Buying a generator is mostly an arithmetic problem wearing a shopping problem's clothes. Work out what you actually need to run, add up the starting loads rather than the running ones, and the right machine usually picks itself. Skip that step and you end up with a unit that either cannot start your well pump or cost twice what the job required.
This guide covers the three categories, how to size one properly, how to connect it without killing anyone, and the safety points that genuinely matter.
The Three Categories
Portable Generators
An engine driving an alternator on a wheeled frame, typically 3,000–10,000 watts. You start it manually, run extension cords or connect through an inlet, and refuel it. The cheapest watts you can buy, and the most common choice for occasional outages.
Trade-offs: loud (usually 65–75 dB at distance), heavy, and they produce power that varies more in frequency and waveform than the alternatives. That last point matters for sensitive electronics.
Inverter Generators
The engine drives a generator whose output is rectified to DC and then re-synthesised into clean AC by an inverter. Two consequences follow. The output is much closer to a pure sine wave, which makes it safe for laptops, TVs and modern appliances with electronic controls. And the engine speed can vary with load instead of running at a fixed 3,600 rpm, which makes them dramatically quieter and more fuel-efficient at part load.
They cost more per watt and top out lower in capacity, though 4,000–7,000 watt inverter units are now common. For anything involving sensitive electronics, camping, or neighbours within earshot, the premium is usually worth it.
Standby (Whole-House) Generators
Permanently installed outside on a pad, wired to the house through an automatic transfer switch, and fuelled by natural gas or propane. When the power fails, it starts itself within seconds and runs until utility power returns. Typically 10,000–26,000 watts for residential use.
This is the only option that works when you are not home, needs no refuelling on natural gas, and handles the whole house. It is also the only one requiring an electrician, a permit, and usually a gas fitter — installed cost commonly runs $5,000–$15,000 all in.
Comparison
| Portable | Inverter | Standby | |
|---|---|---|---|
| Typical output | 3,000 – 10,000 W | 2,000 – 7,000 W | 10,000 – 26,000 W |
| Equipment cost | $400 – $1,500 | $500 – $2,500 | $3,000 – $8,000 |
| Installed cost | None | None | $5,000 – $15,000 |
| Noise at 23 ft | 65 – 75 dB | 48 – 60 dB | 55 – 70 dB |
| Power quality (THD) | 5 – 20% | Under 3% | Under 5% |
| Fuel | Petrol, often dual-fuel | Petrol, often dual-fuel | Natural gas or propane |
| Runs unattended | No | No | Yes — starts itself |
| Refuelling | Every 8 – 12 hrs | Every 8 – 14 hrs | None on natural gas |
| Best for | Budget, occasional outages | Electronics, quiet, RV | Frequent or long outages |
Sizing It — Running Watts vs. Starting Watts
This is the calculation people get wrong, and it is the difference between a generator that works and one that trips.
Anything with an electric motor — a fridge compressor, a well pump, a furnace blower, a sump pump, an air conditioner — draws a large surge for a fraction of a second as it starts, commonly two to three times its running draw. A fridge running at 200 watts might demand 600–1,200 watts at startup.
A generator has two ratings. Running watts (or rated watts) is what it can supply continuously. Starting watts (or peak/surge) is what it can supply for a few seconds. You need enough running watts for everything on at once, and enough starting headroom for the single largest motor to start while everything else is already running.
Typical Loads
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator / freezer | 150 – 250 | 600 – 1,200 |
| Sump pump (1/3 – 1/2 hp) | 600 – 1,050 | 1,300 – 2,150 |
| Well pump (1/2 – 1 hp) | 750 – 1,500 | 1,500 – 4,500 |
| Gas furnace blower | 600 – 800 | 1,200 – 2,350 |
| Window air conditioner (10k BTU) | 1,200 | 1,800 – 3,000 |
| Central AC (3 ton) | 3,500 – 5,000 | 7,000 – 12,000 |
| Electric water heater | 4,500 | 4,500 |
| Microwave | 600 – 1,200 | Same |
| Lights, TV, router, phone charging | 300 – 600 | Same |
| Space heater | 1,500 | Same |
Doing the Sum
List what must run simultaneously. Add the running watts. Then find the item with the biggest difference between its starting and running watts, and add that difference. That total is your minimum running-watt requirement, and you want roughly 20% headroom above it.
Worked example — essentials only: fridge 200 + sump pump 800 + furnace blower 700 + lights and electronics 400 = 2,100 running watts. The largest starting surge belongs to the sump pump at 1,600 watts, a difference of 800. So 2,900 watts minimum, and with headroom a 3,500–4,000 watt generator covers it comfortably.
Worked example — essentials plus comfort: add a window AC unit at 1,200 running / 2,400 starting. Running total 3,300, largest surge difference now 1,200 (the AC), giving 4,500 watts minimum and a 5,500–6,500 watt unit in practice.
Central air conditioning is the load that pushes people into standby territory. A 3-ton compressor can demand 7,000–12,000 starting watts on its own, which is more than most portables can deliver.
Fuel Types
Petrol is the default for portables. Widely available, but it degrades in storage — three to six months untreated, longer with stabiliser — and it is exactly what is hardest to buy during a widespread outage, when pumps need electricity too.
That storage problem is the same one that strands lawn mowers every spring, and the remedy is identical: stabilise the fuel, or run the tank dry before the machine sits for the season.
Propane stores indefinitely, burns cleaner, and is easy to keep on hand in tanks. It delivers slightly less power than petrol, typically around 10% lower output, and cold weather reduces tank vapour pressure.
Natural gas is the standby fuel of choice where a supply exists — unlimited runtime, no storage, no refuelling. Output is lower still than propane, so a natural-gas standby is derated accordingly.
Dual-fuel and tri-fuel units run on more than one, and the flexibility is genuinely valuable in an outage. Being able to switch to propane when the petrol stations are dark is the single best argument for paying the small premium.
Diesel appears mostly in larger commercial units: efficient, long-lived, and harder to start in deep cold without a block heater.
Connecting It Safely
Never Backfeed
Plugging a generator into a wall outlet to energise the house — "backfeeding" — is illegal, and it kills. It sends power back through the meter onto the utility lines, where it can electrocute the line workers restoring your service, and it bypasses every overcurrent protection in the panel. There is no version of this that is acceptable.
Extension Cords
The simplest approach: run heavy-gauge outdoor-rated cords from the generator to individual appliances. Adequate for a fridge and some lights. It cannot power anything hardwired — furnace, well pump, sump pump — which is usually what you most want during an outage.
Manual Transfer Switch
A small sub-panel installed beside your main panel with a set of circuits wired to it, plus an exterior inlet. During an outage you start the generator, plug it into the inlet, and flip the transfer switch, which physically disconnects those circuits from the utility before connecting them to the generator. That mechanical interlock is what makes backfeeding impossible.
Installed cost typically $500–$1,500, and it is the right answer for anyone using a portable generator seriously.
Interlock Kit
A cheaper alternative: a sliding plate on the main panel that makes it physically impossible to have the main breaker and the generator breaker on at once. Costs less than a transfer switch and gives access to every circuit in the panel, at the price of having to manage loads manually. Must be a kit listed for your specific panel, and it needs an electrician and usually a permit.
Automatic Transfer Switch
Standard with standby generators. It senses the outage, starts the generator, transfers the load, and reverses it all when utility power returns — with no one home.
The Safety Points That Matter
Carbon monoxide is the one that kills. Outdoors only, 20+ feet from the building, exhaust directed away from any opening. Not in a garage with the door open — CO accumulates faster than an open door clears it. Choose a unit with a CO shutoff sensor, which is now common and worth insisting on, and fit CO alarms with battery backup inside.
Refuelling only with the engine off and cool. Petrol on a hot exhaust ignites.
Electrocution risk rises with wet conditions, which is exactly when outages happen. Operate on a dry surface under an open canopy or a purpose-made generator tent — never a sealed enclosure.
Cords must be outdoor-rated and heavy enough for the load and length. An undersized cord drops voltage, which damages motors and can overheat the cord itself.
Fuel storage in approved containers, away from the house and any ignition source.
Keeping It Ready
A generator that will not start when needed is worthless, and non-starting after long storage is the most common complaint about every brand.
Run it monthly for 15–20 minutes under some load. This circulates oil, keeps the carburettor wet, and confirms it works before you need it.
Stabilise the fuel or store it dry. Ethanol petrol absorbs water and gums carburettors within months. Either treat every tank with stabiliser or run the carburettor dry before storage.
Change the oil per the manual — often after the first 20–30 hours, then every 50–100. Small engines run hard and hot during outages.
Keep spares: oil, a spare spark plug, an air filter, and enough fuel for two or three days. During an outage none of these are available.
Standby units self-test weekly and still need annual servicing, which most owners contract with the installer.
Which One to Buy
Occasional short outages, tight budget: a 3,500–5,000 watt dual-fuel portable, run on cords or through an interlock kit. Covers fridge, lights, and a furnace blower.
Regular outages, want the essentials seamless: a 6,500–8,000 watt portable with a manual transfer switch. This is the sweet spot for most households and totals perhaps $1,500–$2,500 installed.
Sensitive electronics, close neighbours, or an RV: an inverter generator. Under 3% harmonic distortion and 20 dB quieter is worth real money.
Frequent or long outages, medical equipment, or away from home often: standby. It is the only option that works with nobody there, and the only one that runs indefinitely on natural gas.
Central air conditioning matters: either standby, or a large portable plus a soft starter on the compressor.
Frequently Asked Questions
Add up the running watts of everything that must run at once, then add the largest single starting surge on top. Anything with a motor — fridge, sump pump, well pump, furnace blower, air conditioner — draws two to three times its running wattage for a moment as it starts. A typical essentials list of fridge (200W), sump pump (800W), furnace blower (700W) and lights and electronics (400W) comes to 2,100 running watts; the sump pump's 800-watt starting difference takes it to 2,900, so a 3,500–4,000 watt generator covers it with headroom. Adding a window air conditioner pushes it to a 5,500–6,500 watt unit. Central air conditioning, which can demand 7,000–12,000 starting watts alone, generally means standby or a soft starter.
No. This kills people every year and an open door does not make it safe — carbon monoxide accumulates far faster than an open garage door clears it, and CO is colourless, odourless and can be fatal within minutes. Portable generators must run outdoors only, at least 20 feet from the building, with the exhaust directed away from doors, windows and vents. That includes not running one on a porch, in a carport, in a shed, or in a basement under any circumstances. Buy a model with a CO shutoff sensor, which automatically stops the engine when CO builds up, and install CO alarms with battery backup inside the house.
How the electricity is produced. A conventional generator's alternator output goes straight to the outlets, so the engine must hold a constant 3,600 rpm to maintain 60 Hz — which makes it loud and thirsty regardless of load, and gives a waveform with 5–20% harmonic distortion. An inverter generator rectifies the output to DC and re-synthesises clean AC electronically, producing under 3% distortion, which is safe for laptops, TVs and appliances with electronic controls. Because frequency no longer depends on engine speed, the engine can throttle down at light load, making inverters dramatically quieter (48–60 dB against 65–75) and more fuel-efficient. They cost more per watt and top out lower in capacity.
No — this is called backfeeding, it is illegal, and it can kill utility workers. Power fed backward through an outlet passes through your panel and out through the meter onto the utility lines, where it can be stepped back up to thousands of volts at the transformer and electrocute line crews working to restore your service. It also bypasses every overcurrent protection in your panel and can start a fire. The legitimate options are running heavy-gauge outdoor extension cords directly to appliances, installing a manual transfer switch ($500–$1,500), or fitting a panel interlock kit — the latter two use a physical interlock that makes it impossible to connect the generator and the utility at the same time.
A portable runs until its tank empties — typically 8 to 12 hours at half load, sometimes 14 on an inverter's efficient part-load operation — and then needs refuelling with the engine off and cool. Most manufacturers also want a cool-down and an oil check every 24 hours or so of continuous running. Standby generators on natural gas can run indefinitely as far as fuel goes, though manufacturers generally specify a service interval measured in hundreds of hours for extended outages. In practice the limiting factor for portables during a long outage is fuel availability, which is why dual-fuel capability matters: propane stores indefinitely, while petrol stations need electricity to pump.
For most people, yes — the premium is small and the flexibility matters exactly when it counts. Petrol degrades in storage, gumming carburettors within three to six months untreated, and it is the hardest fuel to obtain during a widespread outage because filling stations need power to run their pumps. Propane stores indefinitely, burns cleaner, leaves no stale-fuel problem, and you can keep several tanks on hand. The trade-off is roughly 10% lower power output on propane than petrol, so size accordingly, and cold weather reduces tank vapour pressure. Being able to switch fuels when the pumps are dark is the single strongest argument for dual-fuel.
Typically $5,000 to $15,000 installed. The generator itself runs $3,000–$8,000 for a residential unit of 10,000–26,000 watts, and the rest is installation: a concrete or composite pad, an automatic transfer switch, electrical work to tie into the panel, a gas line run and connection, plus permits and inspection. Natural gas installations cost less to run than propane and need no fuel management at all. What you get for the money is the only option that works when nobody is home — it senses the outage, starts itself within seconds, transfers the load automatically, and reverses everything when utility power returns. For frequent outages or medical equipment, that autonomy is the whole point.
Almost always stale fuel gumming the carburettor. Petrol containing ethanol absorbs moisture and begins breaking down within three to six months, leaving varnish deposits in the carburettor jets that block fuel flow. Prevention is straightforward: either treat every tank with fuel stabiliser, or run the carburettor dry before storage by shutting the fuel valve and letting the engine stall. Beyond fuel, run the generator for 15–20 minutes monthly under some load — this circulates oil, keeps seals wet, and confirms it works before you need it. Also check the oil level, since many units have a low-oil shutdown that prevents starting, and inspect the spark plug, which is cheap to keep as a spare.