Whole-house or managed-load: decide this first
Everything downstream depends on this, and it is where the money is.
Whole-house means the generator can carry everything the panel can throw at it simultaneously — central air, electric range, dryer and water heater all at once. It is the expensive answer, and it is expensive because it is sized for a coincidence that almost never happens.
Managed-load means the system sheds or staggers large loads so they do not run together. A load management module will hold the air conditioner off while the dryer runs, then let it start. Everything still works; it just does not all work in the same minute.
Managed-load commonly lets a household drop a tier or two, which is the single largest saving available in this purchase — and most people never find out it was an option because they price whole-house and stop.
Ballpark sizes, and what drives them
| What you want | Roughly | The load that decides it |
|---|
| Critical circuits only | 7-10 kW | Furnace blower plus a well pump |
|---|
| Critical plus one air conditioner | 12-16 kW | The AC compressor's starting surge |
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| Most of a gas-heated house | 18-22 kW | Central air plus a range element |
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| Whole house, all electric | 24 kW and up | Electric water heating, range and dryer together |
|---|
These are planning brackets built from the appliance figures Champion and Honda publish, not a manufacturer sizing table. A load calculation done by your installer against your actual panel is the number that matters, and it is part of a proper quote.
Gas heat changes everything
The single biggest variable, and it is worth checking before doing any other arithmetic.
A gas furnace needs only its blower — Champion publishes 300-800 running watts for a half-horsepower fan. An electric furnace or a heat pump with resistance strips is a completely different and far larger load.
The same applies right across the house:
- Gas water heater: often needs nothing at all. Electric: 3,000-4,500W whenever it cycles.
- Gas range: usually lightable by hand. Electric: several thousand watts.
- Gas dryer: just the drum motor. Electric: thousands of watts.
A gas-heated house with gas cooking and hot water can often be well covered by a mid-size unit where an all-electric house of the same size needs the largest residential model made.
Size against your fuel, not the headline
The mistake that costs people a tier. Standby units are rated differently on the two gases.
Generac rates the Guardian 24kW at 24 kW on propane and 21 kW on natural gas. If your calculation landed on 22 kW and you connect to the gas main, that unit does not cover it.
Always compare units on the rating for the fuel you will actually use. The fuel comparison goes into why the gap exists and which fuel to pick.
The air conditioner is usually the decider
In most houses, one appliance sets the size, and it is the central air conditioner.
Champion publishes 3,000-3,500 running and 5,000-6,000 starting watts for a 3 ton unit. That starting surge is what a generator has to absorb, and it is larger than the entire rest of a typical critical-circuit list.
Two ways round it, both worth raising with your installer:
- A soft starter on the air conditioner. Micro-Air publishes 65-75% start current reduction, which can bring the surge inside a smaller generator's capability and save a tier.
- Load management. Let the AC start only when the dryer and range are not running.
Why the starting figure is so much larger explains the mechanism.
Measure rather than estimate
Here is the approach we would actually recommend, and almost nobody takes it.
- Fit a manual transfer switch and use a portable for a season. This costs a fraction of a standby installation.
- Read the watt meters during a real outage. You find out what your house actually draws, with your appliances, in your weather.
- Note what you genuinely missed. Most households discover the list is shorter than they assumed — and occasionally discover one load they had not thought about at all.
- Then size the standby unit against a measurement rather than a spreadsheet.
The portable does not become redundant afterwards either — it becomes the backup to the backup, covering the annual service window and gas supply interruptions.
How to read a quote
- Ask which fuel the quoted kW is for. If the answer is not immediate, ask again.
- Ask whether it includes load management and what it would save.
- Ask for the load calculation, not just the recommendation. A proper one lists your appliances.
- Ask what the transfer switch covers — whole panel or a sub-panel of selected circuits.
- Ask about gas pipe sizing or tank siting. This is where quotes diverge most, and where surprises appear.
The installation guide covers what the whole job involves, and the whole-house comparison covers the units themselves.
Bigger is not automatically safer
Oversizing has real costs beyond the purchase price. A generator running far below its rating burns more fuel per useful watt, runs cooler than it was designed to, and on some engines that leads to incomplete combustion and deposits over time.
The exercise cycle costs more too, every week, for years.
Some margin above your calculated load is sensible. Two tiers is money spent on a coincidence that will not happen, and it is exactly what load management exists to avoid buying.
