Hi @warwick, using an extrapolation like this would get you close to the correct size. If I can add, the generator should be sized based on the load, taking into account losses. Let’s use this example. An 8K Quattro, input current limited to 34A, so max input power is roughly 8KW. Divide this by 0,8 brings us to a 10K Gen, this is to compensate for power factor. Then you’ll need to compensate for elevation (height above sea level) I take 1% for every 100m, so if the site is located 1500m above sea level our 10K gen becomes 11,8, so call it a 12KVA. Also, we don’t want to run the gen at full tilt, therefore a Gen with a continuous rating of 15KVA would be just fine for the above example. Also, does it supply the Quattro only, or additional loads?
note this is a conservative approach, and generator build quality is a just an important factor.
Looking at these Gas Generators - the 13kVA for the 10kVA Quattro:
I put that table in the Generator FAQ to give people a place to start and a reality check (why won't my 500W generator work properly with my 5 KVA multi?) but generator sizing should involve a bit more consideration.
The Australian Standard for generators in stand alone power systems is AS4509.2-2010 clause 220.127.116.11.
The standard says:
The sizing of renewable energy generators is often influenced by a number of design criteria, including the available energy resource, acceptable generating set run time (if used), budget constraints and other factors. For systems in which the renewable energy fraction is less than 75%, other system features may be necessary to ensure satisfactory system performance (see Clause 3.5.2).
I often design around the minimum sized generator. Not necessarily the most cost efficient or otherwise, (eg periodic generator runs to reduce battery sizing) which would require much more site specific design work.
Minimum is big enough to run the essential loads + 10% for battery charging + 10% for kVA/kW power factor + 10% for losses + 20% oversizing for genny duty cycle/decay ( (Loads) + (charger x 0.1) x 1.1 x 1.1 x 1.1 x 1.2)
Maximum Useful is big enough to run the battery charger at maximum power + power to run all loads + 10% power factor + 10% deratings/in-efficiency + allow for 20% oversizing for genny duty cycle/decay - ((Loads + Charger) x 1.1 x 1.1 x 1.2)
Over time if you find the generator isn't keeping up any more (and overloading), adjustments may need to be made to reduce charge power demanded from the charger, or loads run (via the AC input current limit).
Most of the time we want the system to be charging off the solar/renewables. The generator is only ‘required' when the renewable is unavailable to see you through. A bigger generator will mean it will not need to be run for as long. But if the system is designed to only run the generator rarely, money may be better spent elsewhere.
In my view, that relationship is too broad. Useful, but maybe we can delve deeper..
There's no point having a genny much bigger than what the batts can accept, and of course that can vary a lot. Add a little for concurrent baseloads and there's a start point. Add in Murray's fudge-factors and that might do. Load spikes don't matter, they just get 'absorbed'.
Li batts can handle a lot faster charge, so then might come down to genny cost vs runtime. (And which one of those you dislike the most).
Look at the system overall, not just one aspect of it.
Additional resources still need to be added for this topic
MultiPlus Product page / Quattro Product page
MultiPlus 230V Manual / Quattro 230V Manual
MultiPlus 230V Datasheet / Quattro 230V Datasheet
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