Technical & Electrical Generator FAQs

If you’re just getting started, it can help to read the Home Generator Sizing Guide and the General Generator FAQs first. This page is aimed at homeowners who want a more technical view of how electricians and designers think about generator sizing.

How do I convert amps to kW for my house?

The basic formula for single-phase loads is:

kW = (Volts × Amps × Power factor) ÷ 1000

In a typical U.S. home with 120/240 V split-phase service:

• Most 120 V circuits feed lighting and receptacles.
• Most large appliances (ranges, dryers, AC condensers, well pumps) are 240 V loads.

For example, a 240 V, 30 A load with power factor ≈ 1.0:

kW ≈ (240 V × 30 A × 1.0) ÷ 1000 ≈ 7.2 kW

Household load is the sum of individual loads, adjusted for diversity (everything is not on at full draw at the same time). That’s why a 200 A service (up to ~48 kW theoretical at 240 V) can often be backed up reasonably well with a 16–24 kW generator: we’re sizing to what you actually expect to run during an outage, not the absolute maximum your service can deliver.

If you’d like a structured walkthrough, you can plug your loads into the generator sizing calculator and let it do the math.

My home has 200 A service. Does that mean I need a 48 kW generator?

No. A 200 A, 120/240 V service can theoretically deliver up to:

kW = (240 V × 200 A) ÷ 1000 ≈ 48 kW

That is a maximum service rating, not a requirement for generator size. In practice:

• Most homes rarely use anywhere near 48 kW at one time.
• Generator sizing is based on expected load during an outage, not worst-case everything-on-at-once.
• Load management and circuit selection further reduce what must run simultaneously.

Many 200 A homes are well served by a 16–24 kW standby generator when outages are infrequent and heavy electric heat is not being backed up. If you have a large, all-electric home and want true “everything on” capacity, a larger unit may be appropriate, but that’s the exception, not the rule.

For a deeper dive on this topic, see What size generator is needed for 200 amps?

What does “power factor” mean and why does it matter?

Power factor (PF) describes the relationship between real power (kW) and apparent power (kVA). It ranges from 0 to 1:

kW = kVA × Power factor

Many residential loads are close to unity (PF near 1.0), especially resistance loads like electric heat and incandescent lighting. Motor loads (AC compressors, pumps, fans) often have PF in the 0.8–0.95 range.

Generators are frequently rated in kVA with an assumed power factor (for example, “22 kVA at 0.8 PF”). If PF is lower, you’ll hit the generator’s current limit sooner for the same real power. For most homeowners, the key takeaways are:

• The published kW rating already reflects a reasonable assumed power factor.
• Large motor loads with poor PF can stress a marginally sized generator.

What does it mean that a generator is rated for “standby,” “prime,” or “continuous”?

Generator manufacturers often publish different ratings depending on how the unit is used:

• Standby rating: For typical residential standby use during outages. Full rating is available, but only for a limited number of hours per year.
• Prime rating: For applications where the generator is a primary power source and load varies over time. The prime rating is usually lower than the standby rating.
• Continuous rating: For running at a constant load for long periods. This is typically the lowest rating of the three.

Residential whole-house units are generally sized and labeled for standby use. When you see “22 kW standby” on a common air-cooled unit, that rating is intended for intermittent outage duty, not for industrial continuous use.

Why do people say to size a generator for 70–80% load?

Running a generator at or near 100% of its rated capacity for long periods is hard on the equipment and leaves no room for:

• Short-term starting surges from motors.
• Extra loads that may be turned on unexpectedly.
• Output reductions in extreme heat or at higher altitudes.

A common design target is to keep expected peak load in the range of 70–80% of the generator’s rating. That offers a good balance between:

• Fuel efficiency and engine health.
• Capacity for new loads in the future.
• Reduced risk of nuisance tripping or voltage sag.

For example, if your calculated peak load is around 14–16 kW, a 20 kW standby unit is often a sensible choice rather than trying to “force it” onto a 14 kW generator.

How do temperature and altitude affect generator output?

Engine-driven generators are less powerful at higher temperatures and higher altitudes, because warm, thin air contains less oxygen. Manufacturers often publish derating curves, but a rough guideline is:

• Expect some reduction above ~3,000 ft elevation.
• High ambient temperatures (for example, 100 °F+) can further reduce output.

In most residential settings near sea level, derating is modest. But if you live at elevation or in a very hot climate and your load calculation already pushes the generator toward its limit, it’s prudent to:

• Choose a slightly larger unit, or
• Plan more aggressive load management during extreme conditions.

How do electricians calculate load for sizing a generator?

Electricians and designers use methods based on the National Electrical Code (NEC) to estimate total load. The details vary by jurisdiction and code edition, but typical steps include:

• Identifying continuous vs. non-continuous loads.
• Applying demand factors to general lighting and receptacle loads.
• Accounting explicitly for large appliances (ranges, dryers, AC, pumps, EV chargers).
• Considering whether loads are likely to run concurrently.

For residential optional standby systems (NEC Article 702), the generator does not necessarily have to be sized for the entire service rating, as long as load management and transfer equipment are designed appropriately. Your local electrician will follow the version of the code adopted in your area and any local amendments.

For background on NEC and related standards, see the National Fire Protection Association (publisher of NFPA 70, the National Electrical Code) . Always defer to your local licensed electrician and inspector for how these rules apply to your specific installation.

For more on how transfer equipment choices affect sizing and load management, you can also review Transfer Switch vs. Interlock.

Do I need a generator that matches my service size (100 A, 150 A, 200 A)?

No. The amp rating of your service does not dictate the kW rating of your generator. Instead:

• Generator kW is based on the loads you plan to support during an outage.
• Transfer switches and load management equipment make sure the generator is not overloaded.

For example, a home with 200 A service might use:

• A 100 A or 150 A service-entrance transfer switch, if not backing up the full service.
• Or a 200 A service-entrance switch with load-shedding modules on lower-priority circuits.

In all of these cases, the generator itself might be something like 18–24 kW. The key is that the switching and protection equipment keep total connected load within what the generator can safely supply.

Can I oversize my generator too much?

On the residential side, “too much” is usually about cost and practicality rather than safety, as long as the unit is installed correctly. Downsides of a significantly oversized generator include:

• Higher equipment and installation cost.
• Higher fuel consumption at a given load compared with a right-sized unit.
• In some cases, less efficient operation at very light loads.

That said, unlike some diesel prime-power applications (where low load can lead to issues like wet stacking), residential air-cooled natural gas/propane standby units are generally tolerant of lighter loading. Being modestly oversized is common.

What’s the difference between kW, kVA, and kWh?

These three terms often get mixed up:

• kW (kilowatts) – real power; what most generator ratings use.
• kVA (kilovolt-amperes) – apparent power; kW divided by power factor.
• kWh (kilowatt-hours) – energy over time; what your utility bills are based on.

When sizing a generator, we focus on kW and kVA at a given moment in time (how much power you need right now). When thinking about fuel cost over a day or week of runtime, we’re effectively talking about kWh consumed.

If you’d like to estimate your own kW from loads and breakers, open the generator sizing calculator and plug in your home’s details for a structured walkthrough. You can also cross-check high-level concepts in the Home Generator Sizing Guide.

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