What Size Generator Do I Need? A Nashua Sizing Guide

First: What a kW Means for Your House

Generators are rated in kilowatts, kW, the amount of electrical power they can deliver continuously. Your home's loads are measured in watts on their nameplates, and a thousand watts is one kW. Sizing is the exercise of adding up what you want running during an outage, honestly, including the brief starting surges of motor loads, and matching that total to a generator with headroom to spare. Everything else on this page is technique for doing that addition well.

Two kinds of load matter. Running load is what an appliance draws while it works: lights, electronics, a refrigerator between compressor starts, a furnace blower mid cycle. Starting load, also called surge, is the extra draw in the first seconds when a motor starts: air conditioner compressors, well pumps, sump pumps, refrigerators, and furnace blowers all surge above their running draw, sometimes by a large multiple. Generator failures in the field are usually surge failures: the running math fit, and then two motors started at once and the voltage sagged. Manufacturer sizing guidance, Generac's installer sizing guide included, is built on nameplate starting and running requirements for exactly this reason.

The kW Ladder

These bands reflect the planning ranges installers and manufacturers use nationally, cross checked against the home size and load patterns of the Nashua area housing stock. They are starting bands. The worksheet below narrows them to your house, and the installer's load calculation, done from your panel and nameplates, is the final word.

The Worksheet Method

Step 1: List what must run

Walk the house with a notebook. Write down the heating system and its fuel, every pump you own, well, sump, sewage, the refrigerator and any second fridge or freezer, and anything medical. These are non negotiable loads. If it protects the house, your health, or your water, it goes on the list.

Step 2: List what should run

Air conditioning, by system and size if you know it. Kitchen appliances, noting gas versus electric for the range, oven, and cooktop. Laundry, noting the dryer fuel. Water heating, noting its fuel, because an electric water heater is a heavy continuous load and a gas or oil unit is nearly free electrically. Lighting and outlets can be estimated as a block for the rooms you actually live in.

Step 3: List what would be nice

Hot tub, pool pump, workshop tools, EV charger, second refrigerator in the garage, finished basement systems. Nice loads are where load management earns its keep: they can be powered when capacity is free and shed when it is not, which lets you include them without buying a bigger generator for their sake alone.

Step 4: Flag the motors

Put a star next to every motor load: air conditioners, heat pumps, well pump, sump pump, refrigerator, freezer, furnace blower. These carry starting surges. The sizing has to absorb the largest motor starting while the rest of the must run and should run loads are already on. This single step is the difference between a paper calculation and a working system.

Step 5: Add headroom

Do not size to the exact total. Installer and manufacturer practice adds headroom above peak load, with commercial guidance describing units selected about 20 to 25 percent above peak so the generator runs near 80 percent of capacity rather than at its redline. A generator running at its limit through a three day outage is loud, thirsty, and stressed. Headroom is reliability.

Step 6: Check the fuel side of the answer

Confirm the model you land on keeps its rating on your fuel. Some home standby models are rated lower on natural gas than on propane. Then run the consumption figures for that size through the fuel page math so the tank or the gas service matches the generator you just picked.

Worked Examples: Nashua Area Home Profiles

Example 1: The gas heated colonial

A 2,400 square foot Nashua colonial, gas furnace, gas range, gas dryer, city water with a sump pump, one 3 ton central air conditioner, refrigerator, standard lighting and outlets, home office. Must run: furnace blower, sump pump, fridge. Should run: the air conditioner, kitchen, office. Motors to respect: the air conditioner first, then the sump pump and fridge. This profile is the heart of the whole house market here, and it lands in the 18 to 22 kW band with load management watching the air conditioner's starts against the rest of the house. Essentials only sizing for the same home, dropping the air conditioner to a maybe, fits 14 kW.

Example 2: The well and septic ranch

A single story ranch in Hudson or Merrimack, oil heat with an electric blower, a well pump that the whole water supply depends on, a septic system, an electric range, one smaller air conditioner, a freezer in the garage. The well pump is the sizing pivot: it is a real motor with a real surge, and it cycles whenever anyone uses water. Essentials coverage with the well, heat, fridge, and freezer fits 14 to 18 kW. Adding full kitchen use and the air conditioner without management pushes toward 18 to 22 kW. This is the profile where a portable generator disappoints most, because the well pump is exactly what a portable usually cannot start.

Example 3: The all electric home

A 3,000 square foot home with electric heat or a heat pump with electric backup strips, electric range, electric dryer, electric water heater, two air conditioning zones or a large heat pump, and a hot tub. Every heavy load in the house is electric, several are continuous, and two of them are among the largest residential loads that exist. This profile lands at 26 kW and up, with load management staging heat and shedding the hot tub and dryer during peaks, and it is the profile where an honest installer talks about priorities instead of promising everything at once. It is also where fuel planning matters most, because big generators at high load burn fuel at rates that make propane tank size a first order decision.

Example 4: The small home done right

A 1,400 square foot home, gas heat, gas water heat, city water, no air conditioning or one small unit, one refrigerator, modest loads everywhere. Essentials coverage at 10 to 14 kW carries this home completely, heat, water safety, food, light, and outlets, at the lowest installed cost in the standby market. Small homes are where standby generators are cheapest and where the protection per dollar is best.

The Five Sizing Mistakes

• Sizing by square footage alone. Square footage correlates with load. It does not determine it. The all electric 2,400 square foot home can need twice the generator of the gas heated one next door.
• Forgetting the starting surge. Running load math that ignores motor starts produces the classic failure: fine on paper, sagging in the first real outage.
• Ignoring the fuel derate. Comparing models by headline kW without checking the rating on natural gas versus propane overstates some units on some fuels.
• Planning for the first outage, not the tenth year. The air conditioner gets replaced with a bigger one, a hot tub arrives, an EV charger goes in. A size with headroom, or at least a conversation about future loads, is cheaper than a second generator.
• Buying the biggest unit as insurance. Oversizing has costs too: purchase price, fuel burn at light load, and in the worst cases poor engine loading over years of exercises. Right sized with headroom beats biggest available.

What the Installer Does That This Page Cannot

This guide gets you to the right band and the right questions. The installer's load calculation finishes the job from your actual panel schedule, appliance nameplates, and in many cases a measurement of your home's real demand. Cummins describes its sizing inputs the same way this worksheet does: square footage, home essentials such as heating, cooling, garage, and pumps, family essentials such as kitchen and laundry appliances, and added luxuries such as a hot tub, pool, and sauna. If a quoter's sizing process skips those inputs entirely and goes straight to a size, that is a finding about the quoter, not about your home. Bring your worksheet to the site visit. A good installer will be glad to see it, and will show you where their numbers differ from yours and why.

Sizing Notes for Specific Loads

Heat pumps and backup strips

The sizing trap of the decade in New Hampshire. A heat pump alone is a manageable motor load. A heat pump whose electric backup strips engage in deep cold becomes one of the largest loads in residential service, arriving exactly when the outage is worst. If your home heats this way, the strips dominate your worksheet, and the design conversation is about staging, shedding, or accepting a bigger generator with open eyes. Bring your heating contractor's specifications if you have them. This single load decides more 26 kW installs than any other.

Electric vehicle chargers

A home EV charger is a large, patient load: it draws heavily, and it does not care when. That makes it the ideal load management citizen, shed during peaks and restored when the house is quiet, and it rarely justifies a bigger generator by itself. Tell the installer it exists, or is planned, and it goes on the managed list. Forgotten, it becomes the mystery overload in your first outage.

Hot tubs, pools, and workshops

Discretionary loads with real appetites. A hot tub heater and a pool pump together can consume the headroom of a mid band generator. The standard treatment is inclusion with management: powered when capacity allows, shed when the house needs the capacity more. Sized that way, they cost you modules and honesty, not a size band. Workshops add motor surges of their own: a large compressor or saw started during a loaded moment is a surge event, and belongs on the worksheet with a star like any other motor.

When the answer falls between sizes

Load calculations often land near a boundary: the house computes to the top of one model and the bottom of the next. Decide with three questions. How certain is the load list: a young family adding loads argues up, a settled household argues as calculated. What does the step up cost, in equipment and in fuel burn at your typical load, against the headroom it buys? And what does load management already solve: if the boundary case is one shed module away from comfort, the smaller unit plus management is engineering, while the same gap without management is hope. A good installer walks this boundary with you using your numbers. One who always lands on the bigger unit, or always on the cheaper one, is telling you about their inventory, not your house.

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