Generator sizing for PV supplemental power

[Riley5781]

Hi guys -

I am building an off-grid cabin (2000 sq ft) and am nearing the stage of needing to buy my generator. The generator is for supplemental power, topping up batteries and providing power in the winter time. My thinking is that in the winter, I could charge the batteries in the daytime for a few hours and then run off batteries in the evening and not have to listen to the generator 24/7.

My daily load is estimated at 4KW-hours/day. Some say that’s estimated too low but my place is designed for low power consumption and I added up as best I can. The 48V battery bank is comprised of 8 * 6V 440AH batteries and the inverter is a 6.5KW set (XW6048).

My question is, what size of generator would make sense given this setup and my goal to not listen to it for too many hours at a stretch. I know it’s simple math in terms of generator size and run time but I am looking for practical advice here.

Specifically: is a 5KW genset too small? I have a used one lined up that looks good (1800 rpm old school Onan) but I want to make sure I won’t regret having too small a generator.

Thanks in advance

Riley

[Mike90250]

The XW series, in the right hands and programmed correctly, should be just fine with 5Kw. Both my gensets are 3Kw, and allow me to charge, and run loads. In fact, you may have issues with the diesel (if it's diesel) "Wet Stacking" if you don't load it enough,
In Bulk charge mode, you should only run about 10-15% of the battery Ah nameplate [ 44-66A ] which at 60V (late Bulk Voltage) would be 2.6 - 4Kw. Running an older diesel below 50% load can induce buildup of wet carbon because of low chamber temps.
Look into getting the Combox instead of the SCP, if you have a desire to see power graphics.

IMG_20160117_183519036.jpg
Above - SCP with many layers of menus

Next - screen shot of a typical 90 min morning generator run, and then some sun came out and topped off the batteries,
You can see that as loads came on (toaster, blow dryer) that the XW, knowing the limits of the genset, throttled back charging the batteries till the loads eased off
batteryGraph_1-11-2015.jpg



And finally - below: the system screen from the combox, showing the total power flow in the system (my solar charge controllers are not XW gear, and don't work on the xanbus - but they do charge like a dream)
ComboxSystemScreen.jpg

[Mike90250]

PS, I deleted the dupe topic for you.

[Riley5781]

Excellent. I was thinking the XW series would be the ticket based on the sticky thread by Chris Olsen. I took that thread to heart.
I assume that I can set the max draw from the genset on the XW and then if the cabin loads start to draw more, then the charge rate is simply reduced. Then when the loads lighten, the charge rate will go back up. If that's how "generator support mode" works then perfect. I think it should be fine. I have designed the entire cabin (with the help of the people here) to not draw much power and gives me the option to not use the generator for big loads like water pumping (unless I am flush with PV power).

Note I am looking at propane power and not Diesel. Thinking of an old school Onan CCK unit that is 5KW. I will have a small 3KW Honda as a back-up for the Onan.

I will note to get the Combox unit.

And thanks for deleting the duplicate thread. I was going to flag it but the warnings around that didn't seem to cover stupid double posts
my browser seemed to hang and didn't show the post... anyway... thanks Mike.

Perhaps next I need to think about if I should reduce the size of my battery bank in order to reduce reoccurring costs. BTW I have 12 panel system (3KW?).

[Mike90250]

Only covered half the magic of the Gen Support. The other half is, if a load comes online, larger than the genset (firing up the flux capacitor for a time jump), the inverter senses when you exceed the programmed limit AND THEN stops charging, and goes into Load Support, to keep the loads running off the Generator and now the Inverter too!

But you can't just "change the size" of your battery bank. That is supposed to be calculated to be a 3-5 day Stand-Alone size to support your carefully planned loads and expected cycle life.
Go too small and you start doing deep cycles on your bank, and you get a 2 year life from it.
As long as you know the tradeoffs - small battery bank = no vacations Because who will monitor the weather and start up the generator if it's cloudy while you are gone?

[Riley5781]

thanks for the explanation of Gen support. It was the second half of the feature (flux capacitor) that is covered in the manual. It wasn't clear about reducing the charge but I assumed it was the case. I did my battery capacity calculations from Sunkings thread on the sizing topic... so I assume I am good. When I am not there, I plan to shut down the loads to zero except for the fridge in the sunny, summer time...

Having said that, you have convinced me to stay with the larger sized batteries.

BTW - my last question (for this week) will be on input voltage from my main Onan propane genset. I'll start that topic on another thread (but will search for that topic first).

[Mike90250]

.....
BTW - my last question (for this week) will be on input voltage from my main Onan propane genset. I'll start that topic on another thread (but will search for that topic first).

The XW is designed to produce and accept 240V North Am Split Phase power. So if you are in the states, you will want to provide a 240V genset
there is a downgrade procedure to convert the inverter to 120V single phase, but now you cannot power a standard split phase distribution panel.