# Determining Generator size for an off grid community

Discussion in 'Homework Help' started by Sidelosisa, Mar 31, 2016.

1. ### Sidelosisa Thread Starter New Member

Jun 18, 2015
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0
Good Day
I'm doing a research project on energy/fuel consumption and generators.One of the requirements is to determine how much fuel would be consumed and what size of a generator would I need to power a community of at least 30 households, supplied off-grid.

so far I found a 400kW/500kVA generator with a fuel consumption of 107 litres per hour. I doubt that this would be enough.
Any other generator I find does not have the fuel consumption specified.

Thank you.

2. ### BobaMosfet Active Member

Jul 1, 2009
109
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You won't find any generator not owned by a utility company that will support that. It also depends on _where_ this community is. In the USA, you'd have to have a generator that provided between 30 and 90 Megawatts/month, to support the needs of 30 homes.

3. ### WBahn Moderator

Mar 31, 2012
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What is a Megawatt/month?

That's like saying that an engine produces 100 hp/day. It's meaningless.

4. ### WBahn Moderator

Mar 31, 2012
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The first thing you need to do is determine your requirements.

How much does a household in this community use? You have a few numbers at play -- the average consumption and the peak consumption. Then you need to combine those to get numbers for the community. The average you can just multiply by the number of homes, but the peak is a different matter. That will depend on whether most of the homes hit their peak usage at the same time or not and that depends on a number of factors. Then you need to build in a safety factor (or plan to impost restrictions of some kind).

With those numbers you can then determine what size generators you need and whether you want to meet it with a single generator or with several smaller generators. Part of that decision can be the fuel economies of the different sizes of generators (and also their costs, etc). If you use multiple generators, don't forget the issue of synchronizing them (or you might consider the option of using DC for the community with inverters in places that require AC power).

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5. ### WBahn Moderator

Mar 31, 2012
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Really?

A quick Google search shows that Kohler will be happy to sell you a diesel generator set that will output 3250 kW of power.

6. ### BobaMosfet Active Member

Jul 1, 2009
109
11
My original numbers were a little high based on a non-typical assumption-- but your Kohler generator isn't adequate for the OP either. Let's review:

The OP said he wanted to power _30_ homes. 3.25mWh isn't going to do it (Kohlers). Per the EIA (http://www.eia.gov/tools/faqs/faq.cfm?id=97&t=3), the average US home consumes over 10kWh/day. That's basic living. In contrast, my wife bakes, I use power-tools, and am heavy into tech, plus we have HVAC, other fans, other motors, etc-- I use many times that.

So, for 30 homes, basic living, you'd need at least a 10mWh generator.

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7. ### Papabravo Expert

Feb 24, 2006
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You're all confused on units. Small 'm' is the abbreviation for 'milli' or one one-thousandth. Maybe you meant 'M' for 'mega' or million. A generator that produces 3250 kW, may or may not be capable of delivering 3250 kWh (killowatt hours). That may or may not be the same as 3250 kW continuously for an hour, or some profile of power delivered with peaks of 10 kW followed by troughs of 0.5 kW. You may have to look carefully at generator specifications to find the difference between peak power and average continuous power. The results might shock you.

The major problem with power generation is that you have to use it or lose it. It is better if you can use renewables like solar, wind, and geothermal. The next problem is how you expect a community of 30 homes to pay for and manage this infrastructure. This one is pretty much in the 'fat chance' category.

PS. My wife and I are retired and live in Northern Florida; we consumed 825 kWh @7.001 cents/kWh (\$57.76) from 2/18/16 to 3/20/16 for an average of 27.5 kWh/day. The 10 kWh/day estimate seems low by a substantial margin.

If you really want to try making such an off grid community workable there would be way more communal resources than in an average community. This way power could be effectively rationed and managed. For example a communal kitchen, laundry, and bathing facilities.

Last edited: Apr 5, 2016
8. ### WBahn Moderator

Mar 31, 2012
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Generators are not rated in Wh -- they are rated in W. Also, 10 mWh is a trivially small amount of energy -- a typical 9V battery has about 3000 mWh of usable energy.

The average U.S. Residential usage is 911 kWh/month -- which is about 30 kWh/day. That's the nationwide average. The state with the highest average is at about 42 kWh/day and the low average is about 17 kWh/day, so I have no idea where you are getting the 10 kWh/day figure.

Let's call it an even 1000 kWh/mo or 1 MWh/mo. That's an average power consumption of 1.4 kW. The peak-to-average ratio, while climbing, is still under a factor of 2 (according to the Energy Information Institute). Now that is a large ensemble number so the factor for a small group of homes is almost certainly higher. So build in a big margin for error and use a factor of 10. So that's a peak demand of 14 kW/home or 420 kW for a group of 30 homes. That's based on the U.S. average. If you use the highest state average that bumps up to about 600 kW. How is a 3250 kW generator not sufficient for that? The generator that the TS mentions is likely completely adequate -- a 100 kW generator MIGHT be sufficient.

9. ### WBahn Moderator

Mar 31, 2012
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I think most generators (of the diesel/gas variety) throttle on demand, so you are only stuck with the idling consumption as a fixed base, though there is going to be one operating point at which it is at peak efficiency.

The problem there is that these are generally NOT throttle-able and very seldom match the actual demand. So now you have to store the energy for later use. That may or may not be a huge challenge for a community of 30 homes.

A big factor in this is where these homes are located, both geographically (i.e., weather) and culturally (is electricity used for a handful of "necessities" or is it used as a mainstay of life for everything).

Agreed. And you might have slots in which different people are allowed to use different amounts of electricity. That could either be strictly assigned an monitored, or you might be able to have a billing schedule in which each home has time periods with a large premium on energy used above a certain floor.

10. ### BR-549 Well-Known Member

Sep 22, 2013
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From your wording, I assume this is for emergency backup. When main grid is down.

If you want to permanently supply 30 homes, you should consider a small steam turbine.

A lot of industries use to generate their own power.

A boiler can use many different types of fuel.

11. ### BobaMosfet Active Member

Jul 1, 2009
109
11
You funny man *hahaha*. My apologies for getting the important letters in lower-case. I have to switch between several different systems where some of the systems do that (why the mfg didn't follow a standard, I don't know), and so it blurs from time to time. As for Watts .v. Watt Hours... yes, agreed. I myself use 1.2-2.3MW/month.

12. ### #12 Expert

Nov 30, 2010
16,257
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Still having difficulty with units. Is that megawatt hours? megawatt minutes? megawatt seconds?
Taking a guess, I think 1.2 to 2.3 megawatt hours per month.
A bit more than my usage by 30% to 250%
I also think post #8 has some reasonable numbers.
That's a good place to start.

13. ### WBahn Moderator

Mar 31, 2012
17,716
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You still don't understand the basic concepts. What is a MW/month? That's power per time. The only thing I can think of that that would mean would be a rate of increase in the power production or consumption. Go study what power and energy are and the differences between them.