Quick answer: Add the running watts of everything you want on at once, then add the starting watts of the single largest motor. That is Generac’s own instruction. Do it with certified appliance figures and a typical outage kit comes to about 1,400 running watts — not the 2,800 the generator industry’s own table gives you.
What size generator do I need is the most common question in home backup, and every result on the first page of it is a calculator. Seven of them. They disagree with each other about the one rule that matters, none says where its wattages came from, and not one asks the question that can cost you a fifth of the machine you just bought.
So this guide is built the other way round: from the manufacturer’s own manual, and from the sixteen appliance articles on this site — the appliance wattage chart and starting watts versus running watts among them — where every figure is traced to a certification database or a maker’s spec sheet. Where a number is ours, we say so. Where nobody publishes one, we say that too.
Step 1: the loads, and where the numbers come from
List what you want running at three in the morning. Not what you own — what has to be on at the same time. For most households that is a fridge, the furnace blower so the heating works, a sump pump if you have a basement, the internet, and some light.
Then you need a watt figure for each. This is where it goes wrong, because the table most people reach for is the one in the back of the generator manual. Here is that table next to the figures we have verified.
| Load | Generac’s Wattage Reference Guide | What the primary source publishes |
|---|---|---|
| Refrigerator | 700 W | 53 W average, 105–158 W with the compressor running — median of 2,886 certified models |
| Sump pump | 800 to 1,050 W | 598 VA for a 1/3 HP unit, from its 5.2 A at 115 V rating |
| Furnace fan (3/5 HP) | 875 W | 27 to 499 W published by Trane; 90 W on low, 417 W on high |
| Submersible well pump, 1 HP | 2,000 W | 1,210 W published by Franklin Electric |
| Submersible well pump, 1/2 HP | 1,500 W | 670 W, same source |
| Wi-Fi router and modem | not listed | 8.22 W for the pair, across 374 reported models |
Add the four loads a basic outage kit actually uses, plus 200 W of lighting, and the two columns give you this:
2,825 running watts against 1,373. More than double, for the same house on the same night. Both totals are ours; every input is somebody else’s.
We are not going to tell you Generac is lying. Those look like conservative figures for a generic class of appliance, written before the certified datasets existed in the form they do now, and a manufacturer that oversizes you is never going to get a complaint about it. But every one of the gaps runs the same way, and the direction is a bigger generator.
There is a second trap in the same section. The manual tells you that if an appliance does not give a wattage, “multiply volts times ampere rating to determine watts”. That shortcut is exactly what we took apart in how to read an appliance label: on a well pump it overstates the real draw by 72 per cent, because the amp rating on the plate is a maximum the motor never sustains. The full list of what sixty household loads actually draw is in the appliance wattage chart.
Step 2: add one starting figure, not all of them
Motors need a hard pull to get turning and then settle. That gap between the pull and the settled draw is the thing that decides generator size, and it is where the calculators split into two camps: some add every appliance’s surge together, some add only the largest.
Generac settles it in its own manual, in a numbered instruction:
“1. Calculate the watts needed to start the largest motor. 2. Add to that figure the running watts of all other connected loads.”
One motor. The rest at their running figure. Motors do not start in unison, and sizing for a simultaneity that never happens is what starting watts versus running watts costs out at $424 of capacity you never use.
Here is the part nobody writes down honestly. Among the loads above, the largest published starting figure is not the pump. It is the fridge: certification data records a compressor inrush reaching 4,626 VA. That lasts milliseconds — it is a sub-cycle peak, not a motor spinning up — and it is not the figure generator sizing is built around. But no manufacturer publishes how its alternator behaves against a peak that short, so we cannot tell you the margin, only that it exists and that the industry sizes on the motor-start figure instead. The pump’s published surge, 920 VA on a 1/3 HP unit, is the number that governs in practice.
Step 3: the watts on the label are not the watts you get
This is the finding that no calculator on the first page accounts for, and it is printed in the specification table of the manual, immediately under the wattage it is qualifying:
“Maximum power decreases about 3.5% for each 1,000 feet above sea level; and will also decrease about 1% for each 6° C (10° F) above 16° C (60° F) ambient temperature.”
Run that against a 7,500 W unit in Denver, at 5,280 feet, on a 95 °F afternoon. Altitude takes 18.5 per cent. Temperature takes another 3.5. The nameplate says 7,500 W and the machine gives you about 5,850. That arithmetic is ours; the two rates are Generac’s.
At sea level in October it barely matters. In Colorado, Utah, Wyoming or New Mexico it decides whether the well pump starts. Nobody asked you for your elevation, and it is the second most important thing about your generator after the loads.
The same table carries a second qualifier worth reading before you buy dual fuel. The GP7500E is rated 7.5 kW on gasoline and 6.8 kW on propane, with surge falling from 9.4 to 8.5 kW — propane costs you about nine per cent of the machine. It costs run time too: 14 hours at quarter load on gasoline against 7.5 on a 20 lb propane cylinder. Propane stores for years and gasoline does not, which is the trade; the maker publishes both halves of it in one line.
What size generator do I need, by household
With verified figures rather than the reference guide, the tiers move down a step. These are running-watt budgets, before the single largest starting figure is added and before any derating.
| Tier | What it covers | What it will not do |
|---|---|---|
| 2,000–2,500 W | Fridge, internet, phones, lights, a CPAP. The whole communication-and-food kit at about 1,400 running watts, with headroom for one motor start. | Anything resistive. One 1,500 W heater eats the machine. |
| 3,500–4,500 W | The above plus a furnace blower and a sump pump running in the same night, or a microwave used in bursts. | A well pump and a furnace starting together at altitude. |
| 5,000–7,500 W | A well, a sump, the furnace, the fridge and a kettle. This is the tier most households land on and the one the reference guide sells them one size above. | Central air conditioning. That is a different conversation. |
| Above 7,500 W | Central air, an electric range, a workshop, or a house you intend to run normally rather than survive in. | Being portable in any meaningful sense, or being quiet. |
The four loads that change the answer
A well pump
Without water the house is uninhabitable, so this one is rarely optional. The good news is that it is smaller than the charts say: Franklin publishes 670 W for 1/2 HP and 1,210 W for 1 HP, against the 1,500 and 2,000 in the reference guide. Doubling the horsepower does not double the draw.
A sump pump
The one load where failure floods the house, and the one most oversized. A 1/3 HP pump is rated 5.2 A at 115 V with a published 920 VA surge, not the 2,900 W the charts quote. It is also duty-cycled: it runs when the float rises, not continuously, which matters more for fuel than for sizing.
Heat
A gas furnace needs only its blower, and Trane publishes 27 to 499 W for one unit depending on speed and duct pressure. Electric resistance heat is the opposite: a single 1,500 W heater is more load than the entire fridge-and-internet kit put together, and no portable generator makes that arithmetic work for a whole house.
Medical equipment
A CPAP is tiny — 11 W alone, 59 W with heated humidifier and tubing — and a generator is the wrong tool for it. It is noisy, it lives outside, and it cannot run in the bedroom. If this is the load you are solving for, size it with the equipment supplier and read the power station sizing guide instead: a battery is silent and indoor-safe, which a generator is not.
Portable, inverter or standby
Once you have a wattage, the class of machine is the next fork, and the three are not competing for the same job.
A conventional portable is an engine turning an alternator at a fixed 3,600 rpm whatever the load. It is the cheapest way to buy a watt and the loudest way to own one, and its fuel burn falls far less than its output does: at half load it still uses 56 to 66 per cent of what it drinks flat out.
An inverter generator throttles the engine to the load and rebuilds the output electronically, which is why it is quieter and why it sips fuel at low load. It costs more per watt and tops out smaller. If the reason you are asking is a computer or a CPAP, this is the class the question points at — though which appliances genuinely care about the shape of the waveform is far less settled than the marketing suggests, and we went through what three inverter manufacturers actually publish in pure sine versus modified sine. They agree on one item out of everything they list.
A standby is not a purchase, it is an installation: bolted down, wired into the panel, fed from the gas line, starting itself when the power drops. It is the only one of the three that works while you are away from the house, and the only one that brings permits, an electrician and a gas fitter with it. The sizing arithmetic above still applies; what changes is that nothing about it is a weekend decision.
How we calculated this
Every appliance figure in the right-hand column is quoted from the article linked beside it, and each of those traces to a certification database, a federal dataset or a manufacturer specification sheet, with the date it was read. The Generac column, the derating rates, the propane ratings and the run times are quoted from the GP Series owner’s manual, specification table for the GP7500E DF-CO, read on 8 September 2026.
Three sums are ours and are labelled as ours: the two running totals of 2,825 W and 1,373 W, and the Denver derating of about 5,850 W from a 7,500 W nameplate. The load list behind both totals is identical — fridge, furnace blower, sump pump, router, 200 W of lighting — so the only variable is where the wattages came from.
We have not tested a generator and we do not recommend one. Generac appears here because it publishes a manual you can download and check, not because we rate it. No prices are quoted anywhere in this guide: they move weekly and by state, and a figure with no date on it is worse than no figure. Our method is at how we test.
Read this before the sizing
Carbon monoxide kills faster than people expect. Generac’s own manual puts it in capitals: “Using a generator indoors CAN KILL YOU IN MINUTES”, and never in a garage “EVEN IF doors and windows are open”. Outdoors only, well away from windows, doors and vents. Fit a battery-operated CO alarm indoors; the generator’s own CO shutoff is not a substitute for one.
Never run a generator into a wall socket. That is backfeeding, and it energises the line outside your house and can kill a utility worker restoring your street. Generac requires that “a transfer switch must isolate the generator power from the utility power at all times”, installed “in strict compliance with all national and local electrical codes” and “completed by a qualified electrician”. We are not going to describe how any of it is wired, because that is not a job to learn from a web page.
Fuel. Petrol is refuelled with the engine off and cool. Propane cylinders do not go in a building, garage or enclosed space except as NFPA 58 allows. And a generator is not a life-support device: Generac prints that warning on page one.
Frequently asked questions
What size generator do I need to run a whole house?
The question is usually the wrong one. Running a house normally means central air and an electric range, which puts you above 7,500 W and into standby territory. Running a house through an outage — food, heat, water, internet, light — is about 1,400 running watts on certified figures, and the tier that covers it comfortably is far smaller than the charts suggest.
Is 5,000 watts enough?
For a fridge, a furnace blower, a sump pump, the internet and lights, comfortably yes, with room for one motor start. It stops being enough the moment resistive heat joins the list, or if you are at 5,000 feet, where the same machine is delivering closer to 4,100 W.
Do I add all the starting watts together?
No, and the manufacturer says so. Take the starting watts of the largest motor and add the running watts of everything else. Adding every surge assumes all your motors start in the same instant, which does not happen, and it is the single most expensive mistake in generator sizing.
Does altitude really matter?
About 3.5 per cent per 1,000 feet, published by Generac in the same table as the wattage. At Denver’s elevation that is nearly a fifth of the machine gone before you plug anything in, and a hot afternoon takes a little more on top.
Gasoline, propane or dual fuel?
Propane costs about nine per cent of the rated output and roughly half the run time per tank, and buys you fuel that keeps for years instead of months. On the GP7500E that is 7.5 kW against 6.8, and 14 hours against 7.5 at quarter load. Both figures come from the same specification table.
Can I plug it into a socket to power the house?
No. That is backfeeding and it can kill someone working on the line. The manufacturer requires a transfer switch that isolates generator power from utility power at all times, fitted by a qualified electrician to national and local code.
The short version
Get your wattages from something that cites a source, not from the table in the back of the manual. Add the running watts of what is on at once, add the starting watts of one motor, then take off 3.5 per cent for every thousand feet you live above the sea. Most households find the answer is a tier smaller than they were about to buy — and the machine that is one tier smaller is also the one that is quieter, lighter and cheaper to feed.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
