24v solar or 12v

[Craig]

I have a friend who will be fitting solar in the near future and he has been told by the installer that he will fit 24v panels and step them back to suit his 12v system. I cannot see the sense in doing this but is there any real advantage fitting 24v panels?

Craig

[T1 Terry]

The advantages of using 24v panels on a 12v system is the panels are:
(A) The panels are fractionally cheaper
(B) Bigger single panels can be used
(C) Panel with voltages suiting back to grid are even cheaper in bulk purchases
(D) The wiring can be smaller between the panels and the controller.
These is just so the installer can get max profit from the installation, of zero benefit to the customer.
The disadvantages:
(A) The controller that works well is much more expensive, the MPPT controllers under $700 are little more than a con, over a full days input they are no better and often behind the total input of a good PWM controller at less than half the cost.
(B) Bigger panels are effect far more by across the panel shading like an aerial or vent pipe sticking out the roof, the loss is greater the bigger the panel
(C) Heavier wiring is required or a lot more runs of suitable size wiring like 6mm auto cable
(D) More panels to make up the same amount of capacity require more fitting time.
(E) Smaller 12v panels cost a little more than big 24v ones
Advantages of a 12v system.
(A) Panels in parallel will only work when the sun is shining on them without shadows across them, 12v panels will not drain from the other producing panels just because they are shaded, 24v panels will as will panels in series, then bypass and blocking diodes need to be added to the system, these all eat some of the energy the panel produced.
(B) Smaller panels can be mounted in and area that may get sun for only part of the day, like beside a roof top air con unit, one lot gets the sun in the morning, both lots at mid day and the other lot in the afternoon, this is all added input from an area that could not effectively be used by a 24v MPPT system, they require al panels to be the same size.
(C) If the controller fails for some reason, the solar can connected directly to the battery if the max voltage is carefully watched, then disconnected when the battery is charged, even a few panels at a time if multiple cable from the roof to the controller are used, this is not an option with an MPPT controller and series connected panels or high voltage panels.
(D) The excess solar can be shunted to do other work like a 12v water heater element, heating the nights water instead of wasting gas to do it. It is possible to do this with an MPPT controlled system, just much more complicated, there is a 3 page to and fro discussion with circuits that only a telecom tech could follow without a conclusive result, yet a PL20 or Dingo PWM controller comes with the option of programming just that sort of thing with a single cable conection.

Between the swings and roundabout, the 12v system with multiple smaller panels using a good PWM controller will be more work involved in fitting and more cable runs, but a lot better system for the same cost as the 24v panel system using a good quality MPPT controller.
Sadly, the average solar installer will look at minimum effort for max returns, go for a cheap MPPT controller say they are all the same and get the price to a point where the 24v panel idea is cheaper than the 12v panels method nd use B/S figures to try and show it will be a better and more efficient system.

T1 Terry

[John M]

I have a friend who will be fitting solar in the near future and he has been told by the installer that he will fit 24v panels and step them back to suit his 12v system. I cannot see the sense in doing this but is there any real advantage fitting 24v panels?

Craig

Hi Craig,
When we built the bus I initially ran 24v house as well as starter system, without going into details why, it didn't work, I ended up using the 24v solar array through an MPPT controller and a 12v system, I persevered with this system, through several upgrades, for near on 5 years, recently, bit the bullet, rewired the panels back to parallel (a 12v system) and put in a cheap ebay controller as a temporary measure, works quite well, no complaints. though I will upgrade the controller to a better unit (with adjustable charge levels) when we get home.

[T1 Terry]

Did the GSL MPPT unit fail yet again John? I had been tempted to send this paper weight back to GSL again and pay for the latest upgraded, but it was based on your success with finally getting one to work properly, now????

[John M]

Did the GSL MPPT unit fail yet again John? I had been tempted to send this paper weight back to GSL again and pay for the latest upgraded, but it was based on your success with finally getting one to work properly, now????

There was a number of problems one was related to the clash between the Redarc DC to DC charger and the GSL unit. Another problem seemed to be the fact that I cannot set the Bulk rate voltage, then the time algorithm that was used once the batteries achieved the bulk level, and the final straw seemed to be related to the fridge cycling, when the fridge cycled the Gsl would cover the load and consequently diminish the amps to the batteries, but then when the fridge turned off the charging amps and volts would not return to their previous level. (10a input to batteries the fridge would cut in the amps would drop to around 5, the battery voltage would be around 13v then when the fridge cut out the amps would stay around 5a and the volts around 13. The batteries would never reach their bulk rate of 14.5V and even at the end of the day not get much over 13v and certainly never reach float voltage of 13.4) I finally gave up on the whole scenario in Darwin recently, converted back to 12v and fitted a $20 unit and that appears to be working a lot better it does does not "compete" with the Redarc, at times I have seen 30A input while driving, 20 from the Redarc and the balance from the solar, and I seem to be reaching the bulk value of 14.5v by midday most days.

[T1 Terry]

That was part of the problem I was having as well, with so many varying loads running my house system, the GSL would just shut down to trickle charging, shutting off the solar for 5 mins and then reconnecting it would bring it back on line, but that didn't seem to be a very automated system to me. When the smoke finally came out and it died altogether it was a relief really, no more persevering trying to get the thing to work properly. I finally decided it was just a very expensive paper weight