Natural gas home generators vs. electricity

[The Second Stone]

Electricity is one of the coolest things peoplekind have ever come up with. However, air conditioning Stately Stone Manor in the hotter months is very expensive. I'm a wondering if I hooked up my home electrical needs to a standby natural gas generator, more or less using it permanently for electricity if I would be saving money after sunk costs or spending more to generate electricity. I'm paying about 12 cents per kilowatt hour and about $1.09 per Therm for gas.

My Google Fu has not yet revealed the answer to this question, nor even come close. The best I can find is that natural gas generators are at least 30 percent more efficient than gasoline.

The thoughts of the dubious dozens? :hijack:

[Random Precision]

The answer is a fairly straightforward one of how many kW can you generate with a Therm of natural gas. I haven't a clue, but I would like to point out that your $ might be better spent in upgrading to a higher efficiency A/C and more insulation (many utilities are offering rebates).

[Random Precision]

Okay, I googled around a bit and came up with the following info. A Therm is roughly equivalent to 100 ft³ of natural gas. A Cummins Onan 11 kW gas generator (the smallest I found) uses 191 ft³/hr at full load and 103 ft³/hr at 25% load. I estimate you will need roughly 25% capacity to operate a 4 ton A/C system. So the operating cost with a gas generator is near $1 per hour vs. ~$0.24 per hour for electricity.

Back to my original point - a higher efficiency A/C system and better insulation will cut your cooling bill for less out of pocket and you'll stand a good chance of qualifying for rebates from the power company and tax credits from the US government.

[The Second Stone]

Thank you. That is exactly the sort of analysis I was looking for.

[Sybarite]

I dunno; here in western Canada, we heat our homes almost exclusively with natural gas furnaces. As you may or may not know, it gets really cold here, for a long time. I don't think we'd use natural gas if it weren't a cost-effective choice. On the other hand, we have large reserves of natural gas here, too.

[Who_me?]

It would probably be easier, more fuel efficient, and greener to install solar panels to help generate electricity to power your AC. I haven't checked what solar panels cost and what the installation would set you back (and you WILL want anything generating electricity professionally installed), but I've seen solar panels on many homes, so I would assume they're cost efficient after some period of time.

[Lazlo]

Originally Posted by Sybarite

I dunno; here in western Canada, we heat our homes almost exclusively with natural gas furnaces. As you may or may not know, it gets really cold here, for a long time. I don't think we'd use natural gas if it weren't a cost-effective choice. On the other hand, we have large reserves of natural gas here, too.

I did a quick search of natural gas home furnaces and some are in the 90% efficiency range. This means that ~90% of the energy you get from burning the NG actually heats your home. There is very little wasted heat

Generating electricity, on the other hand, is notoriously inefficient. Huge power stations strive to hit 60% efficiency, and that's with multi-stage turbines and heat exchangers to scavenge the wasted heat. With a home-sized generator, you're not going to have any of that. I didn't find a cite, but I'd guess that they're in the 25%-35% efficiency range.

The majority of NG's work potential is going to go right out the exhaust when you generate electricity on a small scale, so you'll almost never see homes being powered that way.

[Random Precision]

Originally Posted by Sybarite

I dunno; here in western Canada, we heat our homes almost exclusively with natural gas furnaces. As you may or may not know, it gets really cold here, for a long time. I don't think we'd use natural gas if it weren't a cost-effective choice. On the other hand, we have large reserves of natural gas here, too.

It's the extra step of using it to run a generator that raises the cost. Using natural gas for heat is pretty darned efficient - up to 95% - because heat is the main product of combusting the gas. But in a generator only a portion of the heat can be consumed to turn the generator, the rest is wasted. I don't have any figures, but I'd be surprised if the efficiency were over 35%.

Originally Posted by Who_me?

It would probably be easier, more fuel efficient, and greener to install solar panels to help generate electricity to power your AC. I haven't checked what solar panels cost and what the installation would set you back (and you WILL want anything generating electricity professionally installed), but I've seen solar panels on many homes, so I would assume they're cost efficient after some period of time.

In the US, solar electric costs ~$9 - $10 per installed watt. The government is offering a $2/watt tax credit and some electric utilities are giving a $2.50/watt rebate. So installed cost runs $4500 to $8000 per kW. Without the rebates and tax credits, solar electric takes longer than the life of the panels to pay for itself. Higher energy costs or lower solar panel costs will be required to change this.

[si_blakely]

If you have Natural Gas Combined Heat and Power (CHP) you can get much better results -

The "ZuhauseKraftwerk" (HomePowerPlant) features a vanilla VW Golf natural-gas engine that generates 20kW electrical and 34 kW heat with an efficiency of 92%.

A planned project in Hamburg will deploy 100000 remotely managed units, delivering benefits to householders and electricity suppliers, reducing transmission losses, improving peak load capacity and decreasing emissions. Link

Si

[Muskrat Love]

The natural gas generators are pretty expensive, with list prices around what you'd pay for a new small economy car. It would be a lot less expensive to upgrade your AC system. If you buy a straight cool AC system with a SEER of 16 or greater and an EER of at least 12, you can get a pretty large federal tax rebate. If you get a heat pump system, you only have to get 15 SEER. Replacing a furnace with a 95% AFUE or greater one will also get you a good tax rebate. It used to be that only the very expensive furnaces with variable speed blowers got that efficiency, but I know at least one HVAC company has come out with new versions of non-variable speed furnaces that meet the 95% requirement to qualify for the tax rebate.

[Sybarite]

We had a high-efficiency furnace at one house; it sucked. We didn't notice any significant savings, either. It was years ago, though - maybe they're getting better at making them suck less.

60% loss when generating electricity, eh? That doesn't sound anywhere near good enough. I'm waiting for home nuclear plants - who's with me?

[Stupid Sexy Flanders]

Now that the factual question is answered, can I jump in with a recommendation?

The most effective thing you can do to start is shade your house with trees (if it isn't already).

[Sybarite]

Seconded - our new house has shade trees on both sides on the back half, and you can feel the temperature differential as you walk down the hallway.

[Muskrat Love]

Originally Posted by Sybarite

We had a high-efficiency furnace at one house; it sucked. We didn't notice any significant savings, either. It was years ago, though - maybe they're getting better at making them suck less.

Can you remember what kind it was? Was it the type that requires you to install a drain for condensate that comes out of the heat exchanger?

I have a bit of book knowledge about furnaces and A/C, but my practical experience is pretty low. I've considered getting a 90% for our house but I'd be interested in hearing about your experiences with them. We currently have an old GE from the late 1960s, it's probably got an AFUE of 50 or so. Even an 80% furnace would be a big improvement (and a lot less work).

[Amblydoper]

What about a NG AC Unit? Do they even make those? I know there used to be NG refridgerators, but they were never very popular.

[plynck]

Originally Posted by Lazlo

I did a quick search of natural gas home furnaces and some are in the 90% efficiency range. This means that ~90% of the energy you get from burning the NG actually heats your home. There is very little wasted heat

Generating electricity, on the other hand, is notoriously inefficient. Huge power stations strive to hit 60% efficiency, and that's with multi-stage turbines and heat exchangers to scavenge the wasted heat. With a home-sized generator, you're not going to have any of that. I didn't find a cite, but I'd guess that they're in the 25%-35% efficiency range.

The majority of NG's work potential is going to go right out the exhaust when you generate electricity on a small scale, so you'll almost never see homes being powered that way.

Exactly. Gas generators are fine for emergencies, but far more expensive than the grid for supplying electricity.

And folks here have been talking about natural gas as a fuel source. My understanding is that propane is more efficient for a generator than natural gas. Although again, not cheaper than power off the pole.

[xman]

Perhaps I missed something however, I did some calculations in my area and it shows that electricity from NG electric generator is cheaper than grid from 16% to 54%, even though in my example the generator is only 26% efficient.

In Toronto, Ontario, Canada
1) NG price including delivery is : 17.6755 cents per cubic metre.
2) Electricity price : avg 10 ¢/kWh
3) Generator : 22kw Generac requires 281k BTU at full load to produce 22kw of electricity

Also other details :
4) 3.4k BTU = 1kw
5) 1 m3 of natural gas = 35k BTU

If you convert the units to same units, then we can compare. For now, lets compare in kw.

281k BTU of energy needed by generator is equivalent to 281k BTU/(3.4k BTU/kw) = 82kw. Since generator produces 22kw of electricity then efficiency is 22kw/82kw = 26.8% efficient.

Generator's NG consumption of 281k BTU/hr of NG to produce 22kw/hr. Its NG cost can be calculated by $0.176755 /m3. x 281k BTU/(35k BTU /m3) = $0.176755/m3 x 8.02m3 = $1.4175751 for 22kw/hr of electricity produced by NG generator.
Per Kwh cost by NG Generator = $1.4175751/22 = $0.0644/kwh

So in summary (in Toronto):
Electricity by grid cost is $0.10/kwh,
Electricity by NG Generator is $0.0644/kwh

That's a 35% savings by NG electric generator.

If the electricity is charged by time of use billing (in my area), then the savings can vary based on grid costs :

14.0 ¢/kWh Highest Price (On-peak) 54% savings
11.4 ¢/kWh Mid Price (Mid-peak) 43% savings
7.7 ¢/kWh Lowest Price (Off-peak) 16.3% savings

So when the generator kicks in during an emergency, I would still be saving money on electricity bills. Wear and tear on the generator, maint. costs are a different thing.

References :
Electricity Prices :
http://www.torontohydro.com/sites/el...cityRates.aspx

NG Price :
https://www.enbridgegas.com/homes/ac...-enbridge.aspx

22kw Generator specs (281k BTU NG fuel consumption on pg 3):
https://www.homepowersystems.net/Por...HSB%282%29.pdf

Kilowatt to BTU (1kw = 3.4k BTU) :
http://www.rapidtables.com/convert/power/kW_to_BTU.htm

1 m3 of natural gas = 35k BTU :
http://www.gowithnaturalgas.ca/getti...y-equivalency/

I haven't decided on the 22kw or the core essentials backup on the 7kw generator yet. If anyone has any recommendations on higher efficiency than 26% to generate electricity from NG, please let me know.

Xman

[Zeener Diode]

I just reported this as spam.

If I'm wrong, let me know.

[hajario]

Originally Posted by Zeener Diode

I just reported this as spam.

If I'm wrong, let me know.

I don't think that this is spam at all.

Thanks for the calcs, xman.

[BJMoose]

Yeah. That's way too much math for a spammer. . . .


Good thread to resurrect, though.

[Jaglavak]

Originally Posted by xman

Perhaps I missed something however, I did some calculations in my area and it shows that electricity from NG electric generator is cheaper than grid from 16% to 54%, even though in my example the generator is only 26% efficient....
Electricity by NG Generator is $0.0644/kwh

Nice first post! Welcome to the veldt, xman. Your calculations are fine as far as they go. But there are a couple more things to think about.

First off, from the Generac manual you linked they also list the half load fuel usage at 5.21 m3/hr.

5.21 m3/hr * $0.177/m3 = $0.92
$0.92/11 kWh = $0.084/kWh

Not quite as good, but still cheaper than the power company.

Originally Posted by xman

Wear and tear on the generator, maint. costs are a different thing.

These numbers change the picture quite a bit. One way to estimate the lifespan of engine driven equipment is to think of highway miles on a car. A car doing 60 MPH will go 6000 miles per hundred hours operating time. So a typical maintenance interval might be every 200 hours. Say an oil and filter change costs $15 if you do it yourself. Let's neglect all other maintenance like air filters and spark plugs.
$15/200 = $0.075/hr

As far as lifespan, let's assume that Generac builds a super-reliable generator that would be equivalent to a car going 1,000,000 miles between overhauls. That's about 16,667 hours operating time. I found the generator you mentioned listed on eBay for $4500. Let's assume you get free shipping and install it yourself for another $1000.
$5500 / 16,667 = $0.33hr

$0.33 + $0.075 = $0.41/hr to turn the crank not counting fuel.


You also might want to give some thought to your usage patterns. When was the last time you pulled 22 kW, or even 11 kW? Most families average more like 1 to 2 kW with spikes to 5 to 10 kW from things like electric ranges, washing machines, and refrigerators kicking on at the same time. The generator efficiency is going to suck pond water at those loads. On the plus side, I think you would be able to get all of your space heat and hot water needs from the engine cooling water with enough left over to keep a hot tub warm.

Finally, the ad for the generator stated, 'Not intended for use as primary power in place of utility or in life-support applications.' That's probably due to air pollution regulations. I severely doubt if the sherrif is going to stop by and give you a ticket for that, but eventually the neighbors might get tired of the constant hum and make some phone calls. I don't know the regulations in your area but it's something to think about.