Quick answer: Add up the running watts of everything you want to power, add the single largest starting-watt surge from your motor loads, then add about 20-25% margin. Essentials (fridge, lights, a few outlets, furnace blower) need roughly 5,000-7,500 watts; running central AC and a well pump pushes you to a 10-22 kW+ standby generator. Size it from your actual loads with the generator sizing calculator.
Running watts vs starting watts
This is the single most important idea in generator sizing. Every load has two numbers:
- Running watts (rated watts) - the continuous power a device draws while it is operating.
- Starting watts (surge watts) - the brief, much larger spike a motor draws at the instant it starts, often 2 to 3 times its running watts.
A generator has to supply the total running watts of everything on plus the largest single starting surge at the same moment. That surge - usually from an air conditioner, well pump, refrigerator, or sump pump - is what pushes the required size up, and it is why two houses with the same running load can need very different generators.
Step 1: List your loads
Decide what you want to power in an outage and write down the running and starting watts of each. Typical figures:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 600 - 800 | 1,200 - 2,200 |
| Sump pump (1/2 hp) | 1,050 | 2,150 |
| Well pump (1 hp) | 2,000 | 4,000 |
| Furnace blower (1/2 hp) | 800 | 2,350 |
| Central AC (3 ton) | 3,500 | ~5,000-11,000 |
| Window AC (10k BTU) | 1,200 | 1,800 |
| Water heater (electric) | 4,500 | 4,500 |
| Lights (LED, whole floor) | 150 - 400 | same |
| Microwave | 1,000 | 1,000 |
| Range/oven element | 2,000 - 5,000 | same |
Note that resistive loads (heaters, water heaters, ranges, lights) have no starting surge - their starting and running watts are the same. Only motors surge.
Step 2: Do the math
The generator's minimum size is:
Total running watts + largest single starting surge
Add up the running watts of everything that could be on at once, then add just the biggest starting surge among your motors (not all of them - they rarely start at the same instant). Then add a safety margin of about 20-25% so the generator is not run flat-out continuously, and round up to the next standard size. Convert watts to kW by dividing by 1,000.
Step 3: Choose whole-house or essential circuits
There are two philosophies, and they lead to very different sizes and prices:
- Essential circuits. Power a chosen set of important loads - fridge, some lights, outlets, furnace blower, sump pump. This keeps the generator small (often a portable of 5,000-7,500 W or a small standby of 8-12 kW). A load-management system can automatically shed big loads so an even smaller unit copes.
- Whole house. Power everything without thinking, including central AC, well pump, and range. This needs a standby generator sized for the full demand, commonly 18-26 kW for an average home, more for electric heat.
Your choice depends on budget, whether you have big motor loads like central AC or a well pump, and how much convenience you want. Many homeowners land on a mid-size standby with load management as the sweet spot.
How many watts to run a house?
| Coverage | Typical generator size |
|---|---|
| Fridge + lights + a few outlets (survival) | 3,000 - 4,000 W portable |
| Essentials incl. furnace blower + sump | 5,000 - 7,500 W |
| Essentials + one central AC | 10 - 14 kW standby |
| Most of the house (avg home) | 18 - 22 kW standby |
| Large home / electric heat / well pump | 24 - 26 kW+ standby |
Why motor starting drives the size
When a motor starts, it draws a large inrush current - several times its running current - to overcome inertia and magnetize its windings. That is the starting surge. If your generator cannot supply it, the motor stalls, lights dim hard, and the generator may trip or bog down. Central air conditioners are the worst offender; a "hard-start kit" can reduce their surge and let a smaller generator handle them. Always size so the biggest surge lands on top of your steady running load without overloading the unit.
kW, kVA, and power factor
Generators are rated in watts/kW (real power) and sometimes kVA (apparent power). The relationship is:
kW = kVA × power factor (usually 0.8 for generators)
So a 10 kVA generator provides about 8 kW of real power. When you compare units, make sure you compare the same thing - running (rated) watts, not the higher surge figure that marketing sometimes leads with.
A worked example
You want fridge, freezer, furnace blower, sump pump, lights, and one 3-ton central AC:
- Running watts: fridge 700 + freezer 700 + blower 800 + sump 1,050 + lights 400 + AC 3,500 = 7,150 W.
- Largest starting surge: the 3-ton AC, roughly +6,000 W over its running draw (without a hard-start kit).
- Peak demand: 7,150 + 6,000 = ~13,150 W.
- +20% margin: ~15,800 W → choose a 16-18 kW standby generator (or add a hard-start kit and load management to use a smaller unit).
Portable vs inverter vs standby
Size is only half the decision - the type of generator matters as much as the wattage:
- Portable generator: the cheapest way to cover essentials (3,000-8,500 W). You run cords or, better, feed a manual transfer switch. You have to start it, refuel it, and keep it outdoors well away from the house for carbon-monoxide safety.
- Inverter generator: a portable that produces cleaner, more stable power - safer for sensitive electronics - and is quieter and more fuel-efficient, though it costs more per watt. Great for smaller essential loads and anything with a circuit board.
- Standby generator: permanently installed, runs on natural gas or propane, and starts automatically within seconds of an outage via an automatic transfer switch. This is the whole-house option (10-26 kW+), the most convenient and the most expensive.
For clean power quality, inverter or standby units are preferable when you are running computers, modern TVs, and variable-speed HVAC. Conventional portables are fine for motors, heaters, and lights.
Fuel type and run time
Fuel affects both sizing headroom and how long you can actually run:
- Gasoline - common for portables, but stores poorly and you must keep refilling; run time on a tank is only a several hours at load.
- Propane - stores indefinitely, clean-burning, common for standby and dual-fuel portables; slightly lower power output than gasoline.
- Natural gas - the usual standby fuel where a gas line exists, giving essentially unlimited run time, but output derates a little and depends on gas supply pressure.
Standby units also derate for altitude and high temperature - roughly 3-4% less power per 1,000 ft of elevation and more in extreme heat - so a unit sized right at sea level may need to be a step larger in the mountains. Factor this in before finalizing the size.
Common generator-sizing mistakes
- Sizing on running watts only and ignoring motor starting surge.
- Adding every appliance's surge instead of just the largest one.
- Forgetting the AC - central air is usually the load that sets the size.
- No safety margin, running the generator at 100% continuously.
- Confusing surge watts with rated watts when comparing models.
- Ignoring altitude and temperature derating for standby units.
Standards and references
| Reference | What it covers |
|---|---|
| NEC Article 702 | Optional standby systems |
| NEC Article 700 / 701 | Emergency and legally required standby systems |
| Key relationships | Running + largest surge; kW = kVA × 0.8 |
| Manufacturer sizing tools | Standby generator selection and load management |
The bottom line
Size a generator by adding your running watts, adding the largest single motor starting surge, and allowing 20-25% margin. Essentials need about 5,000-7,500 watts; whole-house standby for an average home is usually 18-22 kW, driven by the air conditioner's starting demand. Decide between essential circuits (smaller, cheaper) and whole-house (bigger, hands-off). Size your exact load list with the generator sizing calculator or the standby generator sizing calculator, and have the installation and transfer switch designed by a licensed electrician. For the fault-current side of the design, see our guide to available fault current.
Frequently asked questions
What size generator do I need for my house?
It depends on whether you want to run the whole house or just essential circuits. A small portable of 3,000 to 7,500 watts covers essentials like the fridge, some lights, and a few outlets. Running central air, a well pump, and larger loads pushes you to a standby generator of about 10 to 22 kW or more. Add up the running watts of everything you want to power, add the largest motor's starting surge, and size above that total.
What is the difference between running watts and starting watts?
Running watts (or rated watts) is the continuous power a device draws while operating. Starting watts (or surge watts) is the brief, much higher spike that motors draw when they start - often two to three times their running watts. A generator must supply the total running watts plus the single largest starting surge at the same time, which is why motor loads drive the required size up sharply.
How do I calculate the generator size I need?
List every load you want to power and its running watts, add them up, then add the highest starting-watt surge among your motor loads (fridge, AC, well pump, sump pump). The result is the minimum the generator must deliver. Add a safety margin of around 20 to 25 percent so the generator is not run at full load continuously, then round up to the next standard size in watts or kW.
How many watts does it take to run a house?
Essential circuits - refrigerator, freezer, some lights, outlets, furnace blower, and a sump pump - typically need about 5,000 to 7,500 watts. Adding central air conditioning, an electric water heater, a well pump, and a range moves a whole-house demand into the 15,000 to 25,000 watt range or more. Homes with electric heat or large HVAC need the higher end, which is why whole-house standby units are sized in the 18 to 26 kW class.
Why does starting a motor need so much power?
When a motor starts, it briefly draws a large inrush current - commonly several times its running current - to overcome inertia and magnetize its windings. That translates into a short surge of starting watts. Air conditioners, well pumps, and refrigerators are the usual culprits. A generator has to cover this surge on top of everything already running, so the largest motor's starting demand often sets the generator size.
How do I convert generator watts to kW and kVA?
Divide watts by 1,000 to get kilowatts (kW), so 8,000 watts is 8 kW. Generators are also rated in kVA (apparent power); kW equals kVA times the power factor, which is usually 0.8 for generators. So a 10 kVA generator provides about 8 kW of real power. When comparing units, make sure you are comparing the same rating - running (rated) power, not the higher surge figure.
Should I get a whole-house or an essential-circuits generator?
An essential-circuits (or managed) approach powers a chosen set of important loads and lets you use a smaller, cheaper generator, sometimes with a load-management system that sheds big loads automatically. A whole-house generator powers everything without thinking about it but costs more and must be sized for the full demand. Your choice depends on budget, whether you have large loads like central AC or a well pump, and how much convenience you want during an outage.
Is a generator sizing calculator accurate?
A calculator that sums your running watts and adds the largest motor starting surge, then applies a safety margin, gives a reliable minimum generator size for home use. A complete standby-generator design also considers the largest-motor starting method, load management, the transfer switch, fuel type and supply, altitude and temperature derating, and code requirements, and should be confirmed by a licensed electrician or engineer.