System Design questions

[Minnesota]

Hi all,

I've read a ton here and have watched 100s of YouTube vids on solar (and wind) and I do have a few questions.

1. Panel side - I see many people placing multiple panels in parallel (with diodes of course) that have widely varying voltage and currnt ratings, and connecting them to a common charge controller. Doesn't this limit the overall performance?

2. Charging - I see many people with a common battery bank placing multiple charge controllers (of various capabilities) in parallel, either due to input voltage of capacity reasons of the solar feeds. Doesn't this make battery bank management a mess? For instance a "cheap" bulk controller from a starter system (and its own panels) running in parallel with a PWM charge controller of their new/improved "larger" system - both using the same set of batteries.

3. Solar Grid Tie - I have not seen much discussed about how to arrange systems in a way that the grid tie kicks in whenever the batteries are fully topped off. do some charge controllers have a separate output to connect the grid tie inverter?

4. Battery Grid Tie - I have not seen much discussed about how to feed battery energy to the grid in an automated way when the battery bank is underutilized in servicing the daily loads. If a person plans to use a certain amount of KWH out of the batteries each day but doesn't, shouldn't it go to the grid?

Thanks. Great forum.

Tom in MN

[Mike90250]

1 parallel panels must all be the same Vmp (volt max power

2 several charge controllers is a valid way to add solar capacity. When you max out one controller, what else can you do? I've got 54A into my 60A controller. Another array gets a new controller. As long as voltage setpoints are adjusted very close, all is fine. Un-adjustable controllers can be a problem, but not as much as low batteries are.

3 Requires a hybrid inverter (Like the XW6048) If I set my sell voltage to 53V, when my batteries come up to full charge, then the inverter would start selling to grid, but I have no grid to sell to.

4 If you have grid, preferable to keep the batteries in idle/float, and sell as much as possible, without cycleing the batteries. When you cycle batteries, you loose about 40% of the harvest

[Sunking]

Well I will give you my two-cents worth with a different perspective.

1. Mixing panels of different makes and models is not a very effective or efficient process as you can incur some tremendous losses. In a off-grid battery system it can be done by grouping some make and model panels into their own separate system and dedicated charge controller. Best advice is to select a manufacture where solar is only a small fraction of thie product line like BP, Sharp, and Kyocera. These companies have been around for a long time, and will continue to be around for years to come when you need to add or replace panels.

2. No it does not make battery management a mess. It is the only way to have a large capacity system. For example a common MPPT 80 amp charge controllers has limitations. If operated at 12 volts the maximum panel wattage you can run is 1000 watts.
24 at 2000 watts.
48 at 4000 watts

So if you need more than 4000 watt solar panel, you have to cascade contollers to a common battery system.

3. Well if you have a grid tied system, there are no batteries or charge controllers involved. There are what is called a hybrid system that uses a special hybrid inverter, charge controller, but they do not work the way you think they do. Under normal operating conditions the solar panels and inverter work like a conventional grid tied system supplying both power to your home and neighbors. The batteries are charged from utility power, and once charged up they remain that way until needed. Upon a commercial power failure, the inverter disconnects you from the grid, and then the batteries supply power for a very short period of time until the generator starts up and takes over. At that point the generator supplies all power until commercial power is restored, and when restored the commercial power recharges the batteries until needed again.

4. This is where you really have gotten off track. There is no such thing as battery grid tied except as mentioned above. It would be very foolish to have such a system. Why would you want to generate power for $2 per Kwh and sell it to the electric company for 15 cents per Kwh? Only a politician would come up with that idea.

Here is the deal, if you have commercial power, the only system to consider is a grid tied system. It would be very foolish to have a off-grid battery system if you have access to commercial power. Off-grid battery systems should only be considered if you live in a very remote area where no commercial power is available. To go off grid, you have chosen to pay 10 to 30 times more for electricity and have an extremely limited supply that takes constant maintenance and replacement for the rest of your life. You will give up air conditioning, electric hot water, heat, ,cooking, and all other high wattage devices.

On average it will cost you $3000 for every Kwh of electricity to generate with an off-grid battery system. Of that $3000, $1000 of it is batteries that need replaced every 5 years. So if you use say the national average of 24 Kwh per day, it is going to cost you $72,000 to go off grid, and you will spend $24,000 every 5 years in battery replacement. In other words you chose to quit paying your electric company 12 to 14 cents per Kwh, and now pay $1.80 per Kwh. Since you are in Mn your cost would be up around $4,000 per KWh because you live up north where the sun doesn

[Minnesota]

Thanks guys for the answers. You cleared up a few things, however on my Q4 I was thinking of the case where for proper battery maintenance that regular "deep" discharge/cycling was a good thing to extend the battery bank life and that it would be smartest to do this via grid tie.

[Sunking]

Yes if you take care of them and only discharge no more than 20%, they will around 5 years then need replaced.