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How efficient is your generator?

Symbioquine said:
Wow, that's awesome! Seems like you're doing a bit better than spec which seem to be "0.57 gph @ rated load" for that generator.

Converting that to comparable numbers;

Code: 
5 (kWh / hour) / 0.5 (gal / hour) = 10 kWh / gal
$5.718 (USD / gal) / 10 (kWh / gal) = $0.57 (USD / kWh)
10 (kWh generated) / 40.7 (kWh total energy) = 24.6% efficiency
Click to expand...
Diesels are 20-25% more efficient than gas.
 
schmism said:
My 1800w inverter genny runs about a quart an hr on 500-800w.
Click to expand...

@schmism Care to share the generator make/model/fuel-type?

If we assume gasoline then this is:

Best case;

Code: 
0.8 (kWh / hour) / 0.25 (gal / hour) = 3.2 kWh / gal
$5.64 (USD / gal) / 3.2 (kWh / gal) = $1.76 (USD / kWh)
3.2 (kWh generated) / 33.41 (kWh total energy) = 9.6% efficiency

Worst case;

Code: 
0.5 (kWh / hour) / 0.25 (gal / hour) = 2 kWh / gal
$6.42 (USD / gal) / 2 (kWh / gal) = $3.21 (USD / kWh)
2 (kWh generated) / 33.41 (kWh total energy) = 6% efficiency
 
Symbioquine said:
Wow, that's awesome! Seems like you're doing a bit better than spec which seem to be "0.57 gph @ rated load" for that generator.

Converting that to comparable numbers;

Code: 
5 (kWh / hour) / 0.5 (gal / hour) = 10 kWh / gal
$5.718 (USD / gal) / 10 (kWh / gal) = $0.57 (USD / kWh)
10 (kWh generated) / 40.7 (kWh total energy) = 24.6% efficiency
Click to expand...

Rated load details are:

Power Rating5kW, 0.8 pf @ 4000ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I'm testing into a 5kW dummy load (1.0 pf) at 760 ft and temps around 70F. So it makes sense that I'm doing a little bit better than 0.57 gph.

I also have a MEP-803A, which is basically a 802A with 2 more cylinders and a larger generator head. It is spec is 0.97 gph @ rated load, which is:

Power Rating10kW, 0.8 pf @ 4000 ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I bet I can squeeze 10.5 - 11 kWh / gal out of it.

Btw, I buy off-road diesel which is about $0.25 cheaper than on-road. As of this morning, the off-road was $5.44 at my local service station.

You can never have too many generators, lol:

IMG_1379.JPG
 
crazydane said:
Rated load details are:

Power Rating5kW, 0.8 pf @ 4000ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I'm testing into a 5kW dummy load (1.0 pf) at 760 ft and temps around 70F. So it makes sense that I'm doing a little bit better than 0.57 gph.

I also have a MEP-803A, which is basically a 802A with 2 more cylinders and a larger generator head. It is spec is 0.97 gph @ rated load, which is:

Power Rating10kW, 0.8 pf @ 4000 ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I bet I can squeeze 10.5 - 11 kWh / gal out of it.
Click to expand...

Very cool - and nice collection! It makes sense that they'd rate those military generators conservatively.

Any idea how they do efficiency-wise at lower loads? i.e. the 5kW unit at 4kW?
 
Checkthisout said:
Diesels are 20-25% more efficient than gas.
Click to expand...
The diesel engine is not necessarily more "efficient". A gallon of diesel is heavier and has more BTUs than a gallon of gas. Same comparison for natural gas vs LPG/propane where the generator is derated for NG.

A true efficiency comparison is pounds of fuel per kwh. The real world usage comparison is cost of fuel per kwh, including sourcing and transporting the fuel.
 
Last edited:
Delmar said:
The engine is not necessarily more "efficient". A gallon of diesel is heavier and has more BTUs than a gallon of gas. Same comparison for natural gas vs LPG/propane.

A true efficiency comparison is pounds of fuel per kwh. The real world usage comparison is cost of fuel per kwh.
Click to expand...

Yes, diesel fuel contains more btus per gallon. This does not account for the majority of the efficiency difference

Diesels are more thermodynamically efficient than Otto cycle engines. In other words they convert more heat into work at the crankshaft.
 
Symbioquine said:
@schmism Care to share the generator make/model/fuel-type?
Click to expand...
Blackmax (Bmi2100 I think) gas

Volumetric consumption was all I care about because "how long will it run with the supply of fuel I have on hand" is always the question vs what it cost me. As the use case for me is emergency backup as opposed to regular energy production, so the cost of failed fridge/freezer is way more than the cost of the fuel.

but if your looking to do a long term generator use for offgrid use, then comparing a inexpensive big-box-store genny with a commercial diesel isn't a good apples to apples. As the long term cost of replacement/maintenance are not even in the same ballpark.
 
Ideal propane and natural gas provide longest equipment life and lower maintenance costs.

Remember when they ran school busses on propane because it was so cheap to purchase?

They either had to flare it off at the wellhead or refinery or try to make a few pennies condensing it down and selling it.
 
Delmar said:
The engine is not necessarily more "efficient". A gallon of diesel is heavier and has more BTUs than a gallon of gas. Same comparison for natural gas vs LPG/propane.

A true efficiency comparison is pounds of fuel per kwh. In real world usage the comparison is cost of fuel per kwh.
Click to expand...

If you check out the earlier posts in this thread, I've been converting the real-world numbers folks have been providing to both cost and theoretical efficiency (kWh harvested vs total chemical kWh in a unit of each fuel).

Based on the real-world numbers we've been seeing so far, the best gasoline efficiencies are around 19% conversion of the chemical energy in the fuel to usable electric energy compared with around 24% based on @crazydane's numbers.

Depending what strategy you use to calculate, that gives a number of statements we could make;
  • "Diesel generators are about 5% more efficient than gasoline generators"
  • "Diesel generators are about 26% more efficient than gasoline generators"
  • "Gasoline generators are 20% less efficient than diesel generators"
  • "Diesel generators are about twice as cost efficient with fuel compared with gasoline generators"
Aside from pointing out how terrible percentages (and ambiguous ratios) are for comparisons (https://xkcd.com/1102/ https://xkcd.com/985/), I think we can say that there may be a trend forming in terms of efficiency (and fuel cost efficiency) here;

diesel > gasoline > propane
Click to expand...

Another interesting aspect would be total running cost including amortized equipment and maintenance costs, but I suspect that may be beyond the scope of this thread.
 
Delmar said:
The real world usage comparison is cost of fuel per kwh, including sourcing and transporting the fuel.
Click to expand...

Yeah, I don't think we're going to be able to do that.

Total embodied energy - including full supply chain either "notoriously difficult" or "impossible" depending how much of the supply chain you choose to include... e.g. Do we want to include the photosynthetic efficiency of the plants that broke down to form out hydrocarbon fuels? How about the efficiency of that anaerobic decomposition process? How about the amortized fuel equipment costs exploring and proving the oil wells?
 
Checkthisout said:
The craziest part is how your number crunching reminds us of just how inefficient it is to convert heat into mechanical energy and then into electricity.
Click to expand...
Or even worse if you are running a pump or compressor. Heat > rotating energy > generator > electric transmission > AC motor > rotating energy > compressor.

I would like to do a comparison of typical grid powered air conditioning compressor vs a natural gas engine directly coupled to a compressor, also turning a small generator to run light loads and recharge batteries.
 
Delmar said:
Or even worse if you are running a pump or compressor. Heat > rotating energy > generator > electric transmission > AC motor > rotating energy > compressor.

I would like to do a comparison of typical grid powered air conditioning compressor vs a natural gas engine directly coupled to a compressor, also turning a small generator to run light loads and recharge batteries.
Click to expand...

Well, with cars, basically if everyone had a battery powered car and we burned the fuel at a large central plant and then used that power to run the cars, fuel consumption would drop around 60%.

Nuff sidetrack of this thread though.
 
Checkthisout said:
The craziest part is how your number crunching reminds us of just how inefficient it is to convert heat into mechanical energy and then into electricity.
Click to expand...

Checkthisout said:
Nuff sidetrack of this thread though.
Click to expand...

Well maybe... I think it is a key aspect to all this is that the generator isn't the only thing in the system.

Hard to prove (and highly situational), but I suspect the embodied energy of a given off-grid system would be way worse if it had tons of tiny "engines" that burned fuel directly to apply the chemical energy at the point-of-use as heat or mechanical energy.

Another interesting aspect that is a bit more on-topic is what's the total fuel efficiency (or fixed cost efficiency) of folks' off-grid systems. i.e. if you have an off-grid system that produces/consumes 4 MWh/yr but uses 200 kWh of generator-derived power at 10% chemical conversion efficiency and $1.50/kWh to get through the year, how well are you doing overall?

Code: 
0.2 (MWh / yr) / 10% (conversion efficiency) / 4 (MWh) = 50% effective chemical conversion efficiency
200 (kWh / yr) * $1.50 (USD / kWh) = $300 (USD / yr)
$300 (USD / yr) / 4000 (kWh / yr) = $0.075 (USD / kWh)

Obviously, the above doesn't include up-front equipment or ongoing maintenance costs.
 
Symbioquine said:
Well maybe... I think it is a key aspect to all this is that the generator isn't the only thing in the system.

Hard to prove (and highly situational), but I suspect the embodied energy of a given off-grid system would be way worse if it had tons of tiny "engines" that burned fuel directly to apply the chemical energy at the point-of-use as heat or mechanical energy.

Another interesting aspect that is a bit more on-topic is what's the total fuel efficiency (or fixed cost efficiency) of folks' off-grid systems. i.e. if you have an off-grid system that produces/consumes 4 MWh/yr but uses 200 kWh of generator-derived power at 10% chemical conversion efficiency and $1.50/kWh to get through the year, how well are you doing overall?

Code: 
0.2 (MWh / yr) / 10% (conversion efficiency) / 4 (MWh) = 50% effective chemical conversion efficiency
200 (kWh / yr) * $1.50 (USD / kWh) = $300 (USD / yr)
$300 (USD / yr) / 4000 (kWh / yr) = $0.075 (USD / kWh)

Obviously, the above doesn't include up-front equipment or ongoing maintenance costs.
Click to expand...

Heat recovery for heating in the winter which happens to coincide with reduced insolence I think could make it a win.

That is one thing that I haven't seen on this site. Someone with a gen in their system who has come up with a nifty and SAFE way to recover engine cooling and exhaust heat.
 
Checkthisout said:
Heat recovery for heating in the winter which happens to coincide with reduced insolence I think could make it a win.

That is one thing that I haven't seen on this site. Someone with a gen in their system who has come up with a nifty and SAFE way to recover engine cooling and exhaust heat.
Click to expand...
If the engine is water cooled you could use a heat exchanger. Very common for marine applications.

My thought to recover exhaust heat was to repurpose a gas water heater tank. Delete the burner and pipe the exhaust into the bottom. Feed the water outlet into your primary water heater tank. Should make an effective water dampened muffler.
 
Delmar said:
If the engine is water cooled you could use a heat exchanger. Very common for marine applications.

My thought to recover exhaust heat was to repurpose a gas water heater tank. Delete the burner and pipe the exhaust into the bottom. Feed the water outlet into your primary water heater tank. Should make an effective water dampened muffler.
Click to expand...

Yeah but I was thinking along the lines of the smaller air-cooled inverter gens. Off the top of my head, the smallest water cooled engine Honda are the old ES6500's?? but those aren't inverter.

Would be a fun little project. You would definitely want to recover the exhaust heat which would also be easy.

Just have to do it and make it work in a practical way.
 
Local reg gas is 3.78 a gallon. Diesel is 5.53 a gallon. Sorta shoots down the diesels are 20% more efficient and cheaper to run theory.
 
I use a 6kw generator with a 5 gallon gasoline fuel tank.
My battery is a 5KWH eg4 LiFep04
If I run my battery down to %25 I can charge it to %100 on 2.5 gallons of fuel.
I can usually do this twice before it’s empty.
It’s about $20 for the 5 gallons.
So it’s about $10 per charge.
Not the best for my pocket.
The panels help a lot.
 
wme said:
Local reg gas is 3.78 a gallon. Diesel is 5.53 a gallon. Sorta shoots down the diesels are 20% more efficient and cheaper to run theory.
Click to expand...

It's usually recommended to run ethanol-free gasoline in small engines like generators if you care about their longevity and reducing maintenance. I doubt you'll be finding ethanol-free gasoline for $3.78 if regular gas is selling for that. Realistically you're probably looking at a similar price for ethanol-free gasoline and diesel.

That said, even with the numbers you've provided (and the relative efficiencies others in this thread have reported), diesel is cheaper (in terms of raw fuel cost) per kWh than gas.

The math to back up that statement;

Code: 
Diesel generators are being reported as producing ~10 kWh / gal of diesel
Gas generators are being reported as producing ~6.5 kWh / gal of gasoline

So if you needed 10 kWh of power you'd need to buy;

10 kWh / 10 (kWh / gal of diesel) = 1 gallon of diesel @ $5.53 = $5.53
10 kWh / 6.5 (kWh / gal of diesel) = 1.54 gallons of gas @ $3.78 = $5.82
 
I use a champion 8500 whole home standby generator, inverter type, i run it strictly to charge my batteries thru the sol ark in an off grid situation. I usually pull 6000 watts from it while charging. Propane powered. I can tell you that this is an expensive endeavour and am looking for alternatives.

In canada I am paying $0.90 / pound of propane. It consumes 1.1 Gal per hour at half load. If we do the math. 1 gallon is 3.78541 liters. Given my above 50% load I am best guessing that i'm using 1.3 gal per hour. The full load specs show that it uses 1.6 gal per hour at full load.

I just need to figure out why the power is getting "dirty" and the inverter unlocks from the generator when i try to push it past this limit. It shows the hz dropping to about 58 past this point. I feel like I need to adjust the governor on the unit, but my HZ meter bounces all over the place when I try to do this so I have to have someone watching on the sol ark while adjusting (and is the solark hz super accurate? I don't know. )

So in canadian dollars to you americans this is actually a cheap way to go. But for me, solar is the cheapest, pray for more sun :D
 
Mannfamilywoodworks said:
I use a 6kw generator with a 5 gallon gasoline fuel tank.
My battery is a 5KWH eg4 LiFep04
If I run my battery down to %25 I can charge it to %100 on 2.5 gallons of fuel.
I can usually do this twice before it’s empty.
It’s about $20 for the 5 gallons.
So it’s about $10 per charge.
Not the best for my pocket.
The panels help a lot.
Click to expand...

Code: 
5 (kWh / battery) * 3/4 (battery) = 3.75 kWh capacity to charge
3.75 (kWh) / 2.5 (gal) = 1.5 kWh / gal
$6.42 (USD / gal) / 1.5 (kWh / gal) = $4.28 (USD / kWh)
1.5 (kWh generated) / 33.41 (kWh total energy) = 4.5% efficiency

@Mannfamilywoodworks I suspect something is either really wrong with your generator or maybe you're charging way slower than necessary - basically near-idling the generator while you effectively trickle-charge the battery.

How long is the generator running per charge cycle?
 
crazydane said:
Rated load details are:

Power Rating5kW, 0.8 pf @ 4000ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I'm testing into a 5kW dummy load (1.0 pf) at 760 ft and temps around 70F. So it makes sense that I'm doing a little bit better than 0.57 gph.

I also have a MEP-803A, which is basically a 802A with 2 more cylinders and a larger generator head. It is spec is 0.97 gph @ rated load, which is:

Power Rating10kW, 0.8 pf @ 4000 ft/120°F; 110% Max Power; De-rate: 3.5%/1000 ft from 4000 to 8000 ft

I bet I can squeeze 10.5 - 11 kWh / gal out of it.

Btw, I buy off-road diesel which is about $0.25 cheaper than on-road. As of this morning, the off-road was $5.44 at my local service station.

You can never have too many generators, lol:

View attachment 120323
Click to expand...
Off road here is $4.85.

I like those generators but hard to find here over 5kw.
 
myles said:
I just need to figure out why the power is getting "dirty" and the inverter unlocks from the generator when i try to push it past this limit. It shows the hz dropping to about 58 past this point.
Click to expand...
Generators will drop engine speed as loads increase. Falling below 58Hz under full load isn't uncommon as a lot of generators advertise fairly optimistic load capacity specifications. But they can also need some service, things like the carburettor might need a clean if the jets are not pristine it may be slightly restricting fuel flow. Or the governor is not quite operating as well as it should. The governor control springs do lose their capacity for control over time.

My generator is an inverter generator, so the power wave form is high quality.

In terms of power output quality, Inverter generator >> generator with an AVR > no AVR > brushless generator.

No all charge controllers will accept poor waveforms.
 
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