How efficient is your generator?

[Symbioquine]

So about 70% load is the sweet spot?

That's my guess, but I'd love to have more data to support/disprove that!

I guess you could weigh the tank yourself.

Unfortunately, I don't have a scale in the 100 lbs range than is going to be more accurate than the volume measurement from the propane seller.

 

[Checkthisout]

Inverter generators, if they are well designed, should always operate near 100% engine load. I.E. the throttle plate should be open fairly far for the given RPM.

So, this differs from a non inverter which runs at a fixed RPM and then varies throttle plate opening based on load.

Best overall efficiency will be produced at the torque peak so maybe look up similar engines and their power curves and see what RPM is commanded at that given electrical load for your particular gen.

 

[crazydane]

I have a MEP-802A diesel generator. It burns 0.5 gallons of fuel per hour with a 5,000 watt load.

I also have a Kubota 17 kW generator. It has a 4 cylinder turbo charged diesel. Next time I load test it, I'll see about collecting some fuel consumption data.

 

[schmism]

My 1800w inverter genny runs about a quart an hr on 500-800w.

 

[Symbioquine]

I have a MEP-802A diesel generator. It burns 0.5 gallons of fuel per hour with a 5,000 watt load.

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;

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

 

[Checkthisout]

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;

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

Diesels are 20-25% more efficient than gas.

 

[Symbioquine]

My 1800w inverter genny runs about a quart an hr on 500-800w.

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

If we assume gasoline then this is:

Best case;

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;

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

 

[crazydane]

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;

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

Rated load details are:

Power Rating

5kW, 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 Rating

10kW, 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:

 

[Symbioquine]

Rated load details are:

Power Rating

5kW, 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 Rating

10kW, 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.

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?

 

[Delmar]

Diesels are 20-25% more efficient than gas.

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.

 

[Checkthisout]

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.

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.

 

[schmism]

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

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.

 

[Checkthisout]

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.

 

[Symbioquine]

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.

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

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.

 

[Checkthisout]

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.

 

[Symbioquine]

The real world usage comparison is cost of fuel per kwh, including sourcing and transporting the fuel.

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?

 

[Delmar]

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.

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]

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

I was only figuring the user cost of sourcing the fuel and delivering to the generator. IE hauling diesel vs connection to a NG line.

 

[Checkthisout]

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.

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.

 

[Symbioquine]

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.

Nuff sidetrack of this thread though.

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?

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.