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How to choose the right size Diesel generator for backup power supply?

How to choose the right size Diesel generator for backup power supply?

(OP)
I'm looking to provide backup power supply. There's a lot of backup diesel generators out there in the market, ranging from 2 kVA to over 2,000 kVA.

How to pick up the right one? The electrical appliances in the cabin are: one refrigerator, one freezer, one A/C, one cooking machine, one bathroom boiler, a TV-set, and a few light bulbs (altogether a few kVA's maximum). I suppose there are some tips & tricks on how to select the right size/rating for the generator. Can experienced folks provide some guidance? I don't have any experience with generators so far and any help is very much appreciated.

Thank you in advance.

Dejan IVANOVIC
Process Engineer, MSChE

RE: How to choose the right size Diesel generator for backup power supply?

Add the KVA of
a> The cooking machine.
b> The bathroom boiler.
c> Anything else that take 500 Watts/KVA or more.
d> Three (3) times the KVA of the A/C.
That should do it.
I don't have A/C.
I use a 6 KW Light Tower for back-up for my home.
Electric stove, freezer, refrigerator, deep well pump, sewage pump, Microwave, toaster, forced air fans on two furnaces.
We don't use the electric clothes drier on the generator.
During times when we have grid power we also use the tower lights for night time skating in the winter.
We occasionally trip the main breaker.
By far the most fuel is used to keep the engine running during light loading or no loading.
I would rather reset the breaker occasionally than buy the fuel for a larger set.
The trips are always during times of cooking and appliance use.
The load is always lighter at night so we don't have to worry about the set tripping off at night and waking up to a cold house and a cold engine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: How to choose the right size Diesel generator for backup power supply?

Bear in mind that the scope of "needed" loads during a utility outage have been known to increase for various reasons. The first generator I owned was a 500W set that would run a TV, light or two and not much else- during a 4 day outage, my then girl friend was very impressed by same. After we married, a 3.5kW set was obtained, then 12kW, and eventually a 15kW diesel with automated start and switch. Now the 15 has been deemed inadequate, I'm looking for a 30-40kW set.

RE: How to choose the right size Diesel generator for backup power supply?

I'd look pretty hard at solar for that application before dealing with aging diesel fuel and stolen generators. Or, go with an inverter generator that leaves with me when the place is not occupied and that runs on gasoline that can come from a vehicle and be passed thru a vehicle if it starts to age-out.

Keith Cress
kcress - http://www.flaminsystems.com

RE: How to choose the right size Diesel generator for backup power supply?

Other things you forgot

The toaster
The microwave
The chainsaw/saw table/pumps
The kettle
The vacuum cleaner


My off grid house runs off a 3 kW (6 peak) inverter. I have only once kicked the inverter into overload, running a 700W water heater plus my electric chainsaw (1850 W) plus some other unknown loads.

The rule is we only run one 'big' appliance at once.

So if I were you I'd add 3 kW to whatever number you come up with.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: How to choose the right size Diesel generator for backup power supply?

Dejan,

If you think of this like a pipeline project which has a turndown of probably 5:1 you can realise that there is no "right" answer. Its either too small and won't do the max duty or its too big, costs too much and is inefficient.

So you need to come up with a requirement.

As you state it's a "back-up" and you don't have one, I can only assume you have mains power now, but it dies on occasion.

So for me the questions you need to ask yourself and get confirmed by SWMBO are:

How often does the power go off that actually seriously inconveniences you?
How long does the power go off for?
How easy is it to get fuel there?

What are your essentials ( for me it's lights, cooking, + fridge + freezer if outages last > 8 hrs.
Everything else is a luxury - you're in a mountain cabin FGS

Then look at the alternatives.

Battery power is getting better and better year by tear, is silent, charges back up when the mains comes back on or can use Solar and has built in auto switch over. So maybe look at one of those for the freezer if you leave it on when you're not there.

You will need to factor in at the very least a manual switchover from mains to generator supply or go with an auto switchover.

Then power - add up your essential loads and look carefully at how many you can arrange to be not on together. for your fridge / freezer, use an availability of 50% or less.

Your luxury loads are your A/C and that has the biggest impact as the start current on the motor could be 3 x the rating of the whole system, which the mains swallows with maybe a small volt dip, but which could trip your generator. ditto the unknown bathroom boiler.

~The temptation is to go for one with a "bit of spare", but then runs for 90% of the time at < 30% load. This will eat more diesel than if you're running the same unit at 60% and could also struggle if a large load suddenly got switched on. Everyone needs to think differently when running generator loads compared to mains.

good luck - a picture of the cabin would be nice....

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How to choose the right size Diesel generator for backup power supply?

I get by fine with 6 kW/KVA. If I added a decent sized A/C I would need 15 KVA or more.
I have installed a couple of 50 KVA sets in homes to handle A/C loads.
Years ago an extended family all bought 15 KVA backup sets. I was called on to install about 6 of the sets plus some larger sets in commercial buildings.
Within about 5 years I was called back to size and install larger replacement sets.
Some owners opted to keep the original sets and had a 15 KVA set as a backup for a 30 KVA or 40 KVA standby set.
In all cases the automatic re-start on the A/Cs had to be disabled.
The point is that the 15 KVA sets were more than large enough for all the loads UNTIL THE AC KICKED IN.
You should disable auto-restart on the AC. If you chose to live without AC during grid outages you will have a much smaller and more efficient set. Consider disabling electric water heaters during generator use.
The sizing rule that has worked well for me a couple of dozen times is:
Add all major loads in KVA.
Add in the largest motor at 300%. This is generally an AC unit.
There is not much diversity, as during morning meal preparation many appliances will be running simultaneously and any pumps may also cycle on.
An exception is lighting. I often found that the lighting load was small enough to be ignored if the set was sized for an AC.
An exception to the exception: A standby set for a movie theatre where the greatest part of the load was the lighting.
Most owners chose to run only part of their AC units on the standby set. They soon found out how many ACs they could run safely.
ACs and auto-restart; One installer did not disable auto-restart. The first power outage, the set started and the transfer switch closed. The starting load of several ACs was too much and pulled down both the frequency and the voltage. The contactor on the transfer switch dropped out and the set came back up to speed. The transfer switch closed. Repeat. It takes about a minute to destroy the transfer switch. The arc from opening the contactor under overload burns/melts the contacts until there is not enough left to make contact.
A number of sites have special conditions that influence generator sizing.
The phrase;
"You didn't tell me about that.", may be heard from time to time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: How to choose the right size Diesel generator for backup power supply?

If I was doing this for a cabin I would look at a smallish inverter generator and solar/battery for delivering peaks.
The issue used to controllers, especially since you have two inverters feeding the same system but I have seen a number of such packaged systems on the market.
Do you have propane or nat gas on the site? I would use that over liquid fuel.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: How to choose the right size Diesel generator for backup power supply?

Having a 2MVA backup generator for your cottage certainly would be impressive!

Otherwise, maybe invest in a power meter and measure the cottage for a while. Get a meter that will give a 15 minute demand value as well as a peak kVA value. Size the generator for a peak rating above the peak kVA metered and a continuous rating above the 15 minute demand metered.

RE: How to choose the right size Diesel generator for backup power supply?

(OP)
Thanks everyone! Some really cool tips received from all folks.

Best regards
Dejan

Dejan IVANOVIC
Process Engineer, MSChE

RE: How to choose the right size Diesel generator for backup power supply?

Come on Dejan, you know we like a bit more "feedback" than that....

going for the mighty machine or something simple??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: How to choose the right size Diesel generator for backup power supply?

(OP)
Hi LittleInch,

It is definitely going to be something small and handy. Based on the feedback in this thread, I think I will definitely exclude A/C from the generator sizing, and I think it is well worth to consider petrol engine inverter-generator instead of a conventional diesel generator (that's what appears more attractive, as advised by the experts above). Boiler, refrigerator, freezer, bulbs, TV set and a couple of laptops should be the peak consumption case because they run almost all the time (need to see how the stove fits in - if switching off the boiler is sufficient to let the stove kick in, or I have to have that extra power for the stove including the boiler). In simple words, keep the generator for the basic needs only.

The part with solar/battery also sounds very interesting but I think it increases investment significantly? Plus the cabin is in the shade area so not much direct sunlight. Interesting concept in any case.

We don't go there much often so heavy investments are not really justified. The issue is that it gets quite annoying when you get there on a hot summer day, and you have to drink warm beer. This indeed helps you get drunk more easily but then you have a cold shower (because the boiler is off) to get you sober and you can start the thermodynamic cycle again.

Big thanks to everyone!

Dejan IVANOVIC
Process Engineer, MSChE

RE: How to choose the right size Diesel generator for backup power supply?

For standby/backup at a "We don't go there much often" cabin, the small inverter is probably the best solution.
I had a very large house to supply with an badly undersized gen-set. The original set was 15 KVA, replaced in a few years with a 40 KVA.
I built a panel filled with contactors and whit switches on the front cover.
Each of the major loads, hot water, multiple ACs, electric stove, etc. ran trough a contactor. The grid energized all of the contactors.
When on standby power, power to the the contactors was through the switches. (And some relays) The home owner could select which loads he wanted to come on with the standby set. (If he picked too many he was soon in the dark, but it worked well until he upgraded to a larger set.
You may consider some switching arrangement between major loads to prevent them inadvertently being energized together when on standby.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: How to choose the right size Diesel generator for backup power supply?

New AC units use inverter for motor and start-up current is controlled/limited, so no need to oversize needed power.
And most of them have input PFC, with a very good PF.

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