Generator Sizing Calculator: What Size Do You Need?
The power goes out, the fridge goes quiet, and somewhere in the basement a sump pump that was running a minute ago has gone silent too. Buying a generator before the next outage raises a question that a wattage sticker alone can't answer: how big a unit actually covers a house, once the moment a well pump or furnace blower kicks on is counted, not just the appliances sitting there idle.
Shopping by gut feel usually goes one of two ways: a unit too small trips or stalls the first time two motors try to start close together, and a unit far larger than needed costs more to buy, install and fuel than the actual load ever justified. The list below walks through the same running-plus-surge arithmetic the calculator uses, on the exact appliance mix it starts with, so the number it lands on can be checked by hand.
| Appliance | Running (W) | Starting surge (W) | Remove |
|---|---|---|---|
- Total running load
- 4.10 kW
- Worst-case surge
- 6.10 kW
- Recommended continuous rating
- 8.71 kW
Recommended rating targets the surge load at about 70% of the generator's continuous capacity, inside the commonly-cited 50-80% headroom range.
Why running watts alone undersize a generator
Every appliance has a steady running draw, but anything with a motor or compressor (a refrigerator, a well or sump pump, a furnace blower, a central air compressor) pulls several times its running watts for the fraction of a second it takes the motor to spin up. A generator only has to survive that starting surge for an instant, but if it can't, it stalls or trips exactly when the appliance is trying to start, which is usually the worst possible time during an outage.
Sizing around the running total alone ignores this entirely. The calculator above instead checks every motor-driven appliance in turn: for each one, its running watts are temporarily replaced by its starting watts, added back on top of everyone else's running load, and the largest total across every appliance becomes the worst-case surge the generator has to survive.
It's rarely the biggest running load that produces the worst surge, either: a microwave or a bank of lights never starts hard enough to matter, while a modestly sized pump or compressor can dominate the calculation purely because of how much extra current it demands for that first fraction of a second. That's why every motor in the list has to be checked in turn rather than assuming the largest nameplate wins.
From worst-case surge to a recommended rating
Running a generator right at its rated capacity leaves no margin for a surge, a hot day, or a slightly larger load than expected. As general reference, a common approach targets the worst-case surge at somewhere between 50% and 80% of the generator's continuous rating. This calculator uses 70% as its working target, so dividing the surge by that fraction gives a practical continuous rating to shop for.
This is arithmetic for narrowing a shopping list, not a substitute for a load calculation an installer or a formal load-calculation walkthrough would run against the actual panel. A licensed electrician confirms the final unit size and the transfer equipment for any real installation.
Worked example: the calculator's starter list
Refrigerator (700 W running / 2,200 W starting), well/sump pump (1,000 W / 3,000 W), furnace blower (800 W / 2,350 W), lights and misc. (600 W), microwave (1,000 W).
- 1
Total the running watts
700 + 1000 + 800 + 600 + 1000 = 4,100 W (4.1 kW)
- 2
Check each motor's own starting scenario
Fridge: 4,100 − 700 + 2,200 = 5,600 W
Furnace: 4,100 − 800 + 2,350 = 5,650 W
Pump: 4,100 − 1,000 + 3,000 = 6,100 W
- 3
Take the largest surge, the pump starting
6,100 W is worse than the fridge's 5,600 W or the furnace's 5,650 W, so it sets the worst case: 6.1 kW
- 4
Apply the 70% headroom target
6.1 / 0.7 = 8.71 kW recommended
8.71 kW isn't a size any dealer sells, so this list would practically shop in the 8,500-10,000 W continuous range as general reference. The exact model still needs to be confirmed against its own surge rating, not just its continuous number.
Standby vs. portable: which category to shop
Once a target kW is in hand, it still has to be matched against a category of generator, and the two mainstream categories solve the outage problem in very different ways. A portable unit is bought, stored in a garage or shed, and rolled out and started by hand once an outage starts: it typically covers a short list of priority circuits chosen ahead of time, wired through a manual transfer switch or interlock kit. A standby unit lives permanently outside the house on its own pad, watches the utility feed continuously, and starts itself automatically within seconds of losing power, which is why it can realistically cover a whole panel rather than a handful of circuits.
Both categories can be sized with the same running-plus-surge arithmetic above; the difference is in how the power gets delivered and what it costs to install. Figures below are typical/illustrative, not quotes for a specific model.
| Factor | Portable | Standby |
|---|---|---|
| Typical fuel | Gasoline | Natural gas or propane; diesel on larger units |
| Startup | Manual, wheeled out, started, and connected by hand | Automatic, senses an outage and starts within seconds |
| Transfer switch | Manual transfer switch or interlock kit, professionally installed | Automatic transfer switch, installed with the unit |
| Typical continuous range | Roughly 2-10 kW | Roughly 8-25+ kW for whole-house coverage |
| Runtime | Limited by tank size; needs periodic refueling | Runs as long as gas or a large fuel tank lasts |
| Cost and complexity | Lower upfront cost, no permanent installation | Higher upfront cost, permanent pad and permitted electrical/gas work |
General reference only: actual pricing and available ranges vary by brand, region and installer, and change over time; get current quotes from a local dealer or licensed electrician.
Typical starting-surge multipliers by equipment type
The calculator above works best when a real starting-watt figure is available: from a nameplate, a spec sheet, or a manufacturer's published locked-rotor value. When none of those is on hand, a rough multiplier applied to the appliance's running watts is a common fallback for a first-pass estimate, and the table below collects typical multipliers for the equipment that shows up most often on a home generator worksheet.
These are illustrative ranges, not a substitute for the manufacturer's own starting-current or locked-rotor figure. The same logic used for estimating a motor's full-load current applies here too, since a motor's starting behavior and its running current come from the same nameplate data.
| Equipment | Typical starting multiplier |
|---|---|
| Refrigerator / freezer | ~3× |
| Well or sump pump | ~3× |
| Furnace blower motor | ~2-3× |
| Central air conditioner compressor | ~3-5× |
| Purely resistive loads (heaters, lights) | ~1× (no surge) |
Typical/illustrative ranges only, not measured values for any specific model: starting current varies with motor design, load and age, so a nameplate or manufacturer spec sheet takes priority over a generic multiplier.
Load figures a generator has to cover
Questions
Generator sizing FAQ
Questions that come up once a running-plus-surge total has to turn into an actual generator to buy and install.
What's the difference between a standby and a portable generator?
A standby generator is permanently installed outside the home, runs on natural gas, propane or diesel, and starts itself within seconds of an outage; a portable generator is gasoline-powered, wheeled out and started by hand, and typically covers a smaller slice of the house. Standby units generally cover a much wider continuous kW range and can run for as long as their fuel supply lasts, while portable units are limited by a fuel tank that has to be refilled every several hours. As general reference, the tradeoff is convenience and coverage against upfront cost and installation complexity. A licensed electrician and a generator dealer can size either option against a real service panel.
Do I need a transfer switch to run a home generator safely?
Yes, in effectively every real installation, a transfer switch isolates the house wiring from the utility grid before the generator is connected, which prevents the backfeed that can injure utility line workers and damage equipment. An automatic transfer switch handles this instantly for a standby unit; a portable generator typically pairs with a manual transfer switch or interlock kit wired into the panel by a licensed electrician. Backfeeding a generator into a receptacle without a transfer switch is a safety hazard and is not addressed by this page.
What happens if too many motor-starting appliances try to start at once?
If two or more motors happen to start in the same instant, their starting-surge watts stack on top of each other rather than replacing one running load at a time, which can pull the combined draw well past a generator's rated capacity. A generator that trips or stalls under this kind of surge is usually undersized for that worst case, not defective. Staggering large loads (letting a well pump finish its start-up cycle before a furnace blower kicks on, for example) is a common way to avoid stacking surges without buying a larger unit.
Why does a generator have both a kW rating and a kVA rating, and does it matter?
kW is the real power a generator can actually deliver to a load, while kVA is its apparent power rating. The two only match at a power factor of 1.0, which almost no real household load mix achieves. A generator rated at, say, 10 kVA at a 0.8 power factor delivers only 8 kW of real power, so reading the kVA number as if it were the kW number overstates what the unit can actually run. Checking both numbers against the generator's nameplate power factor avoids this mismatch.
How does fuel type affect how long a generator can run without refueling?
Gasoline stores more energy per gallon than propane and is usually the easiest fuel to find right after a storm, but it degrades in storage over months and limits a portable unit to whatever a single tank and any spare cans hold. Propane and, on larger standby units, diesel both store indefinitely and can draw from a large fixed tank or a piped natural gas line, which is why standby generators are typically able to run for as long as an outage lasts rather than for a fixed number of hours. Natural gas offers unlimited runtime as long as the utility gas supply stays up, though it is not available at every property.
Can a portable generator run an entire house at once?
Rarely. Most portable generators are sized to carry a limited set of circuits chosen ahead of time (refrigerator, well pump, some lighting, a furnace blower), not the full electrical service a home is wired for. Running a whole house from a portable unit generally means either a much larger, more expensive generator or accepting that some circuits stay unpowered during an outage. This is one of the main reasons homeowners who want true whole-house coverage move to a standby installation instead.
Does altitude or ambient temperature change a generator's usable output?
Yes: most generator nameplate ratings assume a sea-level, moderate-temperature test condition, and output typically derates as elevation and heat increase because the engine draws thinner air. A unit rated for a given continuous kW at sea level may deliver noticeably less at a mountain elevation or on a very hot day. The manufacturer's derating table, not the nameplate figure alone, is the general reference for sizing a generator installed at altitude or in a hot climate.