Start with the loads, not the brochure
Generator sizes are sold in kilowatts, and the temptation is to pick a number that sounds big enough. The better approach is to work backwards from what you need running during an outage, then add room for motors starting and future plans.
An undersized generator stalls or trips when the furnace and sump pump start together. An oversized one costs more, burns more fuel, and can run so lightly that some engines do not like it. The goal is the right size, not the biggest one.
It also helps to think about how you will use it. A unit that only needs to keep a basement dry and the furnace running for a few hours is a very different machine from one meant to carry a busy household or a small shop for days.
Write the answer down before you shop. It keeps the conversation focused when you start comparing models and quotes.
Running watts and starting watts
Every appliance has a running load. Anything with a motor or compressor also has a starting or surge load that can be several times higher for a second or two. Furnace blowers, fridges, freezers, sump pumps, well pumps and air conditioners all do this.
A generator has to handle the biggest motor starting while everything else is already running. That is often what sets the size.
Look at the nameplate on each appliance for volts, amps or watts. For air conditioners and heat pumps, the label usually lists a locked rotor amps figure, which hints at the starting load.
Pick a coverage level
Most homes and small shops fall into one of three tiers. Write down which loads fit where for your building.
- Essentials keep the house safe and liveable in winter. In Edmonton that means heat first: a gas furnace still needs electricity for the blower, igniter and controls.
- Comfort adds the things that make a multi-day outage tolerable.
- Whole home covers nearly everything. Large electric loads such as ranges, dryers and EV chargers often get load management modules that temporarily shed them so the generator is not overwhelmed.
For small businesses, the list looks different: point-of-sale and network gear, refrigeration, security and alarm panels, some lighting, and anything that protects inventory. Sensitive electronics usually need a UPS as well, because even an automatic generator takes some seconds to pick up the load.
A simple sizing worksheet
- List each load you want backed up with its running watts.
- Add up the running watts of everything that could be on at once.
- Find the single largest starting surge among your motor loads and add the difference between its starting and running watts.
- Add a margin for future additions and so the generator is not running flat out.
- Compare that total to generator ratings on the fuel you will actually use, since output on natural gas is often lower than on propane or gasoline.
| Input | Why it matters |
|---|---|
| Running watts | Sets the steady load the generator carries |
| Largest starting surge | Decides whether motors can start without stalling |
| Fuel type | Ratings often differ by fuel |
| Panel and service size | Limits transfer switch size and wiring |
| Gas meter capacity | A larger standby unit may need a bigger meter or regulator |
This is a rough guide. A licensed electrician can do a proper load calculation, sometimes using your actual utility demand data, to confirm the size.
Things that change the answer
- Electric heat. Baseboard or in-floor electric heat adds a large load fast. Backing up a whole electrically heated home takes a big unit.
- Well and septic pumps. Acreage properties around Edmonton often need the well pump covered, and it has a heavy starting surge.
- Soft starters. Adding a soft start to an air conditioner can reduce its surge enough to allow a smaller generator.
- Older services. A 60 A or 100 A panel may need an upgrade before a whole-home transfer switch makes sense.
- Gas supply. The utility meter and line have a capacity. Your gas provider and gas fitter confirm whether they can supply the generator alongside your furnace and water heater.
If you can, measure or estimate loads on a cold day. That is when the furnace runs most and when you will most depend on the generator.
A common mistake is to add a large new load later, such as a heat pump or an EV charger, without checking the generator. If your plans include either, mention them at the start so the transfer equipment and load management can be set up to handle them.
What to bring to your electrician
- Your must-run list and which tier it fits.
- Photos of appliance nameplates for motors (furnace, pumps, A/C).
- A photo of your panel label and main breaker.
- Where a standby unit could physically sit, and where the gas meter is.
- Any plans in the next few years: EV charger, heat pump, hot tub, shop.
With that, a quote can be put together without guesswork. If you are still choosing between types, read standby vs portable generators.
Common questions
Can I size a generator by square footage?
Not reliably. Two houses of the same size can have very different loads depending on heating type, pumps and appliances. Size by the loads you want to run.
What happens if my generator is too small?
It may stall or trip when a motor starts, or its output voltage may sag, which is hard on electronics and motors. Load management can help, but the starting surge still has to be covered.
Is bigger always better?
No. An oversized unit costs more to buy and run and may need gas supply upgrades. Size it to your loads with sensible headroom.
Does a business need a different approach?
The method is the same, but the priority list is different, and critical electronics often need a UPS to ride through the seconds before the generator picks up.
Codes and permit rules change and every building is different. For work on your system, talk to a licensed electrician and your local permit authority.