Fuel Use vs Load for Gasoline Powered Generators

[Astro_Guy]

Do any of you BITOG folks have personal knowledge of or a good source for fuel use vs load on a gasoline powered inverter generator? I see specifications for run time at 25% load. I've read articles indicating that this is non-linear in the 25-50% range, meaning that the generator won't use twice the fuel when increasing the load from 25 to 50%.

So let me cut to the chase. I am helping some friends with an off grid solar application. They live four months per year in a motor home that doesn't move for those four months. They are powering everything now running a 4500 watt peak / 3700 watt continuous generator for three hours per day. Their generator has a 25% load run time of 18 hours per tank, and they are getting 9.5 hours per tank. My thoughts are that they are running at roughly 50% load based upon 9.5/18 hours. That comes down to 3700 watts x 0.5 load x 3 hours or 5500 watt-hour per day. This is my best estimate of their demands.

Could anyone knowlegeable please comment? Thanks in advance.

 

[dnewton3]

Your calculations are reasonable.

The fuel use is what it is; that's easy to measure. The power consumption is not hard to measure; get an ammeter (clamp on style) and record it several times per hour. You'll get a much more accurate measurement if you actually track the power consumption rather than estimate it mathematically.

I don't really understand your goal here? Are you helping them consider a different generator? Or trying to maximize the run time by balancing power draw vs. fuel used (finding a "sweet spot")?

 

[fsdork]

What is your objective?

As one example, at 50% load the Honda EM5000S uses about 67% as much fuel as it does at full rated wattage. So to your point, it is non-linear.

Honda EM5000S Generator | Honda Generators

The Honda EM5000 generator offers the right features for reliable, home backup power. 5000 watts, electric start and optional remote start, easy transport, & much more.

powerequipment.honda.com

The model of generator will be a big factor, particularly whether it is an inverter generator or not.

Also, the fuel used by a generator through the day will not be an apples-apples comparison to a solar array/charger/battery bank/inverter scenario, if I am inferring your goal correctly.

Similar to @dnewton3 's suggestion, I would be looking at a way to capture data about actual usage and go from there. Depending on the loads involved, perhaps a few smart plugs and an app with the ability to graph data or output to Excel would be enlightening.

 

[Rmay635703]

Most good gensets list their fuel use at different loads.

Your use case sounds like an ideal use for this.

Slow-Turning 5 kW Yanmar Diesel Generator

Inverter gensets have much better partial load efficiency and generally (but not always) better engine designs themselves.

This does not mean they are max effiency at low load, just better than a standard genset.

Most gensets are max efficiency somewhere between 80-100% load but unless you have extremely consistent and predictable demands you cant size your generator to be max efficiency .

The worlds most efficient gensets are slow turn diesels, even the little 5kw ones
are far more efficient than an inverter as small gas engines are pretty abysmal for real efficiency. Even big diesels if they are not slow turn are almost half as Efficient as the little slow turn units.

That is why it’s amusing to watch people draw conclusions from the MPGs of a 12% EFF generator charging an EV, even a small old car engine is twice as efficient as a small gas engine.

People who think their Briggs and Stratton is as efficient as a gas turbine power plant are also on some type of crack, simply no point even comparing the 2.

The reason many large home generators are natural gas is is simply because the fuel is so extremely cheap and because the fuel is extremely clean , if you’ve ever changed the oil on a CNG car it’s always perfectly clean.

It is possible to make any genset dual fuel (propane or natural gas) and is well worth it.

One of the local guys made a little trailer of old CNG tanks to run his off grid home , he got them very cheaply and went to the Kwik Trip to fill up with CNG once a month.

The main cost of this type of power is usually not the fuel but the cost of keeping the generator functional.
Running a traditional genset 24/7 (even a whole house unit) is likely to wear it out rapidly.
Sometimes it’s worth spending more on fuel to have something bulletproof.
Knowing which ones can be abused running on and off for years means finding an old unit likely to do so, new is a crapshoot but some folks run them hard enough to know the ones that are definitely not good.


Things to consider.

If this is something that will be expected to be used constantly for years make sure you get one that will handle it.
If fuel cost is a concern make sure you know how to fit it with a propane or NG bifuel kit.
If it will be used often in cold weather consider a unit that can be located or tapped for heat (water cooled) so the fuel it uses isn’t mostly wasted and you get something useful (heat) out of running the unit
Automatic start? Do they need to integrate into a battery system with an automatic start? (More complex DIY or more costly)

My folks old Oshkosh diesel motor coach had an automatic start diesel genset that shared coolant and fuel plumbing with the main engine.

 

[ctechbob]

Propane/NG also does not plug up carbs after sitting for long periods. That is one of the greatest advantages of it. That and some houses are on municipal supplies of NG so it is an essentially limitless supply. For a backup genny that is a great thing.

 

[Astro_Guy]

I don't really understand your goal here? Are you helping them consider a different generator? Or trying to maximize the run time by balancing power draw vs. fuel used (finding a "sweet spot")?

Neither of the above. I am trying to get a good estimate of their actual power consumption in order to design an off grid solar power system to largely replace the generator. They live four hours away from me so it's not going to be easy to just run over and clamp my meter around the cord. A close estimate will be good enough if I design the storage side of it for five days of autonomy.

 

[nomas]

I think it would be so much easier and more accurate to just take a field trip to their site . Basing your design off of fuel consumption just sounds screwy . They need to have a generator no matter what you come up with even if it becomes the secondary source . Are they not able to provide a current reading ?

 

[Astro_Guy]

I think it would be so much easier and more accurate to just take a field trip to their site . Basing your design off of fuel consumption just sounds screwy . They need to have a generator no matter what you come up with even if it becomes the secondary source . Are they not able to provide a current reading ?

It would be more accurate, and I would do that if this were a paid gig. This is a favor for a friend - actually the sister of a friend.

The generator isn't going away - they just don't want to be bothered with it.

 

[Cujet]

Here is something I put together from published data and some user posted data. It IS NOT my testing data.

A post about generator efficiency and fuel consumption. For understanding.

1) A portable generator is not an efficient device. Regardless of brand or type.
2) Generator produced power (for our purposes, portable and stationary household sized) is very expensive when compared to the utility company, regardless of fuel type and fuel costs. Even with free fuel, the costs exceed the utility company's charges per KWH.
3) Inverter generators are NOT more efficient in many situations. They are quieter and better suited to very low/no load conditions.

Here are some numbers: (note, look at KWH/gal, higher is better)

Honda EU2000i inverter generator

1,600W 4.75 kWh/gallon Run time 2.97 hours/gallon.
555W 4.47 kWh/gallon Run time 8.06 hours/gallon

The Honda is almost as efficient at low loads as it is at high loads. This is fantastic when you need very little power. But it's not as good as a conventional genset at high loads, as we will see below.

Generac GP3250
2300W, 5.53 KWH/gallon
1900W, 5.35 KWH/gallon
1150W, 4.45 KWH/gallon
744W, 3.84 KWH/gallon

Yamaha EF3000is
2300W, 4.50 KWH/gallon
1900W, 4.42 KWH/gallon
1150W, 4.38 KWH/gallon
744W, 3.89 KWH/gallon

Notice the inverter Yamaha is not better than the Generac until it's at 1/4 load or lower. At which point, you will save some fuel.

Listeroid 6/1 diesel with belt drive ST generator head (notice that it's twice as efficient as the Honda/Yamaha inverter generators when heavily loaded, and exceeds the inverter's efficiency all the way down to 500W) (at which point, it consumes about 1/10 gallon per hour, on par with the Honda)
3000W, 8.57 KWH/gallon
2000W, 7.69 KWH/gallon
1000W, 5.71 KWH/gallon
500W, 4.2 KWH/gallon Run time 8 hours per gallon (exactly the same as the Honda, this showcases the diesel's efficiency)

15000W Generac portable (gasoline) (note, the bigger the generator, the more efficient)
15KW, 8.3KWH/gal
7.5KW, 5.9KWH/gal
3750W, 3.8KWH/gal

30,000W John Deere turbodiesel generator (note, diesels are often very efficient at a wide range of loads, and sometimes worse at very high loads)
30KW, 11.4KWH/gal
15KW, 11.5KWH/gal

200,000W JD turbodiesel generator
200KW, 15.5KWH/gal
100KW, 15.5KWH/gal (or roughly 20c/KWH at $3/gal) (still double the local utilities price, and the unit's capital costs are enormous)

 

[Cujet]

Published data may not be perfectly accurate, and some users do not get the same efficiency as claimed. According to one user, the Classic Honda EU2000 may be in fact, a touch more efficient than it's (supposedly more efficient) replacement, the EU2200.

The new fuel injected Honda EU3200 specs claim that it can produce 7.1 KWh per gallon at full load. Maybe under ideal conditions, with non ethanol fuel. Maybe.....

I doubt it actually approaches diesel-like efficiency, although, fuel injection helps considerably.

I included the "Listeroid" diesel in my numbers above. Just to showcase the difference in fuel use between an efficient small diesel and commonly available gas gensets. The BSFC numbers for the Lister diesel engine is quite good and they are 44% TE. Diesel fuel is more energy dense too. By, oh, 20% or so.

 

[Astro_Guy]

Yamaha EF3000is
2300W, 4.50 KWH/gallon
1900W, 4.42 KWH/gallon
1150W, 4.38 KWH/gallon
744W, 3.89 KWH/gallon

The generator in question here is a Westinghouse iGen 4500DF inverter. They are getting 9.5 hours on 3.4 gallons of gas and run the generator 3 hours per day. Using the 4.5KWH/gal figure above, that suggests 4.8 KWH per day of consumption. Theoretically their generator would be a little more efficient. Best case daily solar production is 6X the panel nameplate, worst case is 3X. This implies that they need 800 to 1600 watts of panel array to meet their daily needs. Designing for five days of autonomy would require 24 KWH of storeage - ouch!

 

[meep]

Most good gensets list their fuel use at different loads.

Your use case sounds like an ideal use for this.

Slow-Turning 5 kW Yanmar Diesel Generator

I want it

 

[Rmay635703]

The generator in question here is a Westinghouse iGen 4500DF inverter. They are getting 9.5 hours on 3.4 gallons of gas and run the generator 3 hours per day. Using the 4.5KWH/gal figure above, that suggests 4.8 KWH per day of consumption. Theoretically their generator would be a little more efficient. Best case daily solar production is 6X the panel nameplate, worst case is 3X. This implies that they need 800 to 1600 watts of panel array to meet their daily needs. Designing for five days of autonomy would require 24 KWH of storeage - ouch!

It’s unfortunate 24kwhrs leaf batteries aren’t still $1900 like they were 10 years ago

 

[dnewton3]

Neither of the above. I am trying to get a good estimate of their actual power consumption in order to design an off grid solar power system to largely replace the generator. They live four hours away from me so it's not going to be easy to just run over and clamp my meter around the cord. A close estimate will be good enough if I design the storage side of it for five days of autonomy.

Agreed; I understand now.

Can they help you help them? Can they not get a clamp-ammeter and record some data for you?

 

[Astro_Guy]

Can they help you help them? Can they not get a clamp-ammeter and record some data for you?

They are in upstate New York, helping me with full tank run time on their generator. The first tank went 9.5 hours thanks to all the smoke from Canadian forrest fires. The latest tank ran ten hours. They are living off grid, heating water, washing and drying clothes. I think I have it now, but thanks for asking.

 

[Cujet]

It is good to know that solar insolation in upstate NY may be in the neighborhood of 2.2 hours per day, average, in the winter. In fact, even that can't be counted on, due to the unpredictable, sometimes persistent overcast.

My younger brother has a Tesla solar array on his Burlington, VT home. In wintertime, it produces near nothing. During a winter visit a few years back I saw his 10KW array producing a peak of 500W, or about 5% of rated power. He sized and aligned the panels for good year round performance. What actually happens is that winter snow and overcast leads to no practical output. While his heat pumps churn away, his elec bill climbs into the insane range.

During the summer, he does not need AC, and his panels do a good job producing power, that he can't use.

 

[Astro_Guy]

It is good to know that solar insolation in upstate NY may be in the neighborhood of 2.2 hours per day, average, in the winter. In fact, even that can't be counted on, due to the unpredictable, sometimes persistent overcast.

This is a seasonal May through September application. The weakest solar month is September with about 4.5 sun-hours per day. For those not familar with the term, sun-hours x panel array wattage = average watt-hour production per day. These folks need 4800 watt-hours per day delivered to the generator port on their motor home. That seems to suggest that they need about 1200 watts of solar panels.

I consulted with an engineer at a solar supply house today who took my data and came back with a quote for a 3200 watt array. After a bit of push back, he said that 1900 watts was the bare minimum. When asked about the difference between 1200 and 1900 watts, his answer was "system losses." That seems to be a rather large system loss in my book. I've looked at his recommended components and it appears that he sized the unit with the MPPT controllers and inverter to feed the full capacity of the generator port on the motor home.

 

[Cujet]

For those who might read this thread:

The generator powers a home load (or RV) directly. So what it produces in KWh is always going to be significantly lower than what solar panels need to produce.

As with all things electrical, there are a stack of losses. 5KWh worth of solar panel output in a day, does not equal 5KWh worth of 120VAC power. The charger has losses, the batteries have losses in and out, and over time and temperature, and the inverter has losses. They "stack" up.

The good news is, that one extra solar panel is not all that expensive, and may be all that's needed to size a small system. or a "non-aimed" system, (such as on the top of an RV) for real world functionality.


Also of note, the new crop of fuel injected portable, inverter generators are indeed significantly more efficient than their carburetor equipped cousins. At lower loads, as much as 40%.

 

[Astro_Guy]

My initial estimated demand was 5.5 KWH per day. This was refined to 4.8 KWH per day based upon the fuel usage data supplied by Cujet. The worst case solar month from May through September is September, where the target location receives 4.73 sun-hours. One watt of solar panel capacity multiplied by 4.73 sun-hours yields 4.73 watt-hours of daily production. Using the higher of my figures suggested that 1.2 KW of solar array should get the job done, right?

Armed with my data I went on to have a conversation with an engineer at a solar supply house that I've done business with in the past. I gave him the 4.8 KWH per day demand figure combined with the desire to feed to power into the motor home that same way a generator would. In other words the power would need to be inverted to 120 VAC first. Inverters capable of supplying 4 or more KW of continuous power get expensive rather quickly. Before I knew it I had a quote that included a 3.2 KW array. After a bit of push back I was told that the smallest array to solve the problem at hand would be 1.9 KW but that wasn't recommended because the [anels were a relatively small portion of the cost. The combined inverter / charge controller plus 10 KWH hours of LiFePO4 storage came to about $6000 or 60% of the project cost, minus any labor.

So why the big difference in figures? System losses. There is a loss when converting the raw power from the panels into a suitable charging current for the batteries. There is another loss at the inverter where the stored power is converted to AC power for the motor home. The batteries themselves contribue a loss; supplying them with one KWH of charge does not mean they will actaully store it or give it back. These losses add up.

 

[Jetronic]

I had briefly considered gettin a diesel genset with liquid cooling. The house is heated by an oil fired water heater, so plumbing the genset into the heating system would allow me to recover most of the heat lost through the cooling system, and reduce the oil useage for heating. Winter is when the genset would be most likely required.