What Size Generator Do You Actually Need? The Real Load-Calculation Guide

By ANT HVAC & ELECTRIC · a third-generation Illinois trade family · July 2026

Every generator sizing page I pull up says some version of the same thing: "a 2,000 sq ft home needs a 16-20kW generator." That's not sizing, that's a guess dressed up as a chart. A 2,000 sq ft house with a gas furnace, gas water heater, and gas range pulls a fraction of the amps that a 2,000 sq ft house with electric heat and an electric range pulls — the charts size them identically anyway.

I don't size generators off square footage. I size them the same way I size a service upgrade or a sub-panel: add up the actual loads on the house, run them through the calculation the electrical code gives us for exactly this purpose, and let the number tell me what the generator needs to be. That's what this guide walks you through — not so you can skip talking to an electrician, but so you understand what he should be doing when he quotes you, and so you can sanity-check an estimate that seems too round to be real.

Why Wattage-by-Square-Footage Charts Are Garbage

Square footage charts don't ask what your heat source is, whether you have a well pump, or whether your AC condenser is a 2-ton unit or a 4-ton unit. Two houses of identical size can have real electrical demand that differs by 30-40% depending on appliances alone. A chart can't see that. A load calculation can.

Most charts also quote "starting watts" and "running watts" for a handful of appliances and just say to add them up. That skips the part where code tells you which loads count at full value and which get diversified down, because not every circuit in the house pulls max load at the same second. That's the whole point of a load calculation: it accounts for diversity, not brute-force addition.

The Real Method: NEC 220.82

The National Electrical Code has a defined procedure for calculating the load on a dwelling unit's electrical service — Article 220. For single-family homes there's an "optional method," laid out at 220.82, designed for exactly this purpose.

Here's the structure of that method, in plain terms:

1. Add up the "general loads" on the house — lighting, receptacles, small-appliance and laundry circuits, and the nameplate rating of every fixed appliance (dishwasher, disposal, built-in microwave, sump pump, etc.).
2. Apply a demand factor to that total: the first chunk gets counted at full value, and everything above that gets discounted, because statistically the whole house never draws 100% of every circuit at once.
3. Add the heating or air conditioning load — whichever is larger — at full value, since HVAC is a load you generally can't diversify down for backup power purposes.

That produces a number in volt-amps (VA), which for sizing purposes we treat as roughly equivalent to watts. Divide by your voltage, and you've got the amperage the service actually needs to carry — and from there, the generator size that will actually cover it.

The Demand Factor, Specifically

The optional method counts the first 10,000 VA (10 kVA) of general load at 100%, and everything above that at 40%. That 40% figure is the whole reason square-footage charts oversize everything: they don't diversify anything, so they push you toward a bigger, more expensive unit than the code math actually calls for.

Running Watts vs. Starting Watts (LRA)

Two numbers matter for every motor-driven appliance on your list — your AC condenser, your sump pump, your well pump if you have one, your furnace blower:

• Running watts — what it draws once it's up and spinning.
• Starting watts / LRA (locked rotor amps) — the surge it pulls for a fraction of a second when the motor first kicks on.

A generator has to handle that surge, not just the steady-state running load, or it'll stall or trip when the AC compressor kicks on. An AC compressor's LRA typically runs 4-6× its rated running amps, though it varies by compressor type and manufacturer — which is why an experienced installer reads nameplate data off your actual condenser instead of using a generic "AC = X watts" line from a chart.

Step-by-Step: How I Actually Do This

Step 1 — List every circuit you want backed up. Whole house, or just the essentials? This decision happens before the math, because it changes what you're calculating.

Step 2 — General lighting and receptacle load. Code figures this at 3 volt-amps per square foot of the home.

Step 3 — Small appliance and laundry circuits. Code requires a minimum of two dedicated 20-amp small-appliance circuits plus one laundry circuit, each counted at 1,500 VA regardless of what's actually plugged in.

Step 4 — Fixed appliance nameplate loads. Dishwasher, garbage disposal, built-in microwave, sump pump, well pump, electric water heater if applicable — pull the actual nameplate wattage/amperage off each unit. Don't guess.

Step 5 — Apply the demand factor from 220.82 to steps 2-4 combined.

Step 6 — Add HVAC at full value — the larger of your heating load or your cooling load, not both. You rarely run full heat and full AC simultaneously, and the code accounts for that.

Step 7 — Convert total VA to kW, then to a generator size, rounding up to the nearest standard unit your installer stocks.

Worked Example: 2,000 sq ft Home, Gas Furnace, Central AC, Sump Pump, Typical Kitchen

Here's an actual house — gas heat (so the furnace itself is a small electrical load, just the blower motor), central AC, a sump pump in the basement, and a normal kitchen with a dishwasher, disposal, and built-in microwave. Appliance values shown are typical; on a real job every one comes off the actual nameplate.

15,460 VA is roughly 15.5 kW of calculated running load. Add margin for the AC compressor's startup surge and the standard practice of rounding up to the next unit size the manufacturers actually make (14, 16, 18, 20, 22, 24kW), and this house lands on an 18-20kW standby generator for whole-home coverage.

A square-footage chart would land near this by coincidence at best — it has no idea this house has gas heat instead of electric, a sump pump, or a 3-ton condenser instead of a 5-ton. Change any one of those and the real number moves; the chart wouldn't.

Partial-Home vs. Whole-Home: The Real Tradeoff

This is the decision that actually drives your budget, more than any wattage math does.

Whole-home backs up everything on the panel, sized to the full calculated load like the example above. You never think about what's running during an outage — you also pay for a bigger unit and usually a bigger transfer switch.

Partial-home / essential-circuits backs up a defined subset — furnace blower, sump pump, fridge, some lighting and receptacle circuits, maybe one AC zone — wired through a sub-panel. Smaller generator, smaller price tag, but you live with a mental list of what's covered during an outage and what isn't.

Neither is "more correct." It's a tradeoff between cost and convenience, and I want a homeowner making that choice on purpose, not by accident because a chart told them a number.

Load Management Modules: The Money-Saver Nobody Explains Well

Here's the piece that actually saves people money and almost nobody covers honestly: a load management module (LMM).

An LMM sits between your transfer switch and specific high-draw circuits — most commonly the AC condenser, sometimes an electric water heater or dryer — and temporarily sheds that load if the generator's already near capacity from other demand. In practice: you can spec a smaller generator that covers your whole panel most of the time, and the module briefly drops the AC compressor for a few minutes if the oven, dryer, and sump pump all happen to run at once. Exact behavior and eligible circuits vary by manufacturer and model.

A lot of sales pitches skip this entirely and just sell you the next size up. Sometimes that's the right call, if you never want the AC dropping even briefly. But an LMM is often the difference between an 18-20kW unit and a 14-16kW unit on the same house — a real cost difference worth knowing before you sign anything. (Exact numbers coming in the cost guide.)

Common Sizing Mistakes I See

• Guessing appliance wattage instead of reading the nameplate. Every fixed appliance has one listed. Use it.
• Ignoring diversity entirely — adding every circuit's max draw with no demand factor, which oversizes the generator and the price tag.
• Forgetting the sump pump. Small on paper, but often the most important circuit in the house during the exact storm that's running your generator.
• Not accounting for startup surge on the AC compressor or well pump, and ending up with a generator that stalls the first time the compressor kicks on.
• Sizing off square footage instead of the actual panel. Two identical-looking houses can have real loads that differ by a third or more.

When to Just Call Someone

You can walk through this math yourself with a notepad and your appliance nameplates, and it'll get you in the right neighborhood. But the decision that determines what actually gets installed should be based on someone reading your panel schedule, checking your AC condenser's nameplate, and confirming what circuits you want backed up. That's not a sales pitch — it's how you avoid buying the wrong size in either direction. Once you know your size, the next question is how it connects: read how a transfer switch actually works.