Fuel usage using our generator for an extended period

[Grub Master]

Thought I would post what our experience with a whole house propane generator was, for anyone that’s considering one.

This is our first experience with an extended power outage.

Our power went out Sunday morning at 4:45am. The generator came on automatically. We have a (15 year old) Generac 17KW propane generator, gas heat for downstairs, heat pump upstairs, unvented gas logs and gas stove. Lucky for us, our tank was filled up on Tuesday. The power just came back on (9pm) and I checked our tank. Looks like we have used a little over 125 gallons of propane since Tuesday afternoon. The generator ran approximately 40 hours, we heated the house and had the fireplace going most of the time. According to the gas company, our fireplace burns one gallon every 3 hours and I’m not sure what the furnace uses (90% efficient older unit). We didn’t use the furnace all the time and we did cook some but not a lot. I don’t know how much propane we used from Tuesday night to Sunday morning, but assume we didn’t use enough to matter. I’m overestimating usage, but It looks like we used about 3 gallons an hour. Not really bad when the generator repair guy estimated the generator would use 2.5 gallons and hour by itself. We were warm and figured we had another 3.5 days (400 gallons of propane to start with) if we didn’t make any changes. The downside to our generator is that it runs a constant speed so you can’t conserve fuel by cutting of appliances. Overall, I’m glad we have one, it came with the house) and living in the mountains would never be without one. Probably will have small backup just in case we have an extended outage.

 

[Ruger34]

Good info

 

[keith089]

Outstanding

 

[MegaFan]

The downside to our generator is that it runs a constant speed so you can’t conserve fuel by cutting of appliances.

Good info. Your generator runs at a constant speed in order to maintain a standard output frequency of 60 hz. Adding or dropping loads, such as shutting off appliances, will save on fuel consumption. Think about driving your truck up a hill, especially when towing. If you want to maintain 60 mph, you have to add more fuel, right? When you shutoff an appliance, the reduced load on the generator will result in an increased speed due to less torque/load, so the generator reduces fuel until it again stabilizes to maintain the 60 hz. Most smaller home generators run at 3600 RPM (2-pole generators) and some larger ones run at 1800 RPM (4-pole) but you may not sense the speed change as the unit adjusts to maintain its speed/frequency.

Bottom line - increase the load and fuel consumption goes up to offset the load "trying" to slow the generator. The generator responds by adding more fuel to maintain the speed/frequency. Reduce the load and you decrease the consumption and fuel cost

 

[Grub Master]

MegaFan I need to get a new generator service guy. Based on 2 conversations I had with him, one was today, he was adamant that reducing the load would not save fuel. I should have researched it myself instead of taking his word for it. My owners manual doesn't say anything about it but the Generic website shows that at 1/2 load, 1.6 GPH vs 2.6 GPH at full load. Even .5 GPH would give me another day or so in an emergency. I know from experience that my boat 12.5kw generator uses a lot more fuel under a load and should have known better this time. As Ronald Reagan said "trust but verify" While I know we were probably not at full load, I can minimize the heat pump and shut off the electric water heater next time. Unfortunately, next time might be this weekend. Thanks for the info.

 

[THREEJAYS]

That’s good information, thanks for sharing

 

[moosensquirrel]

MegaFan I need to get a new generator service guy. Based on 2 conversations I had with him, one was today, he was adamant that reducing the load would not save fuel. I should have researched it myself instead of taking his word for it. My owners manual doesn't say anything about it but the Generic website shows that at 1/2 load, 1.6 GPH vs 2.6 GPH at full load. Even .5 GPH would give me another day or so in an emergency. I know from experience that my boat 12.5kw generator uses a lot more fuel under a load and should have known better this time. As Ronald Reagan said "trust but verify" While I know we were probably not at full load, I can minimize the heat pump and shut off the electric water heater next time. Unfortunately, next time might be this weekend. Thanks for the info.

Your service man may have meant "will not reduce load significantly".....by disconnecting non critical loads. The manufacturer load ratings are usually specious at best in that they base it usually on a 100% resistance load. Most motor loads require reactive power, and when running them on low load, will have less than desirable power factor, all of which your generator must supply when running.