A whole house generator sizing calculator converts your home's electrical loads into the generator rating you need to buy, expressed in kilowatts (kW) or kilovolt-amperes (kVA). It works in three connected steps.
What each appliance draws while it operates normally — the baseline for your home's continuous load.
The brief spike a motor-driven appliance pulls on startup, often 2 to 3 times its running load.
The calculator totals your running watts, adds the single largest starting surge, and applies a 20–25% headroom buffer.
The result is the honest number your home needs, not a floor-plan guess. This is the same method every serious sizing source uses. What most tools leave out is the fuel decision, the standby vs portable question, and a path to actually buying the right set. Ours covers all three.
Calculates the surge your generator must handle, not just the running load — so your AC and well pump actually start.
Serves both portable backup buyers and permanent standby installations with automatic transfer switches.
Compares diesel, natural gas, propane, and gasoline. Shows how a micro-air soft-start kit can shrink your AC surge.
Every result maps to a real generator from 8–4,000 kVA and ends in a factory-direct quote with your size pre-filled.
Enter your home details and appliances. The calculator returns running watts, starting surge, required kW and kVA, and recommends a generator size — with fuel and standby vs portable guidance.
Select the loads you truly need during an outage. The calculator will sum running watts, add the largest surge, and size your portable or partial-backup generator.
For permanent standby installation with automatic transfer switch. Enter your home's full electrical profile for a whole-panel sizing recommendation.
Square footage gives a useful first guess, and it's a reasonable check on the calculator. These are typical whole-home standby ranges, in kW.
| Home Size | Essentials Only | Whole-Home (with AC/heat) |
|---|---|---|
| Under 1,000 sq ft | 7.5–9 kW | 10–12 kW |
| 1,000–1,500 sq ft | 10–14 kW | 14–18 kW |
| 2,000–2,500 sq ft | 14–16 kW | 18–24 kW |
| 2,500 sq ft (typical home) | 16–20 kW | ~22–26 kW |
| 3,000–4,000 sq ft | 20–24 kW | 23–36 kW |
| 4,000–5,000 sq ft | 24–30 kW | 36–48 kW |
| 5,000+ sq ft / farm / estate | 30–37 kW | 48+ kW |
Fuel choice shapes the whole installation, so our calculator treats it as an input, not an afterthought. Here is how the common home fuels compare for standby use.
| Fuel | Runtime | Cost to Run | Best For | Watch Out For |
|---|---|---|---|---|
| Natural Gas | Unlimited (utility line) | ~$3–5/hr | Homes with a gas line; no refueling | Fails if the pipeline goes down |
| Propane | Tank-limited | Higher than NG | No gas line, clean burning | Refill logistics and tank cost |
| Diesel | Tank-limited | Mid-range; best $/kWh of liquid fuels | Rural, off-grid, farms, large homes | Fuel storage; emissions hardware on larger sets |
| Gasoline | Tank-limited | High | Cheap portable units | Short fuel shelf life; not standby-grade |
Most convenient standby fuel if you have a gas line. No refueling required. Watch out: fails if the pipeline goes down during a regional outage.
Great for homes without gas lines. Clean burning with long shelf life. Refill logistics and tank cost are the main trade-offs.
Wins for rural properties, farms, and off-grid homes. Starts in ~10 seconds and carries a 20,000–30,000 hour engine life. Dominates worldwide.
Low upfront cost for portable units. Short fuel shelf life and not standby-grade. Best for occasional, short-duration outages.
If natural gas is on your property, it's usually the most convenient standby fuel. But diesel wins for a specific and growing group: rural properties with no gas line, farms and estates with fuel storage, off-grid and semi-off-grid homes, and anyone who wants a backup that starts in about 10 seconds and carries a 20,000 to 30,000 hour engine life. Diesel also dominates whole-house backup across most of the world, where home natural gas infrastructure does not exist. Every diesel size this calculator returns maps to a real model in our 8 to 4,000 kVA range.
Your size only matters once you know how the generator connects to your home. The choice between portable and standby changes everything — from cost to convenience to what your house can run.
Plugs in on demand. You size it to an appliance list, roll it out during an outage, and run extension cords or a manual interlock. It costs a few hundred to a few thousand dollars and covers essential loads, not the whole panel. Use Essentials mode for this path.
Permanently installed, connected through an automatic transfer switch, and starts itself when the utility fails. You size it to the panel, and it covers the whole home automatically. It costs $5,000 to $25,000+ installed, and it's the difference between an outage you manage and an outage the house barely notices.
Our whole house generator sizing calculator sizes both paths. For essentials-only backup, use Essentials mode and pair it with our generator wattage calculator for the appliance-level detail. For whole-home standby, use Whole-Home mode, and we'll quote the diesel set, transfer switch, and enclosure together.
Our engineers see these five errors on nearly every residential sizing job that comes back for help. Avoid all of them and your generator will work when you need it most.
A generator that carries your running load all day can still trip the instant the AC or well pump kicks on. The surge is the number that matters — not the running load.
Two houses of the same size can need very different generators depending on electric heat, AC zones, and pumps. Do the load calculation — never size by floor area alone.
A 1 HP well pump surges to about 4,500 W. Homes on wells undersize constantly because the pump is missing from the appliance list. Check your pump before you order.
A micro-air soft-start kit is cheap compared with buying a generator a full size class bigger. It can cut your AC's starting surge by up to 65%.
An EV charger, a heat pump, or an added workshop load changes the number. Add a 10 to 25% reserve now so you don't have to upgrade in two years.
Not sure your size is right? Our engineers will review your appliance list and confirm the load before you spend anything. Ask our engineers.
Our engineers will review your load calculation, confirm the size, and deliver a factory-direct price — no obligation, no hidden fees. Every quote includes the generator, transfer switch, and soundproofed enclosure for a complete standby solution.
A: For essentials (refrigerator, furnace fan, well or sump pump, lights, internet), about 10–14 kW. For whole-home backup including central AC and the kitchen, plan on 18–24 kW, with 20 kW as the common sweet spot. Run your actual appliance list through the calculator for a firm number.
A: For most homes up to about 3,000 sq ft with one central AC unit, yes, a 22 kW standby covers the whole panel. Two HVAC systems, electric heat, or a well pump push the requirement toward 26–32 kW. Check the whole-home mode result for your exact home.
A: Add the running watts of every appliance you’ll run at once, add the single largest starting surge, then multiply by 1.2 to 1.25 for headroom. Divide by 1,000 to get kW. That is the method our whole house generator sizing calculator applies for you.
A: If you need refrigeration, heating or cooling, and internet during outages and can manage without the rest, a portable sized to essentials is enough. If you want the whole panel covered automatically, or you rely on a well pump or medical equipment, a standby unit with an automatic transfer switch is the right investment.
A: For homes with no natural gas line, and for farms, estates, and off-grid properties, diesel is often the best choice. A diesel standby starts in about 10 seconds, carries a 20,000 to 30,000 hour engine life, and does not depend on the gas utility. It needs fuel storage, and it is the dominant home backup choice in most countries outside North America.
A: Yes. A 30 to 40 kVA diesel set covers a typical whole-home panel including central AC, kitchen, and well pump. We size, build, and ship these as soundproofed standby units to homes worldwide.
A: A portable unit costs a few hundred to a few thousand dollars. A permanently installed standby with transfer switch runs 5,000to5,000to25,000+ installed depending on size, fuel, and site work. A factory-direct diesel set skips dealer markup, and we quote yours with the calculator result pre-filled.
A: Yes. A standby generator connects through an automatic transfer switch that disconnects the home from the utility and starts the generator when the power fails. It is required for safe, code-compliant installation, and we include it in our standby package.
