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Posted By Topic: Genset maximun load

Nov 24 2018 18:50

When calculating the potential maximum demand/load for a residential property we are allowed a certain diversity factor with different item/appliances.

I am presently working on a residential property that will be using a Genset only for electrical supply. We are talking of a load somewhere in the region of 25kw per phase (naturally 3 phase). think I should forget using the diversity factor in my calculation of the load because the Genset will need to be able to provide the maximum demanded of it.

Am I correct, or any other ideas from others would be appreciated? The heavy load is because of 35kw of floor heating to be installed.


Nov 24 2018 19:56

Ouch, resistive heating on a generator? Have they looked at a diesel-fired boiler or something?

Fixed space heating is supposed to be downrated at 75%, but you can basically do whatever you want as far as the standards are concerned.

Nov 24 2018 20:11

The genset maximum load depends on the alternator type used on the genset.

Some gensets have the ability to supply an additional 10% load provided that some detailed comditions are met.

Other gensets will have NO overlaod permitted the maximum rating is the maximum rating.

The voltage dip when a motor load starts also limits the max outpuit available.

And lastly, the engine mechnical output.

In general the best way to find the maximum loading is to use common sense to judge what loads would be operating at the same time, list all these loads also add a margin for any load which has a motor starting when switched on.

BTW VSD required an extra margin and can NOT exceed about 50% of the total loading.

Also for long engine life the continuous loading needs to be about 70% of full loading rating continuously to prevent distroying the engine.

Site unseen you have a major problerm with 35 KW of underfloor heating.

The best solution is likely to be have load control on the underfloor heating at peak loading times, as this is avery high % of the loading. Remembering that you need the egine to be at approx 70% of ther max loading on the genset at all times for long engine life.

I assume that you are not proposing to run the engine 24 hours / day, a battery and inverter for the light night loading is a desirable addition to the system.

Nov 24 2018 21:03

If this for a retirement home, or similar, the inslab heating would be assumed to be a non-essential load.

Nov 24 2018 22:14

Overthinking things a bit. The generator max loading will be in the documentation that is supplied with the unit.

Generators have a standby rating and a prime rating. The prime rating is generally lower than the standby rating.

As others have said you should try to have about 70% load on the generator most of the time.

Nov 24 2018 23:52

From 50 paces it looks like that underfloor load could double your generator suze needs and thus have a large impact on costs.

Given that underfloor heating has a large thermal inertia, some smart load control could significantly reduce the generation requirement.

Nov 25 2018 03:09

It's a jaw dropper for sure!


Nov 25 2018 09:50

Trying to be a bit more helpful, and please excuse any attempt to teach my grandma to suck eggs.

If the underfloor is hydronic rather than direct electrical then you have more options, the first being LPG powered "condensing" "boiler(s)" which have efficiencies in the high 90s, which will reduce running costs and be one less problem for electrical.

The other is CHP generation, which is sized for the heat output, and the waste product is a useful amount of electricity.

Caveat: most of the generation I've actually had first hand experience in is either data centre and building backup power, or in the rock and roll space. Most of what I know about off-grid buildings came from being a Home Power magazine subscriber for many years.

Generators are available in a number of classes, such as standby, and the one that you need, which is "continuous operation". These sets cost more than prime or standby. A standby only rated unit will fall apart quite quickly under this load.

The other important thing about diesel generators is loading, they don't like being underloaded most of the time because this leads to "wet stacking". So one big 75KVA genny will get bunged up very quickly.

And generators need to be maintained, so there has to be planned downtime. And they sometimes refuse to start or fail, so unplanned downtime.

For off-grid installations where all power is produced locally, MD by the book isn't actually that useful a number, what is critical is the power budget, and thus it is necessary to have a detailed look at the possible loads, and figure out where the combinations are.

I'm going to assume this is a high budget build. A big unknown is occupancy, is this a 24x365 premise, or is it a holiday home unoccupied a lot of the time. That can change things.

If I were to design this, I'd split the loads into four; "normal", so lights and sockets, "awkward", eg oven and cooker, "underfloor heating", and "continuous", stuff like sensor lighting, CCTV, alarms, fridges and freezers, stuff needed during unoccupation, even if the house isn't going to be unoccupied.

I'd recommend you think about two generators, a smallish one, 10, 15KVA maybe, and a bigger one, maybe 30KVA. This to prevent underloading and thus wet stacking, and to provide a degree of resiliency. Obviously they'll need to start on command and be able to sync either way.

As its assumed to be a big budget, the "normal" loads I'd run off an industrial scale UPS, 30-50KVA, as this stops annoying stuff like lights flickering, and gives the illusion of always on power.

The awkward stuff requires the big generator to be online, so a button by the cooker and a green confidence light. You may need to consult the manufacturer to get split feed so the timer stays lit up when the big genset is off.

The heating is just a subset of the awkward loads, but the total load is either sectioned into many individually controlled chunks, or has a phase shift controller to adjust the load.

The continuous stuff is either the same as the normal loads, or can be run off an off-grid solar system with battery storage, so that when the place is unoccupied the big gennies don't need to run, just the continuous loads are powered, everything else is off, unless it freezes, whereupon the heating is brought online to prevent frost damage. Even if the current owner will always be there, a little bit of electrician separation and planning upfront with a tiny cost impact will be appreciated by a future owner.

Obviously, this all needs control, so a PLC with current sensors, and the UPS with integration, probably using MODBUS. And contactors, many contactors.

This is an interesting project.

Nov 25 2018 10:09

Thank you, guys. Many ideas and other relevant knowledge from you that I hadn’t even thought about.

This house is a holiday home, which at the most will be used 6 times a year, for a few days at time.

I know the floor heating is the main problem. The customer wants it all on during the day, and at night only in the two bedrooms. The floor heat is underfloor tiling, not in the slab. They have already had the floor heat reduced because of concerns from the 55kw that floor heating installers wanted to fit. Naturally, it will be thermostat controlled. Their attitude is, they just don’t want to be bothered with the thinking required of having to either adjust the thermostats or timers (if set). They just want to walk in and start the Genset and let the place work basically automatically.

The demand/load for rest of the house is about the same as what I call a normal home.
There is one other detail that I didn’t mention yesterday, and that is they are to have 12kw of Solar installed. This is to be installed by others. This will be used mainly at night. Therefore, the Genset will probably be on for 12 to 14 hours a day.

Presently I haven’t even thought much about the Genset or type. Unless others suggest otherwise I thought I might just contact one of the reputable suppliers and look at their advice and recommendations. Can someone who has such knowledge suggest a good Genset supplier? Also, I now realise my suggestion of a 25KVA Genset is far to low.


Nov 25 2018 10:34

Solar to be used at night... ok, I guess that means battery storage.

This project is going to require considerably more thought & coordination in my opinion. And a healthy budget.

Just off the top of my head, I would be looking at having the solar run into dbuckley's industrial scale UPS which will handle the variable load (all of the load), then use a smaller generator to charge the UPS system - having that generator run at a more constant charging load might be able to get around the underloaded diesel issues.

It may also be advantageous to have some load shedding (the heating load probably) when other higher priority loads come on line.

Nov 25 2018 12:04

Dbuckley. For whatever reason your post was not shown when I posted my last post, other wise I would have then noted your comments. You have added other elements that I now need also to seriously consider. By now you would have seen my post about the solar (and with Tesla batteries), and the customer wants and wishes. The Tesla batteries I am told are the best. Is this true?

Because this is possibly getting a lot bigger than I imagined I am thinking maybe I should tell the customer that we need to hire an Electrical Engineer, who has a lot experience in this field?

As others have thought it is “big budget” project.

If any person knows of any Electricians, Engineers, or others who have a never ending amount of knowledge with these situations I would very much appreciate their names or contact details. Though we may not follow this path, I would like to be prepared with names and their experience if required.


Nov 25 2018 12:10

Burning diesel in a genset to run electric heating is about the worst efficiency you could get for converting chemical energy into thermal.
Seems to me an expert in the field of energy usage should have been consulted at design stage, instead of choosing under-tile heating and then trying to find a way to run it

Nov 25 2018 12:19

All good comments , but if this is a new build theres the opportunity to build a house that needs minimal heating . Even if money is no object . wood stove can be effectively heating after 15 minuets .
But thats not the OP Question .
Yes 2 generators


Nov 25 2018 13:07

The solar and battery system for a holiday home type scenario falls in with what I suggested about the continual and critical loads. So that is good thinking, but complicates matters still further.

Are Tesla batteries the best? They have the a much better warranty than traditional cells, 10 years vs 1 year, and they certainly seem work. But.... they need to have internet connectivity, so something else to be thought about.

What needs to be done is a power budget for non-generator activities whilst the home is occupied, ie sleeping. How much KWH of storage is needed? The benefit of solar is when they are not at home for weeks or months the solar can keep the alarm and freezer going.

Having the solar done by others complicates matters, because there needs to be cohesive thinking and planning to make this all work. Exactly how, when, and why and how much power will be fed to the house. Will the PV inverter only run when there is existing power to sync to? When there is existing power will it refuse to connect?

My suggestion of four groups is now five: the original four of "normal (when genset running)", "awkward (when genset running)", "heating (when genset running)", "continual (all the time)", and "night with or without genset but only when we're home".

The "night with or without genset but only when we're home" is potentially the most difficult, because with genset on power should come from the UPS (assuming its fitted), and when the UPS is off overnight, power comes from the PV system batteries via an inverter. I think a Tesla system can do this out-of-the-box by being set up in partial backup mode.

Certainly those nighttime loads should be wired single phase to match typical small solar systems.

Two things I should have mentioned: the first is one big advantage of the UPS, "industrial", and implying "on-line and generator rated", is that it will always present a balanced load to the generator irrespective of the imbalance in the UPS loads. Makes load balancing on the generator so much simpler.

The other thing is probably the most important design characteristic, which is step change in load. If you exclude ugly stuff like cookers and water heaters, then most of the worst that can happen is a 10A change from a socket being switched on. Thus as long as you have 10A of spare capacity on the supply, all is well. If the spare gets used, change the generation tactic, or load shed to get the 10A window back again. With big loads like cookers, there needs to be a much larger spare capacity, which is why the button by the cooker to turn cooker power on.


Nov 26 2018 16:50

Here's my opinion:
For a modern insulated house, that's huge high heating wattage. I'm hoping they are just aiming for fast heat, and that once warmed up the average power draw will be maybe a few kWatt.

It's gotta have at least some insulation underneath the floor heating. Sounds like it inherently has a gap from the slab.

Assuming this is all off-grid, the Powerwall must be rated for supplying everything. Which also means it should be able to absorb any excess from the genset. This will allow the genset to be well loaded while running. And can be kept running until the Powerwall is fully charged.

If there is space for more solar PV then use it. The more panels there are, the lower the fuel costs for the genset.


Nov 26 2018 16:56

It also means the genset only has to be rated for average power. The Powerwall smooths it out.


Nov 26 2018 17:01

And how much time the average covers depends on Powerwall energy capacity verses usage.


Nov 26 2018 18:17

The heating should definitely load-shed to everything else. So then the heating defines the peak demand.


Nov 26 2018 18:53

Hmm, after actually looking at Powerwall feature set, it looks like a Powerwall might have difficulties at running an off-grid setup. ie: not yet supported!


Nov 26 2018 21:12

Yes it is also my understanding the power wall isn't for off grid .
Still Tesla isn't the only system that has lithium battery's
Mercury energy is testing all sorts of things in the solar and battery way , maybe talk to them.

Nov 26 2018 22:41

I'm out of the country at the moment, so this reply is from memory as I don't have access to my library on gensets, PV systems and large battery systems.

All invderters will need to be multi-mode inverters, grid connect inverters never a grid supply system to operate from.

Gensets can NOT withstand 100% loading of a UPS you need some normal (ideally resistive type) loading and a low reactance output.

While a UPS will give load control if will be very expensive an multi mode inverter will be a much better and cheaper approach.

Diue to the limited operating time duplicated gensets will be unnecessary

The best person I know in this field is Hoskings Energy Services based in Caterton.


Nov 26 2018 22:43

If you are looking for an off grid system to run with solar, a battery bank and a genset. Talk to victron energy they have modular systems that can be upgraded over time and even run 3 phase power. Their systems are installed in factories all over the world. The gear is rugged enough for ocean going vessels and factories in third world countries where grid supply is unreliable.

Nov 26 2018 22:43

add to the above a battery system connected to the Muti-mode inverter.

Nov 29 2018 17:53

Many thanks to you all.
This project is so far behind time now the owners have decided to listen to the number of good wisdom offered on the electrical side. I will let you know of the decision, and the outcome sometime in the future. Thanks again