A standby generator is sized by adding the running loads you intend to power plus the starting surge of the largest motor, not by house square footage. Deciding first whether you want whole-home backup or a defined set of essential circuits determines both the generator size and the transfer equipment.
Backup power is one of the few home purchases where the right answer is often "less than you were quoted." The question is not how big your house is. It is which circuits you actually care about at 2 a.m. in January.
What a Colorado outage actually looks like
Front Range outages tend to come in a few recognizable flavors. Heavy wet spring snow brings branches down on lines. Late-summer monsoon storms produce sudden localized hits with lightning and wind. High-wind events along the foothills take out distribution lines, and those can be the longest to restore because crews work a wide area.
Most outages here are hours, not days. That shapes the decision: for many households the real goal is keeping the furnace, the refrigerator, the sump pump and a few circuits alive through a cold night, not running the entire house at full tilt. If your home relies on a sump pump during monsoon season, that single circuit may be the whole reason you are shopping.
Whole-home versus essential circuits
Whole-home backup powers the panel. You notice the outage because the lights blink and the generator starts. It is the most comfortable option, the largest unit, and the most expensive installation.
Essential-circuit backup powers a defined subset through a smaller transfer switch or a dedicated sub-panel — typically heating, refrigeration, well or sump pump, a few lighting and receptacle circuits, and the garage door. You manage what you run. For most Colorado households this covers the real need at a much smaller size.
There is a middle path worth knowing about: a whole-home generator with load management, which sheds big loads like air conditioning or an EV charger automatically so a smaller generator can still back the whole panel.
Comparing backup power options
| Option | What it powers | What you do during an outage | Fuel |
|---|---|---|---|
| Portable generator with interlock kit | Selected circuits in the main panel | Roll it out, connect, start, manage load manually | Gasoline, stored on site |
| Portable with manual transfer switch | A fixed set of pre-wired circuits | Connect the inlet, start, flip the switch | Gasoline or propane |
| Essential-circuit standby | A dedicated sub-panel of critical circuits | Nothing — it starts and transfers automatically | Natural gas or propane |
| Whole-home standby with load management | The full panel, shedding large loads as needed | Nothing; the controller prioritizes loads | Natural gas or propane |
| Battery backup | Selected circuits for a limited duration | Nothing; silent and instantaneous | Stored energy, recharged from grid or solar |
Fuel choice
Natural gas is the simplest if your home already has service: the generator runs as long as the gas line is intact, with no tank to fill and no fuel to stabilize. Running a new gas line sized for a generator is its own piece of the job, because the generator is a significant load on top of your furnace and water heater.
Propane suits homes without gas service and stores well, but you own a tank, you monitor the level, and you need a refill during the event that made you want a generator. Gasoline portables are the cheapest entry point and the most work — fuel degrades, storage has limits, and you are outside refueling in whatever weather caused the outage.
Transfer switch versus interlock
Both exist to do one critical thing: make it impossible to backfeed the utility. That is a lineman-safety issue and the reason this is never a DIY connection.
An automatic transfer switch senses the outage, starts the generator, transfers the load, then transfers back and shuts down when utility power returns. You can be asleep or out of town. An interlock kit is a mechanical plate on the panel that physically prevents the main breaker and the generator breaker from being on at once — far less expensive, entirely manual, and perfectly legitimate when properly installed and permitted.
Sizing by load, not square footage
Write the list. Furnace blower, refrigerator, sump or well pump, a few lighting circuits, internet equipment, maybe a freezer. Add the running load of each. Then find the largest motor — usually the well pump, sump pump or air conditioner — and add its starting surge, which is a multiple of its running draw. That surge figure, not the sum of running watts, usually sets the minimum generator size.
Two Colorado notes. Engines lose output at altitude, so a generator rated at sea level produces less here and the derate has to be in the calculation. And if your heating is a gas furnace, the backup load is just the blower and controls, which is modest — electric heat is a completely different and much larger number.
When a standby generator is more than you need
- Your outages are brief and infrequent. A couple of hours twice a year is a flashlight-and-blanket problem.
- You only need to protect one thing. A battery backup on the sump pump, or a small inverter for medical equipment and a phone, may be the honest answer.
- You travel constantly. A standby unit needs periodic exercise and service; a neglected generator fails when you finally need it.
- You are planning solar and storage. Decide the whole architecture first rather than buying twice.
- Your panel has no capacity and you are not ready for that work. Generator installs often surface a panel conversation — see electrical panels.
Questions to ask before you buy
- What load list was this size based on, and can I see it?
- Which motor sets the starting surge requirement?
- How was altitude derate applied to the rating?
- Is my gas meter and piping sized for this unit plus existing appliances?
- Where will it sit relative to windows, doors and the property line?
- What does annual service involve, and who performs it?
Siting matters more than people expect: clearance from openings, exhaust direction, noise toward a neighbor, and snow accumulation all factor in. Our generator backup installations are permitted and inspected, with both the gas and electrical sides handled in-house.
Frequently asked questions
What size standby generator do I need for my house?
Size comes from a load list, not square footage. Add the running watts of every circuit you want backed up, then add the starting surge of the largest motor — usually a well pump, sump pump or air conditioner. That surge typically sets the minimum size. Altitude also derates engine output and must be factored in.
Should I get whole-home or essential-circuit backup?
Whole-home backup powers the entire panel and costs the most. Essential-circuit backup powers a defined subset — heating, refrigeration, sump or well pump, some lighting — through a smaller switch or sub-panel. Since most Colorado outages last hours rather than days, essential-circuit systems cover the real need for many households.
What is the difference between a transfer switch and an interlock kit?
Both prevent dangerous backfeed onto utility lines. An automatic transfer switch detects the outage, starts the generator and transfers the load without anyone present. An interlock kit is a mechanical plate that stops the main breaker and generator breaker from being on simultaneously — much less expensive, fully manual, and legitimate when properly installed.
Is natural gas or propane better for a standby generator?
Natural gas is simpler if your home has service, since there is no tank to monitor and no refills during an outage. Propane works for homes without gas service and stores well, but you maintain a tank and may need delivery during the event itself. Either way, the gas supply must be sized for the generator plus existing appliances.
Does a standby generator installation need a permit?
Yes. A standby generator installation involves both gas piping and electrical work, so it is permitted and inspected. Siting also has requirements for clearance from windows, doors and property lines, and exhaust direction matters. The inspection record is worth keeping for insurance and for any future sale of the home.