Auditing Power Usage

Thinking about it:

1. Peak demand kVA - generator is oversized most of the time
2. Average demand kVA - generator is under-sized during peak times
3. Somewhere in the middle

Hmmmmm..............

Hi Johan

I used an amp meter set to min/max measuring on my house. I ran the measuring for 3 days and checked the highest peak during normal use. That is using what I normally would use, geyser, stove, aircon etc. In amps. Then you just calculate the amps into watts and you will have the highest peak in wattage that your property will pull from the supplier. You then just work out what wattage you need accordingly if you have to scale down during a power outage. Not using the aircon, swimming pool pump etc.

I suppose it would work exactly the same for a small business. First you can check the main breaker from the supplier. See what amps are rated and already you would know that you couldn't pull more from the supplier than what the breaker is. So your building generator need not be more than what is currently available.

60A Breaker = 230Vx60A=13800W or 13.8Kw +/- Max

Hope this helps a bit

"First you can check the main breaker from the supplier. See what amps are rated and already you would know that you couldn't pull more from the supplier than what the breaker is. So your building generator need not be more than what is currently available."

Be careful with that assumption. Although, for most residential you can probably get away with this method, it could get you into some trouble. Remember that most breakers will not trip intantaniously at their written rating. So, at times, it is possible to draw considerably more than your main is rated at. You may also have to oversize your generator, for startup/inrush currents as well as poor power factor loads. Hence why generators have a surge rating and are usually rated in KVA (include your reactive power as well) instead of simple KW.

Caveat Emptor

Hi Ian

Yes, thx, agreed. Left myself wide open for comment. It comes into play much more for a company/business generator though.

Agreed, it would, for the most part, only be a problem in industrial/commercial. In fact, in a residential application, using the main breaker as a guide, you could end up oversizing your generator by a much bigger margin than you may want. For example, using the main breaker at my house (150A 240V), I would spec at least a 36Kw generator. In reality, I doubt that I would ever load it more than 50% and even that would be an extreme. If it weren't for the noise, I'd bet I could secretly connect my house to a 15-18 KW generator and my family would never even notice a difference.

Generator selection would depend on how much load management control can be exercised by th ebuilding occupants. For a single business or tenant with limited budgets they might elect to only power essential services such as computer center, corridor lighting and of course any essential fucntion rooms plus HVAC.

With larger multi ocupant buildings this may not be feasible. In which case you end up planning for worst case siuation. It also means you are going to be specifying a grossly over sized generator and thus very expensive installation compared to what you could specify if load management is an option.

When I worked for a power utility our office building was only partially powered by a stand-by generator of around 100kW capacity. At full load our building could consume 300 kW.

Operations center, phone exchange, computer center, and of course the main repair shop was powered. Non essential offices were not provided with power. Emergecny lighting was provided throughout the building coridors and internal spaces like washrooms that were essential to continued round the clock operation during a major outage involving the whole region.

You can look at your power bill for usage/day and then math it out from there. Do not forget if you have any motors , A/C or compressors, they take a bit more to start than to run. Best bet would be to inventory critical systems and what you need to run during a power outage. Then add and take away equipment till all your demands are met. You could even break it down to 1 generator for office type management then another for production needs. That will reduce the over all load demands and keep the cleanest power for computers and phone systems. I hope this helps in getting started, after you get some #'s "total watt usage ,office /production, then we can all run the solutions for you.

use a power qualty meter with data loging capasity ,record than you will see your peak and average demand, or simply ask your local power supplyer for a enery audit , most power companies will give you a good discount on rates and in some cases install the back up power , mike kieffer w.fargo nd

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The comments I've read so far cover this subject well. The only thing I would add (in this case just more detail than what was already mentioned) is that the utility company (at least the one in my area) may be able to provide you with a 12 month historical data report for your facility's power consumption.

Also, there is instrumentation available on a rental basis that you can use to monitor power consumption over, say, a 30 day period to look for the moments of peak power that a historical power record may not show. The link

http://www.telogyllc.com/display_model.aspx?bm=FLU(1750)

shows one such instrument.

Using these two readings, the average and peak power needs, and how much of your facility you wish to protect, you should be able to estimate the size of the generator that you need.

You know, something else to think about, try calling one or more of the large generator manufactures (Onan, Generac, Cummins, etc) and ask for their assistance. These folks are very knowledgable about sizing generators to different sites and applications. After all, it is their business. Helping you choose the correct size generator makes you a happy, and hopefully repeat, customer. So, it is in their best interests to help you in this process.

Just a thought.

IanR wrote: try calling one or more of the large generator manufactures (Onan, Generac, Cummins, etc) and ask for their assistance. These folks are very knowledgable about sizing generators to different sites and applications"

REPLY: But don't forget these people will be very happy to sell you the biggest generator they can justify based on the numbers you provide. And they may not tell you how to load manage to use a much smaller genset.

Further to my previous note on needing load management on site. Bear in mind that a back up genset is not like a UPS. Genset start is triggered by a loss of utlity power. There is an unavoidable delay of perhaps 30 seconds while the generator motor starts up, reaches a stable speed with voltage and frequency within normal parameters. At that point every connected load will simultaneously restart. Unless of course the loads are equipped with controls that sequence the restart to keep them shut off until manually restarted or come back on stream at timed intervals.

A client like a major food store with many big freezers and coolers will have all those compressors start as soon as the power from the genset comes on stream. MAJOR SURGE!!

I wouid also argue that a perusal of the power utility billing is not going to give you a complete picture of power demand. Power billing is typically going to give you average readings only.

The only point seemed to have been glossed over is electric motors, without soft start they can take up to 5 times running current when starting, together with odd [power factor] loads this can have a huge effect on gen.set selection.

Motors & unusual loads are the starting point so to minimise gen.set size, these may have to be started first on dedicated circuits, this means splitting circuits into critical and secondary etc.

There is a big difference in load profiles between domestic and (even small) commercial sites.

Gen.sets have become cheaper and many suppliers end up just greatly oversizing the gen.set to get over starting issues, this means the gen.set spends most of its time on a light load, this is not good for a diesel engine.

List out critical & unusual loads, see if they can be left out in a power failure. Power consumption is of interest but not necessarily a direct way to size your gen.set.

Many people have been caught out and found they have insufficient power left to start a critical load whilst supplying the simple basics.