Pseudo Off Grid System: use the grid when there is no sun

Why do you want to put those devices off grid?
Do you experience frequent power outages?

Are your panels angled south?

An off-grid system should be designed for winter days to avoid over-discharge/under-charge situations like this. If you are designing an off-grid system with support from the grid, you might as well just leave all the devices on grid and have a battery back up for the occasional power outage, IMHO.

You can connect a grid-tied battery charger right at the battery posts in parallel with the solar charge controller.
I personally would rather have the battery charger switched on manually, instead of automatic based on battery voltage, as you don't want the batteries dropping to (example) 11.5 volts before they are re-charged.... and what if they only drop to 11.6 volts? The automatic battery charger won't switch on and the batteries will sit un-charged... not healthy.

First, there was the basic idea to save some electricity by avoiding the conversion from AC to DC. Then, of course, it would reduce my CO2 footprint a little bit. Second, because of fun and educational purpose. In the very beginning I even thought I were very smart and that I will save lots of money... well, I know better now but I still want to go through with this because I really want to learn. There are no power outages in my area.

Unfortunately, they are angled east. I have sun from sunrise until ~2 pm.

I am aware of this, but it would be overkill in terms of system size (and costs) compared to the actual need. This is why this came to my mind: to optimize/balance the relation between the needed wattage and costs.

A simple example that should illustrate this: if a 250W panel is sufficient to power everything during 8 months, but in winter I'd need two of them "just" to make sure all the equipment keeps running 100 % off grid, then I have 200 % installation costs (only taking the panels into account) for a situation that only occurs 2/3 of the time. So, to me it makes more sense if I could install e.g. a 350W panel that costs 20 % more and that allows for running completely off grid during 10 months. These numbers are completely random, but I hope I made the idea clear. Furthermore, during wintertime, I still get some electricity from the panels, which reduces at least partially my electricity bill.

If I understand this correctly, both grid-tied battery charger and solar charge controller would operate independently?

This is rather impracticable. The thing is that I have a cloud server running on the NAS and I share it with some friends of mine that also use it for business. If I travel I could not guarantee the availability of the server. That's why it should be fully automatic, however this might be achieved.

I know, that's why I asked if there exists such a device already. The logic of such a device should be that it checks first if the voltage is below a given voltage, which is set high enough to absolutely avoid any under-charging. This value would depend on the battery type, I guess. If the battery voltage is lower, then it should check if the solar panels produce enough output to still load the battery. If both conditions are false then the grid-tied battery charger plugs in to charge the battery.

I conclude from your answer that there does not exist such a device already? If not, is there a DIY solution or could one try to implement it with a Raspberry Pi or Arduino board?

I'll try to answer the best I can as I stand in line at USPS to mail 1040 ...

I for one see nothing wrong with DIY solar for hobby or educational purpose... as long as you're ok with the cost.
I see people wasting more money on vacations and pricey cars that depreciate quicker.

I'm sure you know the consequence of East vs South.
Is your balcony on the east side of the building?
If it is just one 250w panel you are installing, couldn't you tilt it south?

An on-grid battery charger could run in sync with or independent of a solar charger.
You made your need for automatic operation clear.
However, it would be a waste to have an on-grid charger running simultaneously with solar,
so an on-grid device that switches on when solar current is low would be ideal.

I'm sure those devices are available... I have no experience with any ... but I'm sure others here will be able to help.

To the OP, you can use one relative cheap 30 amp automatic switch to switch your power supply between the grid and the Solar off grid system, wire the primary line to the solar, when the inverter shut off due to Low Battery Cut Off, The automatic switch go to the grid so you will have power for your appliances.

How ever make sure you get one off grid inverter that either can program the low battery cut off voltage or at least the one that will cut off the power when battery reach to 12 volt on a 12 volt system. The cheap inverters in the market all have the battery cut off set at 10.5 volt which will kill your battery in no time.

I use this same setup on my house. when the battery bank reach to 48 volts my inverter shut down the grid power come on. when the battery bank reach back preset voltage, the inverter turn on back and energize the automatic switch then change to solar power. hope this help a little.

Cheers

First of all: thank you very much for your quick answers!

I became aware of them, and it's ok.

Yes, I do know the consequences. Actually, the orientation is south-east. The situation is not perfect but could be worse. I will try to install a solar tracking system, if it is feasible. Otherwise I'll follow your suggestion and tilt the panels south. The 250W panel was just a random number for illustration purpose. I'll yet have to properly determine the dimension of the system, but before I do I'd like to know if/how I could achieve the goal of

I hope so. Once again thanks for your help and clarification!

Do you mean something like this? Am I looking for the right thing? Thanks!

From what I can tell, ^^^ that only transfers the appliance power supply from off-grid to on-grid when the inverter has low voltage input.

It does not charge the battery from the grid... which is what I think the op wants so the battery doesn't sit in an undercharged state.

The next day the sun will charge the battery, or he can put a AC charger on the battery. If the inverter has a charger, it can be use too.

No inverter please

One thing I think I was unclear about is that I do not wish to use an inverter. I would install a grid-independent circuitry that is operating at 12V only. This makes sense since most equipment runs at 12V anyways, so why loosing efficiency when inverting DC->AC->DC...

It seems to me that the Phocos PL20 might be appropriate for the task. Chapter 13 in here describes the combination of hybrid solar and diesel/gas generator system. I would replace the generator by a standard 12V battery charger. However, it is quite expensive and I would prefer a cheaper solution.

I can certainly see the usefulness of a small solar array just for modem, router, and laptop.

We have an UPS for our modem and router, and a separate UPS for each laptop. But they are only good for about an hour.

We use wood heat, so our home heating does not require electricity.

Not sure about everyone else, we live in town so we do not lose power as often as folks out in the boonies. We tend to lose power between one and three times every month. Rarely for more than 4 days though.

Because we lose power [like everyone else] it is important for us to have some off-grid ability.


Providing power to your modem, router and laptop make sense to me.

The questions that comes to mind is how cloudy is it there?... and what is the minimum sun hours during winter?

If you built a PV system with a "5day" battery and panels sufficient to charge in one day,
you wouldn't need the on-grid hybrid unicorn...

You said you're trying to offset the cost of "building big-enough"...
but that's like ordering a Corvette with a 3gallon fuel tank... and having a AAA membership just in case.

Use the calculator, do the math... it may not cost you a whole lot more do it right, as you assume.

For best bang for the buck, use grid-tie panels, much cheaper per watt than battery panel, use MPPT CC.

Makes sense to me too...

Man, you spend a lot of time in the dark there.
1 to 3 times a month? ... up to 4 days at a time? ... and people in the boonies have it worse? WOW.
Y'all definitely need a larger UPS.

It would be hellish here if I lost modem for 4days... that means my wife couldn't connect to facebook and pinterest...
and I would have to listen to her talk for hours

We began building our homestead in '05, have not documented a month yet that did not include at least one power-outage.

Most properties that I looked at when I was shopping for a homestead, did not have municipal utilities on site. To get power you need to pay to string power in, often for multiple miles.

Most townships in this state have only dial-up. We were fortunate in finding a town that had just gotten a Federal grant to up-grade the phone lines to dsl. Some days dsl is as high as 1.5Mbps/0.5Mbps, still a lot better than 2400baud

That is what happens when you go off grid and no genny.