calculating gen-size

[evilpsych]

Hey guys.. I'm new to the forum, but have lurked about the various off-grid home-power sites, and I've got a question..

How do you calculate what size generator you need? For instance, my current elec. co sends me a bill with the average kwh per day usage. Is it as simple as taking this number, dividing by 24, and sizing your gen to be slightly larger?

[mobile_bob]

in an ideal world the answer is yes,, but we don't live in an ideal world

for example #1
the electric company might tell you you use 24 kwatt hours of power per day, divided by 24 you end up with 1 kwatt per hour used,
so in an ideal world you need a 1 kwatt genset running 24/7 to do the job,,, but

example #2
what if one hour of the day you use 20 kwatts, and the other 23 hours you only consume 4 kwatts, now for that one hour
of peak demand you need a 20 kwatt genset, in a perfect world not losses figured

so the difference is huge, both examples use 24 kwatts per day, but you need a vastly differing generator capacity to fill the differing needs

so knowing how much you use per day is only one factor or many in sizing your generator needs.

if i might suggest a good place to start is to go back to you bill, find out what you use, and figure how to cut back and conserve by whatever means
are available to you, beit high eff appliances, compact florensent lighting etc. or a comprehensive load scheduling program to spread the peaks out as much as
is practical.

then you are going to have to determine your peak loads, when they occur etc, to be successful in sizing your genset needs.
then i would at least add 50% margin to have some buffer, and provide for mistakes in calculations etc

what do you have in mind, offgrid, standby, grid tied? all are also factors that need to be figured into the equation

good luck
bob g

[hotater]

Welcome aboard---

<mobile bob done said better what I say now>   

The first thing to do is inventory the usage and figure out how much you need at any one *time*.  

With grid electrical there's all the power in the grid at your disposal anytime you flip a switch.  With a generator the loads already on the generator has to be subtracted from the finite total you have available....otherwise a wreck.

There's an inexpensive and easy to use instrument, the  Kill-a-Watt, that's a GREAT help.  Check ebay, beware the high shipping.  

Normally motors that are starting up draw up to double the running load, but some, like well pumps, refrigeration, and other compressors can easily cause problems with very high wattage start-ups.  There are helpers available for hard starters.

Some modern appliances don't like *some* home-grown power.  Microwaves, desktop computers, battery chargers and UPS units are just a few.   Some modern appliances are just too risky to have around....curling irons/hair driers, toasters and toaster ovens, electric space heaters, crock pots and George Foreman grills (drats!), are a few of those.

SO-- If you use 25Kw a day but 20 KW of it is hotshowers, hair driers, toast and a coffee maker with a foot warming heater going, two TVs and a can opener and somebody is pre-heating a curling iron.... and it's all done in two hours inthe morning...and the well pump tries to start....you'll need a 30Kw genset (everything very approx.) to run your house.   But only for two hours a day...the rest of the day can be run on a little $99 special gas job.

You've ask a very complicated question without many clues as to how to answer it.  

Electric supply is impossible to say before you KNOW where and when you use what power you do, and how important it is to use it at that time.

[mobile_bob]

btw:

you want to listen to what hotater tells you, he lives with a genset,
he eats and sleeps genset
and probably knows or rather has forgotten more about gensets than most folks i know

bob g

[evilpsych]

good points everyone. Guess I need a kill-a-watt before I start sizing anything.

Right now, the only 'major' appliances I have constantly running are

2 14cuft Deep Freezers
1 17cuft refrig
4 tvs w/ associated cable boxes, and video players
2 computers and associated equipment
1 SW aquarium
home lighting (on maybe 3 hrs a day)

I guess one way to do it would be to kill-a-watt everything and come up with that total and size the gen that way,

The reason I came to this site is to research alternate energy sources mostly, the ups and downs of each, (came to here via otherpower.com ->utterpower.com (and then interest in listers) -> and then here.. in preparation for building a house. The lister seems to be a very good option (if rebuilt properly) for meeting power demands of homes that don't use electricity for their heating or cooling purposes (i'm really really convinced of annualized geosolar heating, in fact, in certain climates, I think you can do geoshaded cooling as well!!)

I want a grid-tied system with battery backup with a mix of solar, wind, and lister power sources.. (i gave up on micro-hydro as a source of energy - finding a suitable site for it seems nigh impossible or outright cost prohibitive.

[hotater]

Buy a propane fridge and freezer BEFORE building the house  (they cost about the same).   

If you can refrigerate on gas, do so.  To use electric power for the freezer and fridge means you have to have power on to leave home.  THAT is a PITA.

BTW-  Mobile Bob is full of crap...I dont' even know how a generator WORKS, but I live by several, so have learned a few practical lessons along the way.

As an example--- I've got to crank the diesel pick-up to go to 'town' to get mail and use the phone this afternoon.  It hasn't been cranked in a week and the temperature has been between zero and freezing (F) day and night, so I *have* to preheat it before it'll start.  To do that I had to shut off the circuit running the chest freezer because the freezer can't re-start with the extra 1500 watts on the system, and if  and when it tries the computer circuit is affected.
  I  *need* to run the air compressor to pump up a tire, but to run the big one takes the 15Kw genset which would have to also be pre-heated and I don't have that much power available, so after the truck is cranked I can use that 110 circuit to run the small compressor and get going.

  It's fun living off grid, but it ain't easy!!

[Ray C]

Dear Evil,  (just had to open the reply this way  )


The items you're trying to power seem like good candidates for home-rolled electricity.  The only exception might be your video equipment but, who knows, it might be just fine.

As others have said, the applicances you absolutely must avoid are electric stoves/ranges, hot water heaters, clothes dryers, dish washers and space heaters.  Next comes coffee pots and big (> 1000watt) microwave ovens.

Here's a WAG at the power consumption for the items you mentioned.

2 Deep Freezers:  Intermittent load:  About 400 W total
1 refrigerator:  Intermittent load:  About 200 W
4 tvs and cable boxes:  Intermittent load:  About 500 W total
4 video players:  Intermettent load:  About 300 W total
2 computers, monitors, misc periph:  Continuous load:  About 400 W total
1 salt water aquarium:  Continuous load:  About  200 W.
Home lighting:  Intermittent Load:   About 600 W (lot of variability here).

Total:  2600 Watts.

For these items, the demand load will probably be less than this as, the refrigerators and freezers cycle on/off.  

There are other things you need to consider such as your furnace and washing machine etc.  My guess, is that you're a good candidate for solar/battery/generator power for many (but not all) of your electrical needs.  Generally speaking, you have solar panels that charge batteries.  The batteries drive an inverter which supplies electricity to a few of the most common circuits in your house.  The engine/generator is used to charge the batteries on cloudy days.

Keep in-mind, the energy from a diesel generator will almost certainly cost more per kwh than what the power company charges for it.  There's also a fair amount of maintenance involved in generating your own power.  Plan on spending an average of 20 minutes a day for the care/feeding of your diesel genset...

I'm not trying to discourage you but, be forewarned, this is a labor of love, more than anything else.


Regards

Ray

[mobile_bob]

Jack and others have mentioned alot of useful info
Jack mentioned that induction motors draw more than their running or nameplate wattage on startup
refrigerators in my experience can draw up to 3-5 times the running current to start them, so
even if you shed alot of stuff, you will need to figure in these surge currents, and also figure in the possibility that
they may come on at the same time!
(i might also add if the fridge tries to start and can't it will build head pressure and be much harder to start until it has set for a few minutes, but without control
it will continue to try and start until the thermal switch open's. this is a safety switch, not something that was intended to be cycled over and over. damage to the
fridge compressor motor is very likely)

unless...

you impliment some form of priority or scheduling system that takes control of the starting of multiple units.
some things you can control easily yourself, others not so easy.

you can control when you run the washer or dryer, and lights , tv etc...
but it is more difficult to control when the pump starts, when the fridge tstat says start, and when the furnace blower is told to come on.
all these things can be controlled and prioritized by some level of automation, the better the control the more complex and expensive it can be.

so as Jack and others have said, do away with as many electic hogs as you can, this will leave you with fewer hard to control appliances.
the more control you have, the smaller the needed generator.

it is conceivable that in a modern all electric home, without regard to power production you might have to have 200 amps of generator capacity to cover
all possible outcomes, and that is insane in my opinion

i have a book that gives a fantastic overview of total system control, from scheduling, shedding, sharing and all manner of system design.
the moral of the book is ,,, "go at it without a plan and you may need a $10k genset, have a very good plan (and as Jack stated) you might find a
100 dollar setup can get the job done" (the reality is probably not a hundred bucks but you get the idea)

i gotta find that book and post the name of it
it should be in everybodies library that is working on homepower

bob g

[rmchambers]

Some of the inverter sets out there put out very clean power.  I currently use a TRACE DR3824 inverter and 4 big 6 volt trojan batteries.  I use this setup to act as a BIG UPS for my computer and related equipment (routers, VPN boxes, inkjet printers, 2 IP phones, external drives, wireless accesspoints - you get the idea)

The DR series inverter puts out modified sine wave power which isn't as "smooth" as the SW series inverters but is a dang sight cheaper.  My computer equipment runs perfectly on the DR device though.

Moving forward. 

Second the Kill-a-watt meter, it's very handy, easy to use, etc.  I made up a handwritten spreadsheet and went to all the devices I could monitor (which meant everything that had a plug and wasn't 220V like the dryer) and I run the K-A-W for a few days, write down the KWHours used, and divide it by the time it ran for over the course of a few days and come up with a KWH/hour value.  Multiply that by 24 and then by 30 and that's my WAG at KWH per month for that device.  You may be quite surprised.  My computer equipment uses quite a bit of power but my washing machine which I do 4 loads a week in - draws more power for the time it is in use, but is in use very little. 

My bugaboo is the dryer and the stove.  We don't have gas in the street and I don't much relish the thought of a huge tank outside.  I haven't measured how much juice the dryer uses but I know it's a lot.  I'm trying to find a decent priced clamp on ammeter to see how many amps it draws while running, and with that and the voltage and the time I can work out how many KWH that beast uses.

I don't plan to run "off grid" like the venerable Hotater (we are not worthy, we are not worthy) but what I want to do is supplement what I use with some solar AND have some backup power available to me for the next time the power company has lines down after a storm.  I like the idea of having the grid available as a fairly reliable power source but with a diesel genset I can always burn waste motor oil (WMO) or waste veggie oil (WVO) if I get the inclination which would help offset my grid usage.

As of the first of the year (yesterday) we in Connecticut are now the second highest priced electricity victims.  Second only to Hawaii which sorta makes sense.  Politicians got my state into the mess it is in and relying on them to get us out of it is stupid.

If you take anything away from the conversation let it be the word "CONSERVE" - in preparation for our Jan 1 electricity raping my wife and I have embarked on a conscious effort to conserve whenever and wherever we can.  I can count the number of incandescent bulbs in this house on one hand and once I've found suitable CFL bulbs for those uses (dimmable and bright enough) they are going as well.  We are having a very mild winter so far so that skews the electric bill values to the cheaper side but I'm hoping to notice a trend of less and less power usage as we continue on our quest for conservation.

When I started out I averaged about 30KWH/day.  I'm hoping to reduce that by at least 30% and more if I can figure out how to squeeeeeze it.

Sorry for the lengthy diatribe, discard what you don't need and welcome to the forum, it's a good group here.

Robert

[tim]

Conserve, Conserve. Like Robert, Jack and Bob say. I run at night on about 250 watts an hour off of 500 amp hours batteries that including heat with DR 2424.

tim