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If you are asking, “what size generator do I need?” start with the equipment you want to run, not the square footage of your house. A generator must handle the combined running watts of simultaneous loads plus the largest applicable motor-starting surge.
For homeowners considering Electrical service in West Lebanon, NH, and throughout the Upper Valley, that list often includes heating equipment, a well pump, a sump pump, refrigeration, lights, internet equipment, medical devices, and possibly central air conditioning. This guide will help you build a preliminary estimate. A final generator selection still requires an on-site load calculation using your home’s actual equipment ratings.
Quick answer: Add the running watts of everything that may operate at the same time. Then add the largest extra startup demand, which is the equipment’s starting watts minus its running watts. Confirm the result on site before selecting a permanently installed generator.
What Size Generator Do I Need for Essential Circuits?
For essential-circuit backup, size the generator around the systems that keep your home safe and functional during an outage. This approach usually requires less capacity than whole-home coverage, but it may require you to manage when large appliances run.
Typical essential circuits include:
- Furnace or boiler controls, blowers, and circulators
- Refrigerator and freezer
- Well pump and sump pump
- Selected lights and outlets
- Router and communication equipment
- Outage-critical medical equipment
An electrician can map these loads to selected circuits and determine whether the existing panel is suitable. If your service has limited capacity, frequent breaker trips, or an older panel, review Do I Need an Electrical Panel Upgrade? Warning Signs, Costs & Installation Options and Jerm’s Electrical Panel Upgrade service.
Essential-Load vs. Whole-Home Generator Sizing
Essential-load backup powers a planned group of circuits. Whole-home backup is designed to support most household circuits, often with load management to prevent too many large loads from starting together.
| Coverage plan | Usually includes | Main sizing concern |
| Essential loads | Heat, refrigeration, pumps, lights | Which loads must run together |
| Expanded essentials | Essentials plus more outlets or appliances | Pump and compressor startup |
| Whole home | Most household circuits | Total simultaneous demand |
| Whole home with central AC | Most circuits plus cooling | Compressor-starting demand |
A standby generator does not necessarily power every appliance simultaneously. Smart load management may temporarily delay central AC, an electric range, or another lower-priority load. A Residential Electrician can help separate must-have circuits from convenience loads without oversizing the system.
What Are Running Watts and Starting Watts?
Running watts are the continuous watts equipment needs after it starts. Starting watts are the higher, brief demand created when a motor or compressor turns on.
Well pumps, sump pumps, refrigerators, freezers, furnace blowers, heat pumps, and a home AC unit may all have startup demand. That is why adding only the continuous wattage can produce an undersized estimate.
Use this preliminary formula:
- Add the running watts of loads expected to operate simultaneously.
- Find the largest starting-watt requirement among those loads.
- Subtract that item’s running watts from its starting watts.
- Add the extra startup demand to the simultaneous running total.
For example, assume the simultaneous running load is 4,200 watts. A well pump within that total uses 1,000 running watts and 3,000 starting watts. Its extra startup demand is 2,000 watts, making the preliminary requirement 6,200 watts.
This is a planning example, not a final equipment recommendation. Use manufacturer nameplates and documentation rather than generic wattage charts.
Which New Hampshire Home Systems Affect Generator Size Most?
Motor-driven and electric-heating equipment usually changes the answer most. In West Lebanon, cold-weather heating needs and private-well equipment can make two similar-looking homes require very different generator capacities.
Well Pump and Well Pressure Tank
The well pump, not the well pressure tank, is usually the important electrical load. A pressure tank stores pressurized water and reduces pump cycling. Generator sizing should use the pump motor’s voltage, horsepower, running demand, and starting demand.
The pump may start while heat, refrigeration, or a sump pump is already operating. That overlap must be included in the calculation.
Furnace, Boiler, Heat Pump, and Electric Heat
Fuel-fired heating equipment still needs electricity for ignition, controls, blowers, and circulator pumps. Heat pumps and electric resistance heat may require much more power. Because a winter outage can threaten plumbing and indoor safety, heating circuits often receive high priority in New Hampshire.
Sump Pump
A sump pump has a motor-starting surge and may cycle repeatedly during wet weather or snowmelt. If both the sump pump and well pump can start during an outage, the load plan must account for sequencing or overlap.
Refrigerator, Freezer, and Medical Equipment
Refrigerators and freezers have compressor-starting demand. Medical equipment should be listed separately using the manufacturer’s rated requirements, including any continuous-power or battery-backup needs.
Central Air Conditioning
A home AC unit can significantly increase required generator capacity because its compressor draws both running and starting power. Equipment size, number of systems, and approved soft-start or load-management options all affect the result. Do not estimate AC demand from the thermostat or breaker size alone.
How to Complete a Home Generator Load Calculation
A load calculation identifies the electrical demand the generator may need to serve. For preliminary planning, follow these steps:
- List essential equipment. Identify everything that must work during an outage.
- Record rated wattage. Check nameplates and manufacturer documentation.
- Identify motor loads. Mark pumps, compressors, blowers, and other equipment with starting watts.
- Decide what runs together. Separate simultaneous loads from equipment you can stagger.
- Total running demand. Add the running watts of simultaneous loads.
- Add the largest extra surge. Account for the biggest applicable startup event.
- Verify the electrical system. Confirm service, panel, transfer equipment, voltage, fuel rating, and load-management requirements on site.
| Household load | Running watts | Starting watts | Essential? | Runs simultaneously? |
| Refrigerator | ___ | ___ | Yes / No | Yes / No |
| Well pump | ___ | ___ | Yes / No | Yes / No |
| Furnace or boiler | ___ | ___ | Yes / No | Yes / No |
| Sump pump | ___ | ___ | Yes / No | Yes / No |
| Medical equipment | ___ | ___ | Yes / No | Yes / No |
| Home AC unit | ___ | ___ | Yes / No | Yes / No |
| Lights, router, outlets | ___ | N/A | Yes / No | Yes / No |
Important: An online worksheet cannot evaluate every equipment rating, wiring condition, fuel configuration, or simultaneous-load scenario. Contact Jerm’s Plumbing, Heating & Air for an on-site assessment and a generator recommendation based on your actual home.
How Do a Transfer Switch and Fuel Type Affect Sizing?
A transfer switch is equipment that safely transfers selected electrical loads between utility power and generator power. Its configuration helps determine which circuits can receive backup power and whether automatic load management is used.
Never connect a generator directly to home wiring without approved transfer equipment. OSHA explains that improper connections can create dangerous backfeed. Review OSHA’s portable-generator guidance and Ready.gov power-outage safety guidance.
Fuel type can also affect rated output. A propane generator may produce a different rated capacity than the same model operating on another approved fuel. Use the manufacturer’s rating for the fuel configuration installed at the home.
Jerm’s Generator Installation team can evaluate the generator, transfer equipment, fuel configuration, and actual household loads as one system.
Other Electrical Loads to Consider Separately
Do not automatically place every electrical project on generator backup. EV charging, exterior lighting, and convenience circuits may be lower priorities during an outage. Plan them separately so they do not distort the essential-load calculation.
Homeowners considering future improvements can review the Best EV Charger for Home guide and Jerm’s EV Charger Installation service. Other targeted services include Ceiling Fan Installation, Landscape Lighting, and Electrical Outlet Installation.
If the assessment uncovers unsafe wiring or damaged components, schedule Electrical Repair. A Whole-House Surge Protector Installation is a separate protective upgrade and does not replace correct generator sizing or transfer equipment. Explore all Jerm’s Electrical services for help prioritizing the work.
Schedule a Generator Sizing Assessment in West Lebanon, NH
The right generator size is the capacity that supports your real outage priorities, simultaneous running load, and largest applicable startup surge. It is not a number derived from square footage alone.
Contact Jerm’s Plumbing, Heating & Air to schedule an on-site load calculation for your New Hampshire home. A professional assessment can help you choose essential-load or whole-home coverage with confidence and avoid both undersizing and unnecessary capacity.
Frequently Asked Questions About Generator Sizing in West Lebanon, NH
Can I size a whole-house generator by square footage?
No. Square footage does not show whether a West Lebanon home uses a private well, heat pump, electric resistance heat, boiler circulators, central AC, or outage-critical medical equipment. It also does not reveal which appliances will operate together. Generator size should be based on rated equipment demand, starting watts, essential circuits, and household priorities. Two homes of equal size can need very different capacities because their mechanical systems differ. Use a circuit and equipment worksheet for preliminary planning, then have a qualified electrician complete an on-site load calculation before selecting a permanently installed unit.
What size generator do I need for a furnace and well pump?
The correct size depends on the furnace or boiler’s electrical components and the well pump motor’s rated running and starting demand. A fuel-fired furnace may need power for controls, ignition, and a blower, while a boiler may use several circulators. The well pump can add a substantial startup surge. Add the running watts of the equipment that may operate together, then include the greatest extra motor-starting demand. Because pump horsepower, voltage, heating equipment, and household overlap vary, there is no reliable universal size. An electrician should verify both nameplates and the planned backup circuits on site.
Does a well pressure tank use electricity during an outage?
The well pressure tank itself generally stores water under pressure and is not the main electrical load. The well pump motor uses electricity to refill the tank when pressure falls. Tank capacity and current water pressure may affect how often the pump cycles, but they do not replace the need to evaluate the pump’s voltage, horsepower, running watts, and starting watts. For generator planning, identify when the pump might start while heating equipment, refrigeration, or a sump pump is running. This distinction helps prevent homeowners from sizing a generator based on pressure tank size instead of the actual motor demand.
Do I need a larger standby generator to run central AC?
Often, yes. A central home AC unit adds continuous fan and compressor demand plus a brief startup surge. The exact effect depends on the system’s rated specifications, the number of cooling systems, and whether approved load management or soft-start equipment is appropriate. Some homeowners prioritize heat, water, refrigeration, and medical equipment while leaving central AC off backup. Others want cooling during summer outages and need the system designed accordingly. A qualified electrician should compare the AC nameplate data with the generator’s rated output and every other load expected to operate at the same time.
Why does a breaker trip when my generator is running?
A breaker may trip because of an overload, short circuit, ground fault, failing appliance, or incorrect load sequence. During generator operation, several modest loads can overlap with the startup surge from a well pump, sump pump, refrigerator, or furnace blower. Never repeatedly reset the breaker or install a larger breaker to stop the symptom. Turn off nonessential equipment and follow the generator manufacturer’s instructions. If the breaker trips again, stop using the affected circuit and call a qualified electrician. Testing can determine whether the problem is excess demand, damaged wiring, faulty equipment, or an issue with the backup-power configuration.
Can an online generator calculator give me the final size?
No. An online calculator is useful for organizing loads and producing a preliminary estimate, but it cannot inspect your electrical panel, verify every nameplate, measure actual demand, confirm fuel-specific output, or evaluate how equipment starts and overlaps. It may also rely on generic wattage values that differ from your equipment. Use a calculator to prepare questions and identify essential circuits, then request an on-site load calculation. That final review should consider the generator, transfer switch, service equipment, fuel configuration, heating and cooling systems, pumps, medical needs, and the way your household expects to use power during an outage.