Two positions up front, because they are the parts most buying guides get wrong.
Size for the surge, not for the sum. Adding up your appliance wattages and buying a generator slightly larger than the total is the standard advice and it produces the wrong answer roughly half the time. The number that decides whether a generator can do the job is almost always the starting draw of one motor, not the steady-state total of everything.
Buy the CO shutoff. Nothing legally requires a portable generator sold in the United States to have carbon monoxide detection and automatic shutoff. A large proportion of units on shelves still do not have it. Buy one that does anyway, and then behave as though it is not there.
Sizing: the method that actually works
Work through it in this order.
1. List your continuous loads and add them up. Fridge, lights, laptop, fans, the converter or charger if the generator will be refilling a battery bank. That last one catches people out — a 55A converter pulling from the generator is roughly 700W of load that appears nowhere on your appliance list. If you have not done this exercise properly, the full load-calculation method is the place to start, and the answer is useful for far more than generator sizing.
2. Identify the single largest motor start. Compressors and pumps draw several times their running wattage for a fraction of a second at start. A rooftop air conditioner is the usual culprit: a 13,500 BTU unit runs at around 1,300 to 1,600W and can spike to 2,500 to 3,500W on start-up without a soft-start device fitted.
3. Your generator needs to cover the continuous total, plus the surge, minus the running draw of the thing that is surging. In practice: continuous total + (largest surge − that item’s running watts). This is the number to buy against.
4. Add 20 per cent. Generators derate with altitude and with heat, both of which you will encounter. Running any engine at its ceiling for hours is also how you shorten its life.
For most rigs without air conditioning, the answer is a 2,000W-class inverter generator — the category represented by the Honda EU2200i and its many competitors, typically 1,800W continuous and 2,200W starting. That covers a fridge, charging, lights, tools and a coffee maker, though not all at once.
For anyone who needs to start a rooftop air conditioner, a single 2,000W-class unit is marginal at best and frequently will not do it. Your three options are a soft-start module on the air conditioner, two paralleled 2,000W-class units, or one 3,000W-plus generator. The soft-start is usually the cheapest and lightest fix, and it also makes running air conditioning from a battery bank plausible in a way it otherwise is not. Two paralleled small units is the second-best answer, because it lets you run one alone for light loads and get the fuel economy that comes with it.
Inverter versus conventional: what the premium buys
An inverter generator produces AC, rectifies it to DC, then synthesises clean AC from that DC. It costs two to three times what an equivalent open-frame conventional generator costs. Four things justify the premium for this use case, and one does not.
| Inverter generator | Conventional generator | |
|---|---|---|
| Engine speed | Variable — throttles to the load | Fixed, typically 3,600 rpm regardless of load |
| Fuel at partial load | ~0.12 gal/hr at 25% load on a 2kW-class unit | ~0.3–0.4 gal/hr at comparable output |
| Noise | 48–57 dBA depending on load | 65–75 dBA, constant |
| Power quality | Under 3% total harmonic distortion | Often 15–25%, variable with load |
| Weight, 2kW class | 40–50 lb | 90–130 lb |
| Cost per rated watt | High | Low |
The variable engine speed is the whole trick. A conventional generator runs at 3,600 rpm whether it is powering a house or a phone charger, which is why its fuel consumption barely moves with load and why it is loud all the time. An inverter unit throttles down, and at the light loads typical of RV use that difference is a factor of three or more in fuel burn and around 20 dBA in noise.
The power-quality argument matters less than it used to. Under 3 per cent total harmonic distortion is genuinely better for sensitive electronics, but most modern switching power supplies tolerate dirty power adequately. Do not pay the premium for this reason alone. Pay it for the fuel, the weight and the noise.
Where a conventional generator still makes sense: fixed rural backup, sited away from the house, where weight and noise are not constraints and you want maximum watts per dollar.
What the fuel actually costs
This is the calculation nobody publishes, and it reframes the whole question of when to run the thing.
A 2,000W-class inverter generator with a 0.95-gallon tank runs around 8 hours at 25 per cent load, about 5 hours at 50 per cent, and roughly 3 hours flat out. Those runtimes give you consumption figures directly.
At $3.50 a gallon — pump prices swung between roughly $3.00 and over $4.00 during 2026, so substitute your own number:
| Load | Output | Fuel burn | Cost per hour | Cost per kWh |
|---|---|---|---|---|
| 25% (inverter) | ~450W | 0.12 gal/hr | $0.42 | $0.93 |
| 50% (inverter) | ~900W | 0.19 gal/hr | $0.67 | $0.74 |
| 100% (inverter) | ~1,800W | 0.30 gal/hr | $1.05 | $0.58 |
| ~450W (conventional) | ~450W | 0.35 gal/hr | $1.23 | $2.72 |
Set that against a US average residential electricity price of roughly 17 cents per kWh and the picture is stark. Generator power costs between three and sixteen times grid power depending on how well loaded the machine is.
Two conclusions follow, and both are actionable.
Run the generator loaded or not at all. A generator idling to keep a fridge alive is the most expensive electricity you will ever buy. If you are going to run it, run it into a battery charger at high output and fill the bank quickly, then shut it down. This is the core logic behind a properly designed three-source charging system — the generator’s job is short, hard bulk charging, not trickling.
Solar wins on cost the moment it is installed. At $0.74 per kWh from a generator, a 400W solar array producing 1.2 kWh on a decent day is displacing about $0.90 of fuel per day. That is a slow payback on its own, but the array also costs nothing to run, makes no noise, and does not need to be carried, fuelled or stored — and the fuel saving is only part of what you are buying.
The CO sensor question
This section is not a feature comparison. Portable generators kill people every year in numbers that are not in dispute.
The Consumer Product Safety Commission estimates that portable generators cause around 80 deaths a year from carbon monoxide poisoning in the United States. A CPSC review covering 2012 to 2022 recorded 872 CO fatalities associated with engine-driven generators and tools, of which 86 per cent were attributed specifically to portable generators. CO is colourless, odourless, and at the concentrations a generator produces in a confined space it can kill in minutes, before anyone recognises symptoms.
What the shutoff systems do
In 2018 two ANSI-approved voluntary standards adopted CO hazard-mitigation requirements: ANSI/PGMA G300-2018, published by the Portable Generator Manufacturers’ Association, and UL 2201, second edition. Both require a system that shuts the engine down when CO concentrations near the unit reach specified levels — under UL 2201, a rolling ten-minute average of 150 ppm or an instantaneous reading of 400 ppm. UL 2201 additionally caps the generator’s weighted CO emission rate. G300 adds requirements the UL standard does not have, including self-monitoring for sensor faults, loss of power to the shutoff system, and sensor end-of-life, plus a notification to the user explaining why the unit shut down.
CPSC’s 2022 modelling of 511 real fatality scenarios found that generators meeting UL 2201 would have averted close to all of those deaths, and generators meeting G300 around 87 per cent. Both figures are extraordinary for a single engineering intervention.
What is not true
These standards are voluntary, and they remain voluntary. CPSC opened rulemaking in 2016, issued a supplemental proposed rule in April 2023 that would make CO emission limits and shutoff mandatory, and has not finalised it. A CPSC market review found that in late 2021, more than 75 per cent of portable generator models on sale complied with neither standard. That figure has improved since, but the label on the box is the only way to know: look for an explicit reference to ANSI/PGMA G300 or UL 2201, or a named CO-detection system such as Honda’s CO-Minder. Absence of a claim means absence of the feature.
A shutoff sensor does not make it safe to run a generator anywhere you could not run one before. This is the point where the safety case is most often quietly inverted. The shutoff is a last-resort backstop for a mistake that has already been made. It measures CO around the generator, and by the time it triggers, CO has been accumulating somewhere. It does nothing about exhaust that is being carried away from the sensor and into a vehicle by wind, or drawn in through a vent or an open window.
The placement rules do not change because a sensor is fitted:
- Never operate a generator inside any enclosed or partially enclosed space — not a garage with the door open, not a basement, not under an awning or slide-out, not in a trailer’s cargo bay.
- Site it at least 20 feet from the rig or building, with the exhaust directed away from doors, windows and vents.
- Consider where the exhaust goes, not just where the generator sits. Wind changes overnight.
- Fit a working CO alarm inside the living space, and check its expiry date. CO alarms have a service life measured in years and expire whether or not they have ever sounded.
If a generator with a CO shutoff switches itself off, that is not a nuisance fault to be worked around. It is a report that CO reached a dangerous concentration where you put the machine. Move it, ventilate, and do not restart it in the same position.
Dual fuel, and whether it is worth it
A growing share of inverter generators run on propane as well as gasoline. Propane is worth considering for one specific reason and it is not cost.
On energy content, propane is the more expensive fuel per delivered kilowatt-hour in most of the country, and generators produce roughly 10 per cent less output running on it. The advantage is storage. Gasoline degrades in months, gums carburettors, and is the leading cause of a backup generator refusing to start on the day it is needed. Propane does not age. For a machine that might sit unused for a year between outages, that is the whole argument, and it is a good one.
For an RV that already carries propane, the case is stronger still: one fuel, one supply chain, no jerry can rattling in a locker. For anyone who will run the generator regularly, gasoline’s better output and lower cost per kWh win.
Whichever fuel you choose, the storage discipline is the same for the engine. Run it under load for half an hour every month or two, and either drain the carburettor or run stabilised fuel through it before extended storage. A backup generator that has not been started in a year is not a backup, it is an object.
Where you are allowed to run it
Generator hours are regulated more than most owners expect. Campground quiet hours, BLM and National Forest rules, and municipal noise ordinances all constrain when a generator can run, and some of them specify decibel limits at a measured distance rather than just hours. A 48 dBA inverter unit passes almost everywhere; a 70 dBA open-frame unit does not. The state and federal picture on generator noise ordinances and quiet hours is worth checking before you buy, because it can decide the inverter-versus-conventional question on its own.
One practical gotcha
If you plan to feed a portable power station from a generator, turn the generator’s eco or economy throttle mode off. When a motor starts somewhere on the circuit, the generator’s voltage sags while the engine spools up. A power station in UPS mode reads that sag as a grid failure and flips to battery; the load vanishes; the generator throttles back down; and the two fight each other in a loop. It costs fuel and noise to run with eco off, but the combination works. This is one of several reasons the power station versus installed system decision is worth making deliberately rather than accumulating by accident.