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Generator sizing for electrical demand

I am going to live and work in a very remote area in Sudan. The electricity in that place is going to be provided for about 4-5 hours a day as an average. I have to run a clinic (12 rooms) which is less than 2 km away from my office/guest house (12 rooms). The electronic equipment I have to use are:

Office Guest House: 1 fridge, 2 air coolers, 30 neon lamp, 2 computers, 12 fans, 1 printer, 2 power stabiliser, 1 2kW satellite receiver, 2 Radio sets (Motorola), 1 speaker for laptop, 4 tungsten lamp, and 3 mobile chargers.

Clinic: 30 neon lamps, 1 laptop, and 1 fridge (option).

The place is going to be very hot throughout the summer and spring period with an average temperature of between 40-50 degrees.

Could you advisee me on what is the best generator and the most cost effective one? The options I am looking at are: 2 separate generators - 1 for the clinic and one for the office; or 1 generator for both of them

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It's hard to properly size the generators without knowing the power consumption (W or kW) of each appliance, especially, the fridges, 'air coolers' and 'neons'. I'm particularly unsure about the coolers - if these are evaporative devices they might draw 200W or so to drive a fan, but if they are air conditioners with compressors they could draw as much as 5kW each (depending on size) , with start-up surges of 2 or 3 times that. The cooling loads will be by far the greatest demand on the systems so it is worth getting more information about them. 9 & 6 kVA sound reasonable, though the clinic would probably run on less, but you don't get diesel gensets much below 5kW and petrol ones don't last anywhere near as long. If you permanently run the genset at very low load, it won't do it any good but you need extra capacity to allow for fridge compressors starting. Sorry this is so vague - I can give more accurate ideas with more details of the loads.

If the clinic is 2km away from the office, it will not be practical to run both from one generator - cable costs and power losses would be far too high - so you will need two systems.

As you say, the running time could be a worry. As well as general wear and tear, they will use a lot of fuel even if they have little load to meet. I would strongly advise you to consider a generator - inverter - battery system. This way, the generator runs when you have a large load - air conditioners, for example - and also charges a battery bank. When it cools down and you only need lights and a computer, radio, etc you can switch off the generator and run on the inverter, which converts the power stored in the batteries back to 230V ac. This system would cost quite a bit more to buy than a generator on its own, but would be much cheaper, and more reliable, to operate.

In addition, you could also add some photovoltaic (solar) panels that will charge the batteries even if the generator is not running. If you can manage without air conditioning, these could meet most of your loads. If you are buying the equipment, you should get the highest efficiency fridges you can find, and look into evaporative coolers rather than air conditioners. If you are not putting a fridge in the clinic and you only need lights and a laptop, a small solar system will be a far better option than a generator. Even with a fridge, a solar only system is very feasible.

The company I work for (Dulas Ltd, www.dulas.org.uk) has been supplying solar-powered medical refrigeration systems to Africa for about 20 years and is the leading supplier to UNICEF for their vaccine cold chain. I have helped install solar only systems for clinics with fridges up to complete district hospitals in Eritrea and these continue ... (more)

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You are right to be concerned over the long and frequent duty cycles. Generally speaking I would try to stay within the 66% to 75% capacity of the generator especially for a long duty cycle in a hot place, and go for diesels unless diesel fuel is really not available.

I would go for something factory built - may sound obvious but you can still go out thinking to buy something you recognise as a generator and eventually be confronted with units cobbled together locally from random gensets coupled up to old Lister hand start engines.

I can’t work out the size properly from what you have written, there are a few too many variables in power consumption but everything that is to be attached will have info on it somewhere - estimate about 20% of these numbers for anything with a motor or transformer in it (pumps, fridges AC units etc) and then as above rate the generator to run at about 66 - 75%.

Getting two gensets the same can be useful, and may make service parts and familiarity easier, I would try to get something common to the region and as good quality as your budget can accommodate.

One thing: Once you set up long term generators people will come out of the woodwork with reasons to plug in, so either be strict or make sure you can accommodate the increased load when you feel generous to requests (and keep control of the system, no leads running off across the bush, there is no telling what people would consider reasonable use).

Lastly, gensets can be noisy (and usually are). You need to secure them and block the noise but not confine them or you will reduce air circulation and cause yourself problems. I am sure the RedR handbook will have a few pages on how to muffle a generator without burning down the house…

Paul Kirwan

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The size you quoted appears a little small at first sight. As a rule of thumb, (not strictly technically correct, but good enough for this scenario) you can think of KW (kilo watts) as equal to KVA (Kilo Volt Amps). Adding up all the ratings of the different equipment (The ratings are usually marked on a plate or printed directly on each piece of equipment), this will give a slightly high value. You can then size the generator. KVA= 1xstatic load +2xdynamic load. The largest site load gives the minimum size to be bought.

  • Dynamic loads are: fridge, fan, air conditioner (air cooler?)
  • Static loads are: computer, lights, radio, and charger

I would buy generators of the same size and make for both sites plus a spares pack. A third generator, of the same size would be ideal for complete spare coverage. The vendor/manufacturer should be able to advise you of the contents of the spares pack. This has the advantage that you only need one spares pack and staff training in operation and maintenance can be minimized.

You can choose from manual start, or automatic start-up. They both have advantages and disadvantages.

Manual Start.

Advantages: Cheaper to buy, often simpler, therefore easy to maintain. You can control the load. I.e. chose which devices will be turned off during the generator run times

Disadvantages: Delay in start up can sometimes be catastrophic to some medical processes. Will the same person have to do both sites? There will be periods of no electricity (Power Cut) and there may be periods when the generator is running despite the main electricity supply having been restored. Choose a generator that at least will cut out on the restoration of power.

Automatic start

Advantages: Can provide continuous electrical supply without manual intervention. Minimizes power cuts to users. May be more controllable, useful in the scenario that the clinic does not need 24 hour supply for all devices

Disadvantages: Need to track the fuel usage as it is easy to loose track of the number of hours/fuel used by generator. Control system may be more complex and less reliable. Always buy a generator that can be run in manual mode and choose the model with the simplest control system, preferably, electro-mechanical rather than computer based.

Automatic start generators need to be sized to take full load immediately. Seek advice on the start-up load/current with the manufacturer.

It is possible to reduce the start up load on manual type generators, by going around and turning equipment off until the generator is up to speed, but this is time consuming and prone to errors.

Some general points.

The electricity supply from generators is more stable when the generator is working well within capacity. Larger capacity generators do not use significantly more fuel than a smaller counterpart, so do not be tempted to buy the smallest generator. It is more economical to run say a 25KVA generator at 50% capacity than a 9KVA generator at ... (more)

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Hello,

The best generator is one that doesn’t run. And you tend to do better with 2 ‘small’ ones and manage your load (requires discipline) so you always have one ‘spare’.

I think your first step is to go through the list and see where you can cut back (do you need 30 lamps?) . I’ve worked in Sudan myself so I know it can get very hot, but do you really need air coolers (= nice to have but critical for the operation? – get up early, sleep during the hottest part of the day & get lots of shading/ put grass on your roof?). There are several billion people surviving and working without airco or only a fan. There are several evaporation techniques that allow you to have cold drinking water/ food storage.

If you live remote: the logistics of transporting fuel is in general no easy task.

For the clinic: go solar/ kerosene. Running fridges of generators is asking for troubles. You’re much better off on a solar/kerosene one. Burning fuel to generate electricity to generate heat isn’t very efficient. Go to your ‘local’ WHO office (or other medical NGO) to get maybe one or 2 kerosene fridges ‘for free’.

As for the lights in the clinic: solar lights will do perfectly. You either have mobile ones that you take back to the compound if they are not needed or you have batteries/lights at the clinic and take the solar panel back and forward (avoid theft). You then connect the solar panel(s) on a stand on the floor (don’t forget to lock them to the frame!) and in the evening, you take it back to the compound (without the frame).

Provide a few head torches with re-chargeable batteries (solar charger) for emergencies and wind-up torches for the guards. You can always complement with some kerosene lights.

Small generators also have the advantage that they are mobile: if your batteries at the clinic need a topup if there isn’t enough sun: you can take a generator to charge them. Office gest house: go to a 12V system. Often it is cheaper and a better long term option than a generator. If your battery runs low if solar is not sufficient, you can top-up with a smaller generator. That is also the time you can switch on your satellite receiver (are you sure about the 2kW – seems very powerful & most satellite systems work 12V/24V). The Radio set can often be ‘adapted’ to 12V and you can easily find travel adapters for most mobiles.

The 12 V systems avoids also multiple inverters (you probably will always need one). Your 2 computers are ideally replaced by laptops: they have a built in UPS (=the battery) and often you can find 12V travel adapters for those too.

But despite the sales pitch for solar: keep a few 220V lights in case of an emergency/ when you get it wrong.

When working with generators, you may want ... (more)

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Asked:
2013-12-09 10:22:43 -0500
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Last updated:
Dec 16 '13