# Propane generator efficiency: Am I calculating it correctly?

### Re: Propane generator efficiency: Am I calculating it correctly?

lsayre wrote:During the months when we use no air conditioning we consume (on average) 16,738 Watts of electricity per day. But much of that consumption likely comes over only an hour or two per day (when we are cooking and doing laundry). A generator capable of delivering only 14,000 watts per hour might be marginal for our house. (???) It would perhaps take a 20,000 Watt generator to assure that we can cover all of our peak hourly demand needs.

16,738 Watts per day = 510 KWH per month. The electric company (where I live at least) bills you based upon KWH's consumed per month.

16,738 Watts per day should theoretically require about 2.5 gallons of propane per day (on average) at 23.6% efficiency.

Your figure seems to assume that the generator is not running at light or no load. you state that it needs 2.1 gallons at full load, but what does it burn the rest of the time? Does it idle and at what consumption rate or does it run at full RPM all the time? I suspect it will burn double your figure.
franco b

Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

### Re: Propane generator efficiency: Am I calculating it correctly?

franco b wrote:Your figure seems to assume that the generator is not running at light or no load. you state that it needs 2.1 gallons at full load, but what does it burn the rest of the time? Does it idle and at what consumption rate or does it run at full RPM all the time? I suspect it will burn double your figure.

Ouch! Probably realistic though.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

Probably the most fuel efficient set up would have a very small generator and a battery bank instead of a huge generator that could handle full load at any moment. The battery bank would add power during times of higher power draw and then recharge during times of lower power draw. Kinda the same concept as a hot water tank stores hot water when yer not using it, then brings the cold water back up to temp slowly over time after most of its been depleted. This is also how a solar system would be best used. Although, Batteries and inverters would be pricey to install and maintain I'm thinkin.
Lightning

Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

### Re: Propane generator efficiency: Am I calculating it correctly?

coalnewbie wrote:Two of the three electric feeds to the farm are switched off for 7 months so of the year and it all gets fed underground from the house. Orange and Rockland have "visited" me 7 times in this last quarter, replaced meters etc. They can't work out how I do it and want to charge me \$1000 a month to switch on a few lights on in the barn. I use their power for A/C briefly in the summer and then off it goes.....phkm. The last guy asked me how I do it and I mutter darkly, solar...... Solar what a joke, around here that would run a cell phone at best.

Why don't you run the entire complex from the house service?
Rob R.

Stoker Coal Boiler: EFM DF520
Hand Fed Coal Stove: Hitzer 50-93
Other Heating: Dad's 1953 EFM Highboy

### Re: Propane generator efficiency: Am I calculating it correctly?

lsayre wrote:I'm looking at the specs for the Kohler 14 kW RESA Generator, and it consumes 81 CFH of propane per hour to generate its full output of 14,000 Watts/Hr.

1 CFH = 2,500 BTU of LP/Propane (one source said 2,516)

81 CFH x 2,500 BTU's/CFH= 202,500 BTU's per hour of propane consumed by the generator at full output load of 14,000 Watts per hour

Propane has 96,500 BTU's per gallon (so it burns 2.1 gallons of propane per hour to deliver 14,000 watts/hr. of electricity)

14,000 Watts x 3.412 BTU's/Watt = 47,768 BTU's per hour equivalent delivered by the generator at full output

47,768 (BTU's equivalent out) / 202,500 (BTU's in) = 0.236

0.236 x 100 = 23.6% efficiency in converting propane into electricity for this generator

Overall that seems to be quite good. But am I calculating the correctly?

Are you planing on buying one? and what do these cost?
grumpy

### Re: Propane generator efficiency: Am I calculating it correctly?

Batteries are amazingly costly, bulky, and even sometimes dangerous. They also need replacement roughly every 5 to 7 years, or much sooner than that if you discharge them any further than 33.3% of full rated capacity. And for this I'm considering the most advanced AGM or Gel-Cell deep cycle types.

I ran the numbers, and to run my home on batteries for 3 days would require over \$30,000 worth of them. I would need more than 3,000 AMP hours of storage at 48 volts.

17,000 Watts per day to run my home x 3 days of run time = 51,000 Watts required

51,000 / 0.3334 = 153,000 Watts actually required to be stored in the batteries (so they never have to discharge more than 1/3 in order to run my home for 3 days)

153,000 Watts / 48 volts = 3,187 AMPS required

3 battery "units" of 1,062 AMPS each ~= 3,187 AMPS

Each unit requires 3 x 354 AMP, 6V batteries wired in parallel with 8 batteries in series for each of these 3 (since 48V/6V = 8 batteries) = 24 x 354 AMP batteries/bank.

3 x 24 battery banks required to achieve 3,187 AMPS = 72 batteries total, with each of these 3 x 24 battery units again paralleled to achieve 3,187 AMPS

72 of these batteries if purchased as sealed AGM deep cycle types at ~\$425/battery = \$30,600 in total batteries needed (before racks and wiring)

Looking at this a different way as a quick proof of the above confusion I get:

72 x 6V, 354 AMP batteries = 72 x 6V x 354 AMPS = 153,000 Watts total x 0.3334 (for 1/3 max discharge) = 51,000 Watts / 3 days required = 17,000 Watts per day delivered
(whereby my home requires a nominal 17,000 Watts per day)

PS: If I wanted enough batteries to run our home for only one day instead of three, I could cut the cost down to \$10,200 for the batteries.
PPS: If you are brave enough to wire 3,187 AMPS worth of batteries together, be totally dry, and wear super good long sleeve rubber gloves while doing so.

For the battery guru's among us, does this sound right?
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

I think Lightning's point was that with battery back up a far smaller generator could be used. With peak load both the batteries and generator would be drawn from and with light load the generator alone could handle it. It's a mess though no matter what you do.
franco b

Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

### Re: Propane generator efficiency: Am I calculating it correctly?

franco b wrote:I think Lightning's point was that with battery back up a far smaller generator could be used. With peak load both the batteries and generator would be drawn from and with light load the generator alone could handle it. It's a mess though no matter what you do.

Yes exactly... Not 3 days worth of batteries You said peak draw is a couple hours a day. You would need enough battery power to cover the peak hours.. The remainder of the hours would be less than what the genny is putting out, which would be stored in the batteries again.. SO, actually you need a generator that would generate 20,000 watts per day like a little wee Handa genearator.. and a battery bank that could handle peak draw for a few hours not a few days.. Gen would run 24/7 to keep the batteries charged. Voila - off grid... Work with me partner

Lets say you have a gen that makes 1000 watts per hour and a battery bank that can hold 24,000 watts.. That would more than cover your power for the day..
Lightning

Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

### Re: Propane generator efficiency: Am I calculating it correctly?

Lightning wrote:
franco b wrote:Lets say you have a gen that makes 1000 watts per hour and a battery bank that can hold 24,000 watts.. That would more than cover your power for the day..

A battery bank that can deliver 24,000 Watts is actually a battery bank that can hold 72,000 Watts at 1/3 of rated capacity for delivery, or about half of what I described above.

Also, charge controllers for batteries are only about 75% efficient. The generator would likely need to be larger to factor this in. And don't ever let your batteries get cold, as they loose capacity rapidly as the temp drops.

Or are you suggesting a battery bank of 24,000 Watts, that will never be drawn down by more than 8,000 Watts (1/3) over any given time period before it is brought back up to full charge. 16 x 250 AMP hour 6V batteries (in two banks of 8 each) would accomplish that. It would require 16 batteries at about \$325 each, or about \$5,200 worth of AGM type batteries.

How does your system know when to draw from the batteries and when to draw from the generator? I presume there would have to be a battery cut-out switch set at 1/3 discharge, with simultaneous switching to fire up the generator at that juncture.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

grumpy wrote:Are you planing on buying one? and what do these cost?

Not sure about buying one. I believe they cost about \$4,500 (give or take). On Amazon there is a Generac with the same output for about \$3,300.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

I think that could be shrunk even more Larry. It would depend on peak draw and the duration of that peak. AND if you spread out the use of the peak draw appliances you could make due with even less battery storage. But then you would be compromising to make the system work.

It would be tough to get off grid unless you could replace appliances that have heavy current draw for short periods of time, like an electric stove or hot water tank with appliances that burn fuel instead.
Lightning

Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

### Re: Propane generator efficiency: Am I calculating it correctly?

Lightning wrote:I think that could be shrunk even more Larry. It would depend on peak draw and the duration of that peak. AND if you spread out the use of the peak draw appliances you could make due with even less battery storage. But then you would be compromising to make the system work.

It would be tough to get off grid unless you could replace appliances that have heavy current draw for short periods of time, like an electric stove or hot water tank with appliances that burn fuel instead.

Until I installed the coal boiler, our home was all electric. The boiler handles home heat and DHW. We still cook and dry clothes (and do everything else) with electricity.
Last edited by lsayre on Sat Oct 19, 2013 5:33 pm, edited 1 time in total.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

The generator and batteries would be in parallel. There wouldn't be any switching between the two. When you drew more power than the generator is producing, the batteries would supplement. Then when you drew less power than the generator the surplus power would charge the batteries back up.
Lightning

Hand Fed Coal Furnace: Clayton 1537G
Coal Size/Type: Nut/Stove Size Mix

### Re: Propane generator efficiency: Am I calculating it correctly?

lsayre wrote:
grumpy wrote:Are you planing on buying one? and what do these cost?

Not sure about buying one. I believe they cost about \$4,500 (give or take). On Amazon there is a Generac with the same output for about \$3,300.

The Generac consumes 111.6 CFH of propane to deliver 14,000 Watts, whereas the Koehler consumes 81 CFH for the same output. You save money up front with the Generac, but you save money over the long haul with the Koehler.
Last edited by lsayre on Sat Oct 19, 2013 7:17 pm, edited 1 time in total.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)

### Re: Propane generator efficiency: Am I calculating it correctly?

Getting back to batteries, their advertised amp/hr ratings are all listed for draw down over 20 hours. Draw them down in only one or two hours and you will find that they have far less capacity than you imagined, as well as far less life expectancy. Plus I would imagine they can get down right hot if drawn down rapidly. These factors would mean that well over-sizing your battery bank(s) would likely be prudent.

Update, I just sourced where the 1 hour draw down capacity of a battery is only 59% of the standard 20 AMP Hour rated capacity. Also, at 54 degrees F they can only deliver 85% of their rated capacity. Put them together and you only have half the battery you thought you had.
lsayre

Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Anthracite Pea
Other Heating: Resistance Boiler (if I ever get it fixed)