What Is an Automatic Standby Generator (kW Load Sizing)

An automatic standby generator is a permanently installed backup power system that starts when utility power fails and connects to selected home circuits through an automatic transfer switch. Size it by adding the loads you want to run, then checking motor-starting demand, voltage, fuel supply, and equipment ratings. A qualified installer must confirm the final design and code requirements.

The U.S. Energy Information Administration reported that electricity customers averaged 5.5 hours of power interruptions in 2022, excluding major events. That national figure cannot predict outages at any one home, but it shows why people consider backup power. The sizing challenge is choosing a generator that can handle the loads you need, including the brief surge when certain motors start.

Define Backup Loads and Generator Sizing

Generator sizing means matching the generator’s usable electrical output to the loads it must serve. Start by deciding which circuits need backup, then total their running demand and check the largest motor-starting demand. The generator, transfer switch, fuel supply, and electrical service must also be compatible.

A generator’s kilowatt (kW) rating describes real power. Its kilovolt-ampere (kVA) rating describes apparent power, which includes the effect of power factor. The relationship is kW = kVA × power factor. For instance, 10 kVA at a 0.8 power factor equals 8 kW. Use the generator’s rated output, not a model number or brief surge claim.

Many U.S. homes use 120/240-volt, single-phase, 60-hertz power. Still, check the generator nameplate and your equipment: some loads need 120 volts, some need 240 volts, and a few may need both. The generator must deliver the right voltage and frequency as well as enough kW.

Which loads should a standby generator power?

A backed-up load is a device or circuit you choose to keep running during an outage. The list might include a refrigerator, lights, a furnace blower, a sump pump, or selected kitchen circuits. Start with needs, not the assumption that every home circuit must run at once.

Write down each selected load’s voltage and running watts or amps. Check nameplates and equipment manuals when available. If a device lists amps but not watts, volts multiplied by amps gives volt-amperes; it does not always equal watts. Power factor affects the conversion, so use manufacturer data or ask an electrician when it matters.

Also note which loads may run together. A water heater, oven, clothes dryer, and air conditioner can each use substantial power. If you do not need all of them during an outage, exclude some circuits or plan to manage their use.

How do you account for motor starting?

Motor-starting demand is the extra electrical demand some motors need for a short time as they begin to turn. Air-conditioning compressors, well pumps, and sump pumps can have a higher starting demand than running demand. A generator that handles their running load may still struggle to start them.

Identify the largest starting load and check its manufacturer information. Some generator specifications state motor-starting capability, often in terms of starting kVA, and may describe voltage dip. Compare that information with the motor’s requirements; do not assume a general “surge” figure proves compatibility.

If several motors could start together, ask whether the system can sequence or shed loads. Approved load-management controls can delay or disconnect selected loads, while a compatible soft-start device may reduce a motor’s starting demand. Use only equipment approved for the generator and the specific load.

These are broad examples, not equipment specifications. Actual demand varies by model and operating conditions. Use nameplate or manufacturer data for the equipment being considered.

Worked sizing scenarios: what does the calculation show?

A worked scenario is a planning example, not a final equipment recommendation. It shows how running loads and motor starting affect the decision, but it does not replace a load calculation, manufacturer data, or an installation review.

Scenario A: Essential circuits. Suppose selected loads total 4.5 kW while running. A pump in that group uses 0.8 kW while running, and its starting demand is listed as 2.4 kW. If the other loads are already running when the pump starts, the added starting demand is about 1.6 kW: 2.4 minus 0.8. The system may therefore need to support about 6.1 kW during that start, subject to the generator’s starting capability and other limits.

Scenario B: More whole-home use. Suppose a household wants to run the essential circuits above and also use a 5 kW appliance. The running total becomes 9.5 kW. If the pump could start while that appliance and other loads are operating, its starting demand still matters. The homeowner might choose a larger generator, exclude that appliance, or use approved load management. The best option depends on priorities, service design, and equipment ratings.

These examples show why adding running watts alone can mislead. They do not show that any particular generator model is suitable.

Site, Fuel, Transfer Switch, and Installation Safety

An installation is the complete system, not just the generator. The generator output, automatic transfer switch (ATS), electrical service, fuel supply, location, and local rules must work together. Ratings and placement rules vary by model and site, so the installation manual and local code guide the final design.

What does the automatic transfer switch do?

An automatic transfer switch detects a utility outage and transfers the connected loads to generator power. When utility power returns, it transfers those loads back. Correct transfer equipment prevents the utility and generator from supplying the home at the same time.

The ATS ampere rating and pole configuration must match the service and system design. “Pole” refers to the switching arrangement for the conductors in that design. Do not choose an ATS based only on the generator’s kW rating; the installer must check the service, backed-up circuits, and applicable requirements.

Never connect a generator to household wiring through a wall receptacle or an improvised backfeed cord. That can energize utility wiring and create a serious hazard. Use correctly installed transfer equipment.

How do fuel and location affect generator sizing?

Fuel availability can limit the generator’s output. Natural-gas and propane models have specific fuel-pressure and fuel-consumption requirements. A generator may have different rated output on different fuels, so compare the rating for the fuel you will actually use.

Ask the installer to verify that the gas supply, meter or tank, regulator, and piping can meet the generator’s stated demand. Do not assume an existing gas line has enough capacity. Required pressure and consumption are model-specific, as are exhaust direction and clearance limits.

Placement also matters. Exhaust must not enter a home or other enclosed space. Follow the exact manufacturer instructions and local fire and electrical codes for clearances, openings, and installation location. There is no safe universal clearance figure for every generator and site.

Commissioning, Load Testing, and Routine Maintenance

Commissioning is the set of checks performed when the system is installed to confirm that it transfers power and operates as designed. It should include the generator, transfer switch, fuel supply, alarms, and representative loads. A qualified installer should follow the equipment instructions and applicable code.

Have the installer verify the load calculation, generator rating, ATS configuration, fuel capacity and pressure, grounding and bonding design, permits, and required placement. These checks help catch mismatches before an outage.

During commissioning, test the automatic transfer when utility power is interrupted, the return to utility power, and any load sequencing or shedding. Run representative loads and confirm the system does not overload when motors start. Review how to read alarms and what to do if the generator fails to start or transfer.

Follow the manual’s exercise schedule and service intervals. Keep the unit unobstructed, inspect alarms as directed, and have the fuel system, battery, and transfer operation checked. Do not open or repair electrical or fuel components unless qualified to do so.

Seasonal Readiness and Long-Term Storage

A fixed standby generator is normally installed for ongoing use, but it may still be taken out of service for repairs or other reasons. Seasonal readiness means following the manual so the unit and its supporting systems remain ready when needed. Do not use generic storage steps in place of model-specific instructions.

Before a storm season or other period when outages are more likely, check the unit’s status and alarms, confirm that vents and access areas are clear, and review the exercise and service schedule. Make sure you know whom to call if an alert appears.

If the generator will be shut down for an extended period, follow the manufacturer’s storage and restart instructions. Fuel handling, battery care, and restart steps depend on the model and fuel type. Have a qualified technician inspect it before relying on it again.

Frequently Asked Questions

How many kW does a house need for a standby generator?
There is no single kW size for every home. Add the running demand of the circuits you want powered, then account for the starting demand of motors and verify the generator’s rated output.

Is a 10 kW generator enough for a house?
It may suit some selected-load plans, but the number alone cannot confirm that. Check voltage, actual running loads, motor starts, and the generator’s rating on the intended fuel.

Should I size a generator by running watts or starting watts?
Use both. Running watts show the ongoing load; motor-starting demand shows whether a pump or compressor can start while other loads are operating.

What is the difference between kW and kVA?
kW measures real power, while kVA measures apparent power. They are related by power factor: kW equals kVA multiplied by power factor.

Can a standby generator run central air conditioning?
Possibly. Check the air conditioner’s voltage and starting requirements, then confirm that the generator can start it while supporting the other planned loads.

Does a larger generator always work better?
Not automatically. The generator still needs compatible voltage, transfer equipment, fuel capacity, and installation. A larger unit may also have different fuel and site requirements.

Do I need an automatic transfer switch?
A standby system needs correctly installed transfer equipment to switch between utility and generator power safely. The ATS rating and configuration must fit the service and system design.

Can I connect a generator to a wall outlet?
No. Never use a receptacle or backfeed cord to connect a generator to household wiring. Use properly installed transfer equipment.

Can I use the same fuel rating for natural gas and propane?
Do not assume so. Output and fuel requirements can differ by fuel and model. Check the nameplate and manufacturer specifications for the fuel in use.

Who should confirm my generator size and installation?
A qualified electrician or generator installer should verify the load calculation, generator and ATS compatibility, fuel supply, placement, permits, and local code requirements.

(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)