Sizing a genny

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kaspaas

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Hi,

how does one size a generator for a small office?

5 workstations (desktops + flat screens)
Server
PABX with 7 extensions

A few lights (Low energy)

All the PC's are on UPS - so I guess one has to take into account that these will grab some power as well to recharge as soon as the generator starts to run.
 
You're looking at around 2kva for that setup but you want a bit of headroom so 3kva generator at least. Agreed with rvanwyk that diesel as preferable as it's cheaper to run and designed to run for longer periods. Unfortunately the smaller diesels start at around 5/6 kva, but it does give you some headroom should your office grow.
 
i've seen some nice diesels at makro for between R5k - R6k

EDIT: actually im not sure if they are diesel.

http://www.makro.co.za/ProductDetail.aspx?cat=-1&prod=34729&SKU=000000000000093349|EA

http://www.makro.co.za/ProductDetail.aspx?cat=-1&prod=27324&SKU=000000000000079868|EA
 
Last edited:
Pretty cool question. How big would you guys recommend to run a big house?
 
Apparently things like fridges spike high when the compressor kicks in but after that draw relatively little power?
 
well, it would be mostly a 2 fridges & 2 freezers & an aircon or 2 in the wine cellar. And the Home auto box and a few lights, & the alarm. Oh yeah, & probably the garage door.

what do you reckon?
 
dotVIBE said:
well, it would be mostly a 2 fridges & 2 freezers & an aircon or 2 in the wine cellar. And the Home auto box and a few lights, & the alarm. Oh yeah, & probably the garage door.

what do you reckon?
Click to expand...

Bwana is right about the fridges and freezers but the generator needs to be able to handle those spikes when the compressor kicks in. Air conditioners are usually huge power hogs. Best thing to do is check the power ratings on all those appliances and add them up, add about 20% - 30% headroom and that should give you a ballpark.
 
Perdition said:
Bwana is right about the fridges and freezers but the generator needs to be able to handle those spikes when the compressor kicks in. Air conditioners are usually huge power hogs. Best thing to do is check the power ratings on all those appliances and add them up, add about 20% - 30% headroom and that should give you a ballpark.
Click to expand...

Uhm, yeah. But I actually need to buy the genny first, as it is for installation in a new house.
 
3x-a-d3-u5 said:
Why do you want a genny? get a UPS, WAY quicker and quieter!

You can contact these guys: http://www.powerbackups.co.za/contact_us.php, they have bigger systems but don't advertise them. I have a UPS from them way cool got a little lcd display.
Click to expand...

Got UPS's on each and every PC.

But to provide a UPS for a 3 hour rolling blackout requires a massive UPS.

A genny is more practical.
 
To calculate the kva / kw that a gennie needs to be able to supply, by calculating the power needs of a home, or only certain items / appliances in a home is tricky ...

...because you will have both resistive loads, and inductive loads, in your home.

As example, a resistive load, such as an incandescent light bulb, will draw a constant wattage from the gennie, e.g. a 100W bulb will draw 100W always - both on initial switch on, and while remaining on.

An inductive load has varying power demands .... i.e. anything with an electric motor, or compressor (e.g. swimming pool pump, or fridge/freezer, aircon, washing machine, etc).

Inductive loads draw more watts on start-up, and then reduce power consumption once the motor / compressor is up to speed.

The problem is that few fridge / freezer manufacturers, or pool pump manufacturers, give detailed technical info about start-up power demand, and normal run power demand.

It's good sense to build in a pretty hefty safety margin in terms of what KW you think you will need, and what the chosen gennie can constantly supply. So if you think you need say 5 KW, then buy at least a 7 KW or bigger gennie.
 
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