Originally Posted by Gebo
I realize there are many factors to take into consideration in attempting to answer this question. Just humor me, pretty please??
How long would a 3500 watt generator run on 5 gallons of gas running a freezer and 2 refrigerators?
What would the minimum time be?
Reason I'm asking is the generator is running 1 1/2 hours from my house. I'll have to go and put more gas in before it runs out. In other words, how long can I know it will run with a 90% certainty?
Im going to assume that the refrigerators are modern and pull something like 700W.
So let's say all three items pull about that.
Maximum continuous load is 2100W.
1 gallon of gasoline is around 33.7 kWh electrical.
These constant speed generators are very inefficient, so let's call it 15%.
So 2100We (Watts-electric) requires 14000Wth (Watts-thermal) to get the job done, assuming 15% generator efficiency.
So on an hourly basis, if all three items were running continuously, your loads would pull 2100W for 1hr, and consume 2.1kWh(kiloWatt-hours) electrical. However, the thermal value of the fuel going in would need to be 14000W for one hour, or 14kWh.
So if all three loads were running continuous, you would be consuming a little less than 0.5gal/hr.
See how I got that? We have to correlate the thermal energy in a volumtric unit (gallon) to an electrical draw, adjusted by generator efficiency. Thermal value going in relates to electrical value going out by the factor of efficiency, and we know the volumetric thermal value of gasoline.
So by that logic, 5 gallons would get you a bit more than 10 hours.
BUT!!!!!
We know that for one thing, its not that hot out, and for another, that none of these loads operate all the time. The newer high efficiency refrigerators like I used will pull less energy, longer (my 700W number is from a HE refrigerator), but kept closed it will run what? half time? quarter time?
Interestingly, a 2000 research report indicated the following:
Quote
One can begin by recognizing two very common characteristics of most refrigerators. Over a broad range of models and vintages, the operating characteristics of refrigerators appear the same. A refrigerator turns on, runs for a while, turns off and is off for a while. It then turns on again and the cycle repeats. A typical, steady-state cycle is about 40 minutes with the duty cycle, or per cent running time, being about 50 percent for standard temperature differences between the food compartment of the refrigerator and the room in which the refrigerator is placed. In other words, a typical, properly operating refrigerator runs about 20 minutes and is off 20 minutes. For automatic defrost models, a second cycle is superimposed on top of the primary cycle. The defrost cycle commonly runs with an 18 to 36 hour period and displays a sharp peak for only 10 to 20 minutes. The characteristics of the primary cycle has been remarkably stable at 40 minutes for thirty or more years.
https://publications.anl.gov/anlpubs/2000/04/35529.pdf
Perhaps more interesting is the logged data, which shows a much smaller load. The chart shows 10 minute periods of logging, and the average load across those 10 minute periods. My assumption above is based upon a small margin above the sticker on my refrigerator... FWIW.
So, from here, its "in God we trust, all others bring data". You NEED to get a kill-a-watt meter and determine average power draw, at least steady state (non-defrost), to get real numbers.
But my assessment, is that if we assume a 50% duty cycle, you can get 20 hours out of the generator. My error bars would be small on the side saying youll get less time, and big on the side saying youll get more time (IOW, if you cant run that long, it wont be much less, and quite possibkly you can run MUCH longer).
Plus if you leave the doors closed, the food wont defrost or warm up in the subsequent time.
In each freeze a cup of water and put a coin on the top of the ice. If the coin is still on top when you return, all is well - if its not, you know it got somewhat warmer...
Good luck!
