A standby generator that is too small can leave you without heat, water, refrigeration or the ability to run critical business equipment when the power fails. One that is too large can add unnecessary purchase, installation and fuel costs. To size a standby generator correctly, start with the loads you truly need to keep operating, then account for the way those appliances and systems start.

For homeowners, the right answer may be enough power for the heating system, sump pump, well pump, kitchen refrigeration, lighting and internet. For a commercial property, it may mean protecting a server room, emergency lighting, access control, refrigeration, pumps or a defined part of the operation. The goal is not simply to buy the biggest generator available. It is to install a dependable system that supports your property safely when utility power is unavailable.

Start With Essential Loads, Not Every Appliance

The first decision is whether you need whole-property backup or selected-circuit backup. This choice has more impact on generator size than nearly any other factor.

A selected-circuit system powers only the equipment identified as essential. In a typical home, that might include the boiler or furnace, refrigerator, freezer, a few lighting circuits, internet equipment, garage door opener, sump pump and well pump. This approach often keeps generator capacity and fuel use reasonable while covering the equipment that matters most during an outage.

Whole-home backup is designed to carry most or all normal electrical use. It can be appropriate for larger homes, properties with medical equipment, homes that rely on electric heat or owners who want the least disruption during extended outages. It also requires careful load management. Central air conditioning, electric ranges, clothes dryers, electric vehicle chargers and electric water heaters can quickly push demand beyond what a practical residential generator should carry at one time.

For commercial and industrial sites, essential loads should be defined with the same discipline. A standby system may be intended to keep a building safe and secure rather than maintain full production. Emergency egress lighting, fire-related systems, communications, critical refrigeration and building controls may take priority over convenience loads. Facility operators should identify what must remain online, what can be shed and what can wait until normal power returns.

How to Size a Standby Generator From Electrical Demand

Generator capacity is generally measured in kilowatts, or kW. Your electrical panel, however, contains circuits that may be labelled in amps, while appliance nameplates may show volts, amps, watts or horsepower. A professional load calculation brings these figures into one clear demand number.

As a simple starting point, watts are calculated by multiplying volts by amps. A 120-volt appliance drawing 10 amps uses approximately 1,200 watts. A 240-volt appliance drawing 20 amps uses approximately 4,800 watts. But adding every breaker rating in the panel is not a valid way to size a generator. Breakers are sized to protect wiring, and most circuits do not operate at their maximum rating all at once.

Instead, review the actual equipment that will run during an outage. Nameplates on pumps, HVAC equipment, refrigerators and other major loads provide useful information. Utility bills can help show general consumption, but they do not identify the moment when several high-demand appliances start together. A site assessment is the reliable way to confirm the demand.

The generator must also have enough capacity for start-up demand. Motors often require a higher surge of power when they start than when they are running. Well pumps, sump pumps, compressors, air handlers, refrigerators and commercial refrigeration equipment are common examples. If a generator is sized only for their running load, voltage can dip when a motor starts. That can trip protection devices, interfere with electronics or prevent the motor from starting at all.

Modern inverter-driven HVAC systems and soft-start equipment can reduce starting demand, but they do not eliminate the need for proper calculation. The equipment at your property determines the answer.

Load management can reduce the required capacity

Load management allows a generator system to prioritise selected equipment and prevent too many large loads from operating at the same time. For example, a generator can be set to run heating equipment before allowing central air conditioning, or to temporarily hold off an electric water heater while a well pump starts.

This can make a whole-home or larger commercial backup plan more practical without oversizing the generator. It is not a shortcut around sound design. The transfer equipment, managed loads and generator controls must all be selected and installed correctly.

Common Loads That Change Generator Size

A few loads frequently change the scope of a generator installation. Heating is one of them. A gas- or oil-fired boiler may need relatively modest electrical power for controls, circulators and burners, while electric resistance heat can require substantial generator capacity. Heat pumps can also have high demand, especially when auxiliary electric heat activates in cold weather.

Water systems deserve close attention in Southern New Hampshire and Northern Massachusetts. A private well pump, pressure tank controls and water treatment equipment may be essential for a home to function during an outage. A sump pump is equally important where groundwater or stormwater creates a flooding risk. If both may run at once, their start-up loads need to be considered together.

Kitchens and laundry areas are another source of surprises. A refrigerator is usually manageable, but an electric range, electric dryer and dishwasher heating element can consume a significant portion of generator capacity. The same is true of hot tubs, pool equipment and electric vehicle charging. These loads may be excluded, managed or included in a larger system depending on the property owner's priorities.

At commercial sites, elevators, large rooftop units, process equipment, commercial kitchens and refrigeration systems require a more detailed engineering approach. Some equipment has manufacturer-specific requirements for standby power. In certain occupancies, life-safety systems may also be subject to code requirements that go beyond a typical optional standby generator installation.

Generator Size Is Only Part of the Installation

Selecting the right kW rating is essential, but the generator itself is only one part of the system. A reliable installation also includes a correctly rated automatic transfer switch, appropriate wiring and overcurrent protection, a suitable fuel supply, grounding and bonding, and proper placement outdoors.

The automatic transfer switch separates the property from the utility during an outage and transfers the selected loads to generator power. It must be matched to the electrical service and the generator design. This is critical for safety. A portable generator connected improperly can backfeed utility lines, creating a serious hazard for utility crews and neighbours. A professionally installed standby system prevents that risk and restores power automatically when it is needed.

Fuel capacity also affects the decision. Natural gas offers continuous fuel where service is available, but gas piping must be sized for the generator's full demand alongside existing appliances. Propane offers dependable on-site storage, though tank size, delivery access and expected outage duration should be considered. Diesel is common for many larger commercial applications but requires fuel maintenance and storage planning.

Local permits, manufacturer clearance requirements and electrical code requirements should be addressed before installation begins. Generator location needs to account for exhaust, service access, snow conditions, noise and clearance from openings in the building. A low-cost installation that overlooks these details can become expensive to correct later.

Plan for the Outage You Are Most Likely to Face

A generator should be sized for realistic conditions, not a best-case scenario. In this region, outages may occur during winter storms, heavy rain, wind events or extreme summer heat. Think about what the building needs during those conditions.

A homeowner may decide that reliable heat, water, sump protection, food storage and communications matter more than running every appliance. A property manager may prioritise common-area lighting, heating controls, domestic water pumps and security systems. A business owner may need to protect inventory, communications and controlled shutdown procedures before considering general convenience loads.

It is also worth planning for how the property may change. An upcoming panel upgrade, heat pump, finished basement, workshop, well pump replacement or electric vehicle charger can alter the load calculation. If those changes are close, include them in the design conversation now rather than finding the generator is undersized later.

Dalpe Electric can assess your electrical system, identify the loads that matter most and recommend a standby generator system built around your property, fuel source and outage priorities. Licensed installation and proper testing give you confidence that the system will respond when utility power does not.

The most useful next step is simple: write down what must stay on during an outage, including heating, water, pumps, refrigeration, safety equipment and business-critical systems. With that list and a professional load assessment, you can make a generator decision based on real needs rather than guesswork.