# electrical questions

Registered Users Posts: 8
i have a question about the way the voltage works with this.

does the voltage going into a combiner need to be the same as the voltage in the charger and the voltage size of the battery bank, or can these be different?

I am just starting with solar and am trying to learn some things so I dont make too many mistakes

thanks.

• Banned Posts: 17,615 ✭✭✭
Re: electrical questions

You're confused because it is confusing. Feel better now?

First off, there are "nominal" Voltages and "actual" Voltages. For instance you may have a PV with a "nominal" Voltage of 12, but it has at least two other "actual" Voltages: The Voltage Open Circuit (Voc) and the Voltage at Maximum Power (Vmp). For the same "12 Volt" panel the Voc may be 23 and the Vmp 17.5.

And then it gets more confusing.

In between the solar panels and the battery bank (assuming off-grid) there's a charge controller. This could be one of two (three, actually) different types: Pulse Width Modulated or Maximum Power Point Tracking. The PWM just connects the panels "straight through" until certain battery Voltage set points are reached, then it starts to pulse to maintain that set point. The MPPT can take a much higher Voltage array of panels and "down convert" it to the relevant charging Voltages.

Which is where we get into more differences. A "12 Volt" battery actually gets charged at about 14.2 Volts, and then settles back to "float" at about 13.8. Furthermore, there is another available charge state called "equalization" which pushes the Voltage up to 15 in order to force the individual battery cells to "even out" or equalize (hence the name) as batteries don't always stay at the same Voltage per cell as they should.

So for the example "12 Volt" system you could have a photovoltaic array of 17.5 Vmp running through a PWM charge controller to bring the batteries up to 14.2 Volts for charging, or you could have a "24 Volt" array of 35 Vmp running through an MPPT controller to do the same thing. Just as an example; not written in stone.

Hopefully I've given you some understanding of the source of confusion. Does it help or is it more confusing?
• Solar Expert Posts: 59 ✭✭✭✭
Re: electrical questions

I'll give you an example of my soon to be set-up.

Five 12 volt panels wired in series to make 60 volts nominal. As noted in the above post the actual voltage is higher.

The flow then goes to an MPPT controller which converts the sixty volts to my 24 volt battery. The actual battery voltage is slightly higher and varies a little depending on the state of charge of the batteries.

In the above example an MPPT type of solar charge controller is needed because other types of controllers are not as flexible.

The same controller could also convert the sixty volts to a 12 volt battery if that were my preference.

Also the same controller could be used with voltages up to 150 volts and convert to 12 or 24 or 48 volts depending on your preferred battery voltage.

It cannot go the other way. The solar panel voltage has to be higher than the battery voltage.

Zeuspaul
• Registered Users Posts: 8
Re: electrical questions

Also how does the amps coming off the panels affect the battery system?

I know that when wired in parralell that they are doubled, but is there a benefit to it and how so?
• Banned Posts: 17,615 ✭✭✭
Re: electrical questions
puresolar wrote: »
Also how does the amps coming off the panels affect the battery system?

I know that when wired in parallel that they are doubled, but is there a benefit to it and how so?

It's a bit complicated. More along the lines of "do you need to". In the case of using the PWM controller which won't "down convert" Voltage, you can only parallel panels that are at the nominal system Voltage. Vis: two "12 Volt" 135 Watt panels in parallel = 270 Watts; 12 Volts @ about 22.5 Amps (for example - not an exact number). Whereas with the MPPT you could put them in series and still have 270 Watts, but it would be "24 Volts" @ 11.25 Amps which could charge a "24 Volt" system at 11.25 Amps or down convert to the same "12 V 22.5 A" as the PWM controller. (Note I've left out the actual Voltages and the efficiency losses.)

The reason for coming up with a particular current is the need to charge batteries at between 5 and 13 percent of their Amp hour rating. As in a 100 Amp hour 12 Volt battery needing at least 5 Amps @ 14.2 Volts to charge and can take up to 13 Amps (depending on the battery). If it were a 24 Volt 100 Amp hour battery bank you still only need the 5 to 13 Amps, but at 28.4 Volts:

10 Amps @ 14.2 Volts = 142 Watts
10 Amps @ 28.4 Volts = 284 Watts
• Solar Expert Posts: 59 ✭✭✭✭
Re: electrical questions
I know that when wired in parralell that they are doubled, but is there a benefit to it and how so?

For a given pair of solar panels if you wire them in parallel you get double the amps as you noted. If you wire them in series you get the same amps but double the voltage coming out of the panels.

Either way you get the same amount of energy delivered to the battery. 2X as many amps at twelve volts equals 1X amps at 24 volts.

Zeuspaul
• Registered Users Posts: 8
Re: electrical questions

so if I had two or four 24 volt panels then my best configuration would be to wire them in parallel to a single charge control and then into the 24 volt batteries.

correct?
• Solar Expert Posts: 1,571 ✭✭
Re: electrical questions
puresolar wrote: »
so if I had two or four 24 volt panels then my best configuration would be to wire them in parallel to a single charge control and then into the 24 volt batteries.

If you have a PWM charge controller then this is the only configuration possible. If you have an MPPT controller then you can still wire it this way, or you could wire the panels in series instead which means a higher voltage and therefore you can use thinner cables between the panels and the charge controller.

Take note of what cariboocoot said about nominal and actual voltages. A panel with an actual voltage of 24V will not charge a 24V battery!
• Solar Expert Posts: 59 ✭✭✭✭
Re: electrical questions
if I had two or four 24 volt panels then my best configuration would be to wire them in parallel to a single charge control and then into the 24 volt batteries.

Maybe not. A 24 volt battery wants about 29 charging volts for the absorbtion stage and about 27 volts in float. Also 30 volts or more to equalize them which is done once a month or so depending on your batteries.

The 24 volt panels should provide at least 30 volts. The panel voltage is reduced with higher temperatures so 30 volts at 77 degrees will become less than 30 volts at higher temperatures.

Look at the voltage spec for panels you are interested in. The Vmp is significant for battery charging. 24 volts is nominal which means in name only. Now you need to look at actual voltage in various conditions.

I would wire them in series and use an MPPT charge controller. You would have a lot of head room over the battery charging voltage with an MPPT charger. There is an optimum charge voltage for a battery and it varies depending on the state of charge of the battery. The MPPT controller can always provide the optimum voltage to the battery as long as you give it enough surplus voltage to work with. It will choose the best voltage and amp combination which optimizes the energy into the battery.

With two 24 volt panels I would wire them in series and use an MPPT controller to charge a 24 volt battery. With four panels you have other options.

String calculators helped me understand this. Try experimenting with the Morningstar string calculator.
http://www.solar-electric.com/mochco.html

Zeuspaul
• Banned Posts: 17,615 ✭✭✭
Re: electrical questions

To add to what Zeuspaul said and possibly further confuse you ...

There is also an efficiency factor in array size and going up in Voltage when using MPPT controllers. For a small array (<400 Watts) there's not much gain to be had from MPPT. Also, the efficiency drops off as the array Voltage goes up in relation to the system Voltage. Usually it's best to keep the down conversion to a factor of 2, as in a 24 Volt system charging from a 48 Volt array.

And yes there are some "not quite 24 Volt" panels out there with a Vmp that is too close to 24 to do any good for charging the system.

To really get a handle on this you have to start with loads, and design from there. Then you can see how different configurations work out for the proposed need and what the advantages/disadvantages of each potential design are.
• Registered Users Posts: 8
Re: electrical questions

So can you tell me if this would be a good setup:

2X -- Kyocera KD205GX-LP
1X -- Tristar-MPPT-60

This would give me 410 watts wired in series.

I could then get 2 more panels in a series and wire them in parallel with the first series for 820 watts.

This would leave room to do this one more time for a third series in parallel for 1230 watts.

According to the string calculator, these are optimum settings.

To go bigger than this with more panels, I would have to start over with a second mppt controller.

Does all this sound right to you guys, and can I continue to increment a system this way, or would it create a problem?
• Banned Posts: 17,615 ✭✭✭
Re: electrical questions

You're skipping some very important steps. The first being answering the question "how much power do I need?" You have to get that worked out before you can determine how to supply it.

Off-grid systems are battery based. The panels and charge controller re-charge the batteries. Until you know how much battery you need you can't determine what panels/controller you should get.

Growing a system over time is one of the most frequently asked about aspects of solar. It's nearly impossible to do. It's best if you look at it long-term, and see what the power requirements might be down the road and then see if there's any practical way to build up to that "one stage at a time" or if it's going to require a complete re-work.

There's a couple of reasons for this. One is that aging batteries don't mix well with new batteries, so if enough time has lapsed you have to buy an all new battery bank even though there may be years of usable life in the old ones. Another is that as over-all loads increase it is better to go up in system Voltage rather than try to supply 4 kW off 12 Volts - the current draw gets enormous and difficult to deal with. If it's lots of small loads you can sometimes get by with adding parallel systems.

But you really need a long-term plan, starting with your present loads (including both maximum draw at any one time and total Watt hours used per day) and extending in to what you want to include en toto come the future.
• Registered Users Posts: 8
Re: electrical questions

I figure that I will be needing between 600 and 800Kwh a month.

There is no way at all that I could purchase upfront for a system of that size.

Also, I do not actually need the system yet. I am grid tied, and want to build up till i can produce at the 800Kwh range.

At that time I want to relocate the system to an off grid loccation.

I could probably survive at 600Kwh but would prefer 800+.

Is this doable? I know that some items would need to be swapped as the system increased, and went from grid tied to gridless. I know I would also need the batteries once I was off the grid. It is my understanding that while I am on the grid though that I would not need the batteries as all the power I make and dont use goes straight to the power company.

I know that there are probably a million things I am missing, so please help out and explain. The more detailed the explanatin the better.
• Banned Posts: 17,615 ✭✭✭
Re: electrical questions
puresolar wrote: »
I
Is this doable? I know that some items would need to be swapped as the system increased, and went from grid tied to gridless. I know I would also need the batteries once I was off the grid. It is my understanding that while I am on the grid though that I would not need the batteries as all the power I make and dont use goes straight to the power company.

Whoops! System incompatibility! Error 404!

There's quite a difference between a Grid-tie system and an Off-grid system. Different inverters, for one thing. The basics:

Grid-tie. Solar panels feed directly to a grid-tie inverter, which sends whatever AC it can produce to the house wiring. Anything not consumed by the household loads is sold back to the grid. Providing such an option is available in your area. This requires permits and the co-operation of the local utility.

Off-grid. Power is supplied by a battery bank which is recharged by solar panels regulated by the charge controller.

That said, there are "hybrid" systems available (look at Xantrex XW inverters) which can feed the grid and also supply battery-based "back up" power if the grid goes down. Normal grid-tie inverters shut down with no grid to synch to. Of the three, the hybrid is probably the most expensive, followed by off-grid and finally grid-tie.

Now here's where planning comes in. If you know you want an off-grid system capable of 'X' Watt hours per day, there is no reason you can't build it in your own home and shift loads to it by taking them off the utility power. You'll save on electric bill, but you won't be selling anything back to the grid. In \$ terms it's a lousy plan, but if the end goal is to get a system with all the bugs worked out for that remote cabin - it works. You just have to be sure your inverter AC Out is never connected to the grid wiring.

I also hope you don't mean 800 kilowatt hours per month. It's doable, but talk about expensive! Normally an off-grid set-up would be 30 kW per month to 60 kW per month. Some are "full size" household systems, but with off-grid power costing around \$1 per kW hour over the life of the equipment it gets expensive real fast.

You can start getting a handle on your plan by purchasing a Kill-A-Watt meter and start checking everything you use now on utility power. You'll get a very good idea on what uses the most power and where you could cut back. About \$30 for the meter, and you'll be seeing things to shut off right away. Probably pay for itself in the first month!
Re: electrical questions

The only way to do such a large system in any sort of "tranferable" manner...

Purchase enough solar panels for your Grid Tied home (on the order of 7-10kW of solar panels) and a Grid Tied inverter...

When you go off-grid, move the panels to the new location, sell the GT inverter on Ebay, and setup an off-grid inverter + battery bank + genset + etc. for your off grid setup...

Even then, you may need to almost double the size of your off grid array do to off-grid system losses and the ability to only store ~3 days of "sun" in your battery bank (cost effective size).

As 'Coot/Marc says... Every time I try and run the numbers--the retail cost of panels+batteries+hardware results in the off grid power costs of ~\$1-\$2+ per kWH... For an 800 kWH per month off-grid system, your up front capital costs (plus replacement batteries+electronics ~10 years out) would give you an effective \$800-\$1,600 per month utility bill (and \$100,000-\$200,000+ upfront system cost.

We can certainly work the details out based on where you live, your summer/winter power needs, and how much genset runtime per year you are willing to accept...

But it will not be cheap.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8
Re: electrical questions

right now on the grid, I average about 800 Kwh per month.

I know for a fact that I can bring this down by at least 200 Kwh.

Some of the things that is killing me is lights in the kids room. They get turned on in the mornings, and stay on all day long. I was thinking of putting sensors on them so that they come on when someone enters the room and once movement stops, they go off after about 3 minutes. This alone would cut out about 600 - 1000 watt hours per day * 10 + lights = 10000 wh daily or 30Kwh per month.

This is just one example. This is also a reason why I want to build my system. If I can build it and either have it tied to grid, or slowly replace the grid a piece at a time.

The reason I want it to be off grid eventually is that I am looking into purchasing land in some areas that have no access to electricity and build a house there for me, my wife, and our 3 children.

I am guessing that eventually it will have to be about a 600Kwh system. Once again there is no way that I could pay for this upfront, but over the next three years could piece it together.

This is my intention anyways.

I know that things like the inverters would have to be replaced, but what else would have to be replaced? I am not including the batteries in this as i would get them when I got ready to go gridless.
Re: electrical questions

With a pure Grid Tied system--the only components are the solar panels, panel mounts, GT inverter, some electrical wiring, and a circuit breaker...

And while it is possible to move a GT inverter over to an off-grid system--It is debatable if it is worth the trouble (basically, a GT inverter is connected to the off-grid inverter's AC output as an "AC Coupled" system. The GT and OG inverter share the AC loads, and if the GT inverter output is greater than the loads, it drives AC energy back through the OG inverter and recharges the battery bank). It can be done--but this is a little "cutting edge" at this time (there is the Sunny Island GT inverter--but it is also not cheap and still requires other support equipment too).

I would suggest you do a paper design of both types of systems to support your needs--then evaluate their costs and "transferability".

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 59 ✭✭✭✭
Re: electrical questions
So can you tell me if this would be a good setup:

2X -- Kyocera KD205GX-LP
1X -- Tristar-MPPT-60

This would give me 410 watts wired in series.

I could then get 2 more panels in a series and wire them in parallel with the first series for 820 watts.

This would leave room to do this one more time for a third series in parallel for 1230 watts.

You are using the string calculator with a twelve volt system. Consider 48 volts for a system as large as the one you are contemplating.

With a 48 volt battery the string calculator lets you have four strings of four panels for a total of 3280 watts. You get a lot more panels on the same controller. Also you will need fewer breakers in the combiner box. Wiring sizes on the battery side of the inverter will also be significantly smaller.

The 60 amp rating for the controller is 60 amps for twelve volts, 60 amps for 24 volts, or 60 amps for 48 volts.

Zeuspaul