what size genny for 7-8kwh prime power?

[srryan]

what size genset should i be looking for a 8kwh prime power?  looking to spend 10-12k. please help

[Ronmar]

OK, are you saying you have a need to supply a 7-8KW electrical load?  Well then you need to have a 7-8KW generator set?  For sustainable power generation, you need 2HP per KW, so an 8KW generator will have a 16HP engine...

Perhaps if you elaborate on your needs(8KW peak, or 8KW sustained/average), we can deliver a little more specific of an answer...

[srryan]

 lets just say 8 KWH sustained.  2 hp per 1kwh? i thought there was around 700 watts in a hp?

[mobile_bob]

a rough figure is 745 watts per hp, and that is mechanical

so for two hp you would have approximately 1500watts mechanical power
available to drive a generator with...

an average number for efficiency of a generator/alternator in this class is about 75-80%

so 1500watts mechanical  x  .75 alternator eff.  =  1125 watts electrical output

then time you factor in losses due to drive systems, windage, transfer etc. you get down another 2-3% , which nets you a bit over a 1000 watts

therefore the reality is about 2hp for every kwatt electrical generated.

so if you need 8kw continuous you ought to be looking at a minimum of a 16hp diesel or a 20hp gasoline engine... and that only if you are not at high elevations, high temperatures and not burning low quality fuels, all of which will derate the hp capability and require an increase in installed hp to get you the needed 8kw continuous...

then there is power factor concerns, will you be running lots of inductive loads?

in my opinion, if you need 8kw electrical continuous, you out to be looking at a genset capable of at least 10kw continuous, which means a minimum of 20hp diesel
or about 24hp with a gasoline engine.  not many generator sets like to be run right up to maximum rating for protracted lengths of time, even those that can stand up to this sort of service generally are not as efficient as they would be if run at ~80% of full rated output.

what elevation are you at?  what ambient temperatures maximum will you have to encounter? what quality of fuel?  and what are your intended loads?

bob g

[srryan]

250 feet, 100 F, #2 diesel, PC, Amps, Speakers, Lighting, Fans.

[Carlb]

Pc, amps, speakers, lighting and fans will not require near that much power unless your running several thousand watts through those amps.

[Ronmar]

I would say get a Kill-a-Watt and plug in a sample of all these devices to get an actual value for each in operation, then multiply those values by the total numbers of those devices and that will give you a realistic estimate of need...  Then add 20-30%  to size your generator...

[Horsepoor]

One of the most useful items I purchased is a "Cent-a-Meter" which has a remote RF link to the portable monitor (total kit about $100). I am sitting here now watching TV and the remote power meter is measuring my household power draw every 6 seconds or 60 seconds (user selectable). As you glance over at the meter several times a day / night, over a period of time (I.e. Weeks) you will get a very good idea of what your power demands are. It is neat to see the well pump kick on, the electric hot water heater, down stairs AC and/or upstairs AC, etc.

Then when I switch the house off the grid and onto emergency power, I move the monitoring clamps over to onto my generator leads. This gives me an RF link from the generator shed 150 feet away from the house to the remote monitor inside my house and allows me read instantly the generator load. All of this for about $100.

http://www.energymonitors.com/product.php?p=cent-a-meter-wireless-whole-house-electricity-monitor-split-core-connectors_eco-response_cm113a&product=175282&utm_source=google&utm_medium=Product_Search&utm_campaign=google_base

[Derb]

Hi Boss. Also factor in wether or not you are likely to be starting any electric motors during your "peak-load" running. Startup current draw is pretty horrible (roughly amps x 3) on motors during startup. Could have a negative affect on sensitive equipment if Hz/voltage takes a momentary dive. Cheers.

[Tom Reed]

That Cent-a-meter is interesting. Our current system has most of the functions it has, except for the alarm. I would buy one if it had ability to show usage vs. time charts. To bad it doesn't.

[cgwymp]

That Cent-a-meter is interesting. Our current system has most of the functions it has, except for the alarm. I would buy one if it had ability to show usage vs. time charts. To bad it doesn't.

You mean this sort of thing?

[Tom Reed]

Ah so the TED system will do that. Looks like for around $240 it would be ready to go.

[cgwymp]

Ah so the TED system will do that. Looks like for around $240 it would be ready to go.

Yep -- no alarm, though, that I'm aware....

Here's a write-up from someone who's done a lot more with his TED than I've done with mine:
http://physics.ucsd.edu/do-the-math/2012/07/ted-stravaganza/

Sorry to the OP for hijacking the thread!