals cluster at $7,000-11,000 for 10-14 kW units, $9,000-16,000 for 18-24 kW, and $13,000-19,500 for 26 kW — equipment plus transfer switch, gas line, pad, permit, and labor. High-cost metro areas run higher.
Do I need an electrician or a permit?
Yes, for every path that connects to your panel. Standby installs need permits, gas work, and an electrician; an interlock or transfer switch for a portable or battery is electrician work too (typically $400-850). This calculator produces an educational estimate — final sizing must be confirmed by a licensed electrician.