Total Noob question......

[Scorch]

I'm trying to get my head around all this, so would appreciate some help.... I'm looking at an offgrid situation and I'm just trying to understand how power will flow....

So I'll need about an 11kW system for my needs... so lets say 50x300 watt panels.
I'll also planning on about 8, 100aH 48V batteries. and then putting on a propane generator as backup.

So let's say on a cloudy day I'm getting 6kW from the panels, But I'm drawing 8kW of power..... my assumption is that I get 6kW from the panels, and 2kW from the batteries. Then when my AC shuts off, and I only need 2kW of power, I'll get the 2kW from the panels, and the excess 4kW being generated is used to charge the batteries. - Is this correct (more or less)?

So then my assumption is that when the batteries are empty, or can't supply enough power to make up the difference between load and solar, that's when the generator kicks on. So at that point the generator will put out 10kW of power (assuming a 10kw generator) which will make up the difference in load with any remaining power charging the batteries, but will it then shut down once the load is less than the solar generation or will it shut down once the batteries are filled to a certain level?

Or do I have this all wrong in my head? any enlightenment would be greatly appreciated.

 

[Q-Dog]

WIth the right components you can make the system do exactly what you propose. As for the generator, you can set it up to charge until batteries reach a certain voltage, or a certain state of charge, or until full, or ... any number of things you can think of.

 

[boondox]

Yes, a system can work that way, as long as it is designed to do so. There are a couple of little things that might be different. One is that many higher power inverters work in "pass through" mode when using a generator. Since the inverter is being used as a charger, it can't invert. So all of the AC loads are "passed through" to the generator. So you wouldn't be charging at 10kW, you would be charging at say 6kW and leave the other 4kW for whatever loads are running at the time.

Also, very few generators can run at their max rating for a long time. For best fuel economy and life they are genreally run at 2/3 to 3/4 capacity.

 

[Scorch]

Very helpful.... wasn't sure about all that.

So in "pass through" mode, that is only when power is supplied with a generator or other secondary source?

So if I have this right.... Let's say I'm using a continuous 4kW base load and the array is providing 6kW, then I'm charging with 2kW (6-4) and of course the generator stays off. Then when night comes, and I'm still using 4kW it will draw all that from the batteries until the battery voltage drops to some voltage at which time, the generator kicks on and supplies any loads required, but will not charge the battery (since it's in pass through), and will continue to run until the sun comes up, and the panels start generating the 4kW required at which point the generator would either shut off, or continue to run to charge the batteries depending on how everything is set up.

does that sound about right?

 

[OffGridInTheCity]

.... but will not charge the battery (since it's in pass through), and will continue to run until the sun comes up, ...

I don't see it this way. If you have a battery -> inverter -> AC power... then this would run all the time. As it runs, the battery voltage will drop toward cut-off voltage (e.g. inverter needs to be turned off).

You avoid voltage drop to cut-off by charging the battery... using PV and/OR a generator. You don't need to use the generator to power loads - you just need to charge the battery directly. Yes, there will be Generator-AC->DC-charging losses (15%) but the approach is simple. If PV is not enough, then turn the generator on at low voltage X and off at either:
1) PV input is 'enough' and generator not needed.
or
2) high voltage Y - e.g. battery is charged 'enough' for the moment

Note that you can simultaneously charge the battery from PV and Generator at the same time as long as the battery can accept the max generator + PV charge rate.

In all cases the generator would turn on at low voltage to avoid shutdown of the inverter -> AC power.

For #1 above (nuanced): If you have instrumentation you could turn off the generator once you detected PV was high enough to 'take over' or the battery was full charge.

For #2 above (simpler): You could turn off the generator at 52v or 53v or 54v (battery partially charged) no matter what PV is doing - e.g. manually reduce the overlap - and maybe PV will take over. If PV isn't enough you just have more cycles of 49v generator on up to 52v off.

 

[boondox]

Very helpful.... wasn't sure about all that.

So in "pass through" mode, that is only when power is supplied with a generator or other secondary source?

So if I have this right.... Let's say I'm using a continuous 4kW base load and the array is providing 6kW, then I'm charging with 2kW (6-4) and of course the generator stays off. Then when night comes, and I'm still using 4kW it will draw all that from the batteries until the battery voltage drops to some voltage at which time, the generator kicks on and supplies any loads required, but will not charge the battery (since it's in pass through), and will continue to run until the sun comes up, and the panels start generating the 4kW required at which point the generator would either shut off, or continue to run to charge the batteries depending on how everything is set up.

does that sound about right?

No. In my system anyway the generator will be charging the batteries at whatever rate I specify and then do its best to support any other loads that are applied. If the generator can't keep up with demand then the voltage and frequency will drop and the system will kick the generator off-line. It will then try again in a few minutes.

So you need to leave enough overhead in generator capacity to allow for extra loads. In my case I have a 7.5kW genny. So I pull about 4kW off it to charge. This leaves me enough overhead most of the time. Occasionally if somebody is making a pot of coffee and the water pressure pump kicks on the genny may get kicked off-line. But by the time it tries again the pump is off or the coffee is done and the genny starts charging again.