Generator Capacity and Selection

The generator has to be capable of supporting the peak load. It is not possible to size it without looking at the load pattern at the premises, which cannot be seen from here.

Unless you are planning to only run certain items on that list for certain time periods you should add up the wattage of each item and the total would be the amount of current the generator would need to be able to handle.

Under rating the generator is a common mistake.

Calculate the wattage of each appliance that you wish to run.

Those appliances that have an electric motor in them such as Air conditioners, refrigerator, pumps etc; you should multiply their wattage by 3 to ensure sufficient startup power.

Once you have totaled (including startup) all the requiered wattage that will be drawn from the generator, then add a 25% safety margin and the answer is the size you need. Remember you cannot expect any engine to run at 100% load continuously.

You would need to derate the result further for alltitde compensation, approx 1% extra wattage req for every 100 meters above sea-level. In hot climates you will need to derate further.

Remember that you must evenly split the load (balance) between the 3 phases, and you will need a qualified Electrician to do that for you

Dear Basavaraj,

I suggest you do not use a 3ph generator set as at these low powers you will find that you will have problems balancing the load between the phases. Better stay with single phase.

What do you intend to draw from your sockets? This could change verything.

You haven't allowed for washing machine, dishwasher, clothes iron etc.

If you are running air-conditioning from this then you should look for a 1500rpm generator for certain and really you will need a back-up unit. Running a generator for 24hrs/day, which is often the case where air-conditioning is concerned, means you will need to have the unit serviced every 10days! (Typical service intervals of 250hours run).

I suggest you take a look at several threads posted here and also other forms on "off grid power supply" and solar power. Contact specialist solar contractors in your area for better advice applicable to your situation.

My advice on the power is to be careful not to put too big a generator in because it has serious consequences on the life of the generator set. Running a generator at below 60% is abusive of the engine and results in glazed bores, coked exhausts and a smokey exhaust. Running under loaded is the most common cause of failure on most yacht generators.

Using the calc method suggested by guest (post#3) will result in massive over specification. The air-conditioners will not all start at the same time and, to avoid even the possibility it is quite customary to fit "de-synchronisers" which stop any two units starting within some seconds of each other-

Regards

Chas

Chas has some very good points. The poster who said generators must be sized to peak loads but failed to mention that several peak load reduction methods are now available. One of them being to drive big loads like air conditioning with 3 phase power but then use inverters to handle the rest of loads that are single phase with inverters.

Chas is also correct about his comment regarding service intervals of approximately every 10 days. One solution is to have a second generator but it may be cheaper to use an inverter / battery system.

My uncle in law runs a whole off grid resort from one generator and has been doing this for 25 years. He does the service during the off night shut down period. Even in tropical climates it is usually possible to schedule down time in the early morning before daily power generation is resumed. Piping and battery powered pumps to facilitate oil exchange is a valuable time saver and does not cost much.

Inverters can absorb the starting surge of 2X and even 3X much better than a genset can.

Do you not have local solar system experts who can advise you on system design?

This job looks pretty straight forward. Not very unusual just a question of sizing everything to match the demands of the system.

Dear basavraj,

To add to what chas said, you should also look for the LPG fired generator which invariably comes with the electronic governor (ussually not available in diesel stas) this way you will be able to cut costs when on low load and also avoid emmission of toxic gases which will be intolerable by your neibours.

pls get in touch on mallu_solar@yahoo.co.uk, i can put you on the suppliers depending on the area you are in.