Questions to ask before you buy a standby generator
Gen Size Desk — Educational directory for residential standby generator planning.
Sourced for homeowners gathering quotes. Not a substitute for licensed professional advice.
Quick takeaway
Before you sign a quote, you need to verify four things: the electrician’s load calculation method, the transfer switch type, the fuel specification, and the installer’s credentials. This checklist walks you through each question, the reasoning behind it, and the red flags to watch for.
1. Load calculation: the foundation of your quote
The single most important number on any proposal is the continuous kW rating the generator will actually run at under normal conditions. If that number is based on guesswork or “rough estimates,” the rest of the system—wiring, fuel piping, automatic transfer switch (ATS) sizing—will be mismatched.
The question to ask
“Can you show me the load calculation worksheet and confirm it was performed by a licensed electrician using NEC Article 220 methods?”
Why it matters
* NEC Article 220 provides the standardized method for calculating the total load a service will carry. It accounts for demand factors, largest-load exclusion, and continuous load percentages.
* A proper calculation lists each circuit or appliance, its wattage, and whether it is “continuous” (expected to run 3+ hours at a stretch). Generators must be sized for 125% of continuous load plus the full non-continuous load per NEC.
* If the dealer cannot produce a worksheet signed or stamped by an electrician, the quote is likely based on “appliance names only” rather than actual calculated demand. This can lead to under-sizing (nuisance trips, equipment damage) or over-sizing (wasted fuel, higher upfront cost).
Red flags
* “We’ll just size it to cover the big items”—vague language that skips the math.
* A calculation that totals nameplate wattages without applying Article 220 demand factors.
* No mention of a licensed electrician performing the work.
What a solid answer looks like
“Yes—our licensed electrician performed a NEC Article 220 load calculation. The worksheet lists each circuit, applies the appropriate demand factors, and calculates the required continuous kW. We can share that document with you before the quote is finalized.”
2. Automatic Transfer Switch (ATS): the brain of the system
The ATS is what makes a standby generator “automatic.” It senses a utility outage, starts the generator, and switches your home’s electrical load from the grid to the generator without manual intervention.
The question to ask
“What type of ATS are you specifying, and will it handle the full home load or a circuit sub-panel?”
Why it matters
* Full-home ATS: Routes all circuits (including 240V appliances like ranges or dryers) through the switch. Requires a larger generator and more complex installation.
* Sub-panel ATS: Only a selected set of circuits (lights, receptacles, furnace, well pump) are connected to the generator via a dedicated sub-panel. The rest of the home remains on utility power during an outage. This is a cost-effective way to keep essentials running with a smaller generator.
* The ATS must be listed for the application and properly matched to the generator’s output and the calculated load.
Red flags
* No discussion of an ATS, or an implication that a manual transfer switch is acceptable for automatic operation (it is not—manual switches defeat the purpose of standby power and can be dangerous if operated incorrectly).
* Guaranteeing “whole-house” coverage without performing the load calculation in Step 1 (you may not have enough generator capacity).
What a solid answer looks like
“We typically recommend a circuit sub-panel ATS for most homes, which keeps the generator size manageable and focuses power on essential circuits like your furnace, refrigerator, and lights. If you want full-home coverage, we’ll size the generator accordingly after the load calculation. The ATS we specify is [brand/model] and is listed for the calculated load.”
3. Fuel type: natural gas vs. propane
Generator proposals often list fuel options. The headline kW rating can differ significantly between natural gas (NG) and liquid propane (LP) due to energy content and fuel delivery pressure.
The question to ask
“Is the headline kW rating based on natural gas (NG) or propane (LP), and what are the continuous run implications for each?”
Why it matters
* Natural gas is typically delivered at constant pressure through municipal lines. The rated kW on the generator nameplate is usually the NG continuous rating.
* Propane systems rely on stored tanks. The engine may derate (produce less power) as the tank level drops or in cold weather. The LP rating is often lower than the NG rating for the same physical unit.
* Fuel availability during extended outages: municipal gas may be interrupted if main lines are damaged, while propane tanks store fuel on-site. However, if the tank is empty or the regulator freezes, the generator will not run.
Red flags
* The sales quote lists a single kW number without specifying which fuel it represents.
* Pressure: “Our generator runs on either fuel”—this is technically true for dual-fuel units, but the kW output will differ by fuel type. You need to know which rating you are comparing.
* No discussion of fuel storage, supply line sizing, or local codes regarding NG vs. LP installations.
What a solid answer looks like
“The quoted kW rating is based on natural gas continuous operation. If you opt for propane, the continuous rating drops to [X] kW due to fuel energy content differences. We’ll size the fuel piping and regulator per [local code/NFPA 54] to ensure proper delivery.”
4. Installation and safety: never backfeed
Backfeeding—plugging a generator into a wall outlet to power the home—is extremely dangerous and often illegal. It can energize utility lines, endangering linemen and causing fires.
The question to ask
“Will this installation include a proper Automatic Transfer Switch? I want to confirm no backfeeding will be possible.”
Why it matters
* A legally installed standby system physically isolates the home from the utility grid when the generator is running. The ATS switches the load to the generator and disconnects the utility feed.
* If a quote or installer suggests using extension cords into wall outlets, or does not mention an ATS, walk away. This is a code violation and a severe safety hazard.
Red flags
* “We can just hook it up to your existing panel”—absent an ATS, this constitutes backfeeding.
* No mention of utility company notification or permit pulling.
What a solid answer looks like
“Every installation includes an Automatic Transfer Switch that isolates the home from the utility grid. Backfeeding is not possible with this setup. We also coordinate with your local utility and pull the necessary permits before commissioning.”
5. Installer credentials and ongoing support
A generator is a long-term investment. The installer’s expertise affects not just the initial install but the system’s reliability for years.
The question to ask
“Are your technicians licensed electricians with standby generator experience? Do you offer maintenance plans after the sale?”
Why it matters
* Standby generators involve fuel piping, engine exhaust, electrical grounding, and ATS programming. A licensed electrician ensures the work meets NEC and local building codes.
* Regular maintenance (oil changes, filter replacements, exercise cycles) is critical for reliability. Check your OEM warranty booklet — some manufacturers condition coverage on documented service by qualified technicians (do not invent terms; read the warranty).
Red flags
* “Any licensed guy can hook this up”—electrical work on standby systems is specialized.
* No offered maintenance plan, or a plan that only covers the engine and ignores the electrical components.
What a solid answer looks like
“Our installation team consists of licensed electricians with specific standby generator certification. We also offer maintenance plans that include annual exercise testing, oil and filter changes, and transfer switch inspection. Manufacturer warranties typically require this annual service to remain valid.”
6. Comparing quotes: a quick checklist
When you receive multiple proposals, line up these five data points:
| Item | What to look for |
|------|------------------|
| Load calculation | NEC Article 220 worksheet by a licensed electrician |
| ATS type | Full-home vs. sub-panel; listed for the calculated load |
| Fuel kW rating | NG continuous kW vs. LP continuous kW (headline numbers must be comparable) |
| Safety | ATS included; explicit statement that backfeeding is not possible |
| Credentials & service | Licensed electrician install; maintenance plan offered |
If a quote is missing any of these, ask for clarification before proceeding.
Soft-links (for your reference)
* Choose the right size – Interactive guide to estimating your essential load before requesting quotes.
* Maintenance planning – What annual service includes and why it matters for warranty compliance.
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Sources & further reading (public educational links)
- 1. National Fire Protection Association (NFPA): Standby Power Systems – Overview of NFPA 110 and NFPA 54 requirements for generator installation and fuel piping. nfpa.org
- 2. National Electrical Code (NEC) Article 220: Calculation of Loads – The official method for calculating service and feeder loads. Secondary educational guides referencing Article 220 are acceptable for homeowner context if labeled as such.
- 3. U.S. Department of Energy: Backup Power for Homes – Consumer guidance on sizing, fuel types, and transfer switches. energy.gov
- 4. American National Standards Institute (ANSI): Product Standards for Engine-Driven Generators – Technical performance and safety criteria. ansi.org
All links are to publicly available educational or standards-based resources. No proprietary pricing or vendor-specific claims are included.