How to Figure Out What Size Generator You Actually Need
The two most common sizing mistakes are opposite problems. One is buying too small to run a fridge and furnace at once. The other is paying for far more power than the house will ever use. This guide covers the difference between starting and running watts. It covers a simple calculation method. It also covers how sizing changes depending on how much of the house needs to stay powered.
Running Watts vs. Starting Watts: The Numbers That Actually Matter
Running watts are what an appliance draws once it's already on and operating normally. Starting watts are the short surge many motor-driven appliances need the instant they switch on. Starting watts can be several times higher than running watts, depending on the appliance. A typical refrigerator may use roughly 150 to 200 running watts, but actual consumption varies by model.
Sizing around running watts alone is the single most common mistake. It's the same miscalculation that leaves a sump pump undersized during a storm. A closer look at starting watts versus running watts for sizing breaks this down appliance by appliance.
Not every appliance has a starting surge. Lights and electronics without motors draw a steady amount from the moment they're switched on. It's specifically motor-driven appliances, fridges, pumps, furnace fans, that need this extra consideration.
Knowing the difference between the two numbers is step one. Actually, adding them up for a specific house is step two.
How to Calculate the Generator Size You Need
List every appliance that needs to run during an outage. Write down its running watts for each one.
Add up the running watts for everything that would realistically run at the same time, not everything in the house. Take the single highest starting-watt appliance on that list and add it on top of the running total. Only one thing usually starts at a time if appliances are staggered.
A Simple Worked Example
Appliance | Running Watts | Starting Watts |
Refrigerator | 150–200 | 1,200–1,600 |
Sump pump | 800–1,050 | 1,300–2,900 |
Furnace fan | 600 | 1,600 |
Microwave | 1,000–1,500 | 1,000–1,500 |
LED lighting | 50 | 50 |
Running the fridge, sump pump, and lights together comes to roughly 1,000 to 1,250 running watts. Adding the furnace fan's starting surge on top brings the total closer to 2,600 to 2,850 watts needed at once.
A fridge alone is one of the most searched sizing questions on its own. A closer breakdown of refrigerator wattage specifically is worth checking against a specific model's rating plate.
That calculation covers a handful of essentials. Whole-home coverage is a different sizing question entirely.
Generator Size by Home and Coverage Type
Select-circuit backup, covering just a fridge, some lighting, and a sump pump, generally needs the least capacity. It's the most common setup for a smaller budget.
Whole-home backup needs to account for HVAC, water heating, and every circuit in the panel running at once. This is why whole-home systems are sized many times larger than a select-circuit setup.
Home size alone is a poor sizing shortcut. Two homes of the same square footage can have very different loads depending on whether heating and hot water are gas or electric.
A practical guide to picking a household backup generator walks through this same select-circuit versus whole-home decision in more detail.
Even with the math done correctly, a few sizing mistakes are easy to make without realizing it.
Common Sizing Mistakes to Avoid
Oversizing "just to be safe" wastes money without adding real protection, since most outages don't require running every circuit in the house at once.
Forgetting seasonal loads is a common miss. A furnace fan and sump pump matter far more in a Canadian winter or spring than they do in a mild summer outage.
Assuming all refrigerators or sump pumps draw the same wattage is a mistake. Checking the actual rating plate on the specific appliance is the only reliable number.
The same sizing logic applies whether the plan is a gas generator or a battery-based one. A look at sizing a solar generator specifically confirms the watt-hour side of the math works the same way once running watts are known.
Where to Find Your Appliance's Actual Wattage
Guessing wattage is how sizing mistakes happen. Every appliance has a rating plate somewhere.
Refrigerators usually have the plate inside, near the top or on the back wall.
Sump pumps list wattage on a sticker on the pump housing itself.
Furnaces show fan motor specs on a plate inside the access panel.
Small appliances often print wattage right on the bottom or back.
If a plate only shows amps, multiply by the voltage (usually 120 V for most household circuits) to get watts. This five-minute check across your key appliances beats guessing every time.
Conclusion
Sizing a generator correctly comes down to knowing running watts versus starting watts, adding them up honestly for what will actually run at once, and matching that to select-circuit or whole-home coverage.
Do the math with real appliance ratings rather than guessing. Size for the biggest realistic outage scenario, not just a mild summer one. The DELTA 3 Ultra Plus and DELTA Pro Ultra cover select-circuit and whole-home sizing respectively.
FAQs
What size generator do I need to run a fridge and a few lights?
Around 1,000 to 1,250 running watts covers a fridge and basic lighting together, though the starting surge from the fridge itself can briefly spike closer to 1,600 watts.
Is a whole-home generator worth it over a smaller select-circuit one?
It depends on budget and how often outages happen in your area. Whole-home coverage costs significantly more but removes the need to choose which circuits matter most during an outage.
Do I need to account for starting watts or just running watts?
Both. Sizing around running watts alone is the most common mistake, since a generator that can't handle the starting surge of a motor-driven appliance will fail the moment that appliance switches on.
What size generator does a furnace need during a power outage?
A typical furnace fan draws about 600 running watts but can surge to 1,600 watts on startup, so the generator needs enough headroom to handle that spike on top of everything else running.
Does home square footage determine what size generator I need?
No. Two homes of the same size can have very different loads depending on whether heating and hot water are gas or electric, so actual appliance wattage matters more than square footage.
Can a battery power station be sized the same way as a gas generator?
Yes. The same running-watts-plus-starting-watts method applies, though a battery system typically doesn't need the same starting-surge buffer a gas engine does.