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I'm looking to buy an inverter generator for backup of a few essential devices during power outages. I've already tested for & calculated my required wattage (including starting wattage where applicable). Still, I'm debating over size:

On one hand, buying a higher wattage generator would give me more leeway to add additional appliances in the future if desired. But on the other, if I don't end up needing the extra wattage, it would consume more fuel & therefore effectively shorten my "off-grid" time limit (given a fixed stockpile of propane).

...Or would it? This is my actual question:

Given the same load, would a higher-wattage INVERTER generator burn more or less fuel per hour than a lower-wattage one? For instance, let's say I run a load of exactly 1000 watts. Will a 4000-watt inverter generator burn more fuel per hour than a 3000-watt inverter generator on a 1000-watt load?

On one hand, it seems like the bigger one might burn more due to it having a physically larger engine. But on the other hand, it seems like the bigger one might burn less, as it's farther away from its max output & can therefore throttle down & work less hard. I've asked a couple manufacturers and searched around online, and have gotten conflicting answers.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike
    Jun 8, 2020 at 17:15

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There is a sweet spot in between overloading and underloading the generator (running it a bit higher or lower than its optimum operating point).

It means that if you overload the small generator, or underload the large generator then your fuel efficiency will not be as good as it could be. Which one is worse is up in the air because it becomes a matter of degree as to whether the small generator is more overloaded than the large generator is underloaded.

A larger generator certainly will burn more fuel than a small one if you're just idling. So if you are idling more often than not, perhaps it is better to pick a smaller generator that gets is loaded a bit higher than its operating point for maximum fuel efficiency. On the other hand, if your generator is heavily loaded most of the time, this is probably not the best solution and you instead want to pick something that is nearer its maximum fuel efficiency when loaded.

In your example of a 1000W load, I would lean toward it being more fuel efficient on the 3000W generator just because 3000W is already so much larger than the required 1000W, and 4000W is even larger so you're spinning around a lot of dead weight.

The rated wattage on the generator is probably the point of maximum continuous power output, not the point of maximum fuel efficiency. In the unlikely event it is the operating point of maximum fuel efficiency, then 3000W will obviously be better since 3000W is closer to 1000W than 4000W is.

But in the more likely event that the rated wattage is the maximum output power, I still lean towards 3000W because I find it hard to believe that any machine's optimum fuel efficiency occurs at something as low as 30% rated load, let alone 25%. It could be, but my gut feeling is that it doesn't. Someone who knows more about generators and engines will have to chime in.

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  • \$\begingroup\$ Excellent answer! Accepted. \$\endgroup\$
    – J23
    Apr 1, 2020 at 21:42

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