1 big panel or 2 smaller ones in series - which is better to buy?

samppa

Hello!

In the process of purchasing solar panels for a small off-grid cabin. System will be 180Ah fla battery 12v, 30A mppt controller and panels.
Panelwise, I'm thinking of GWL power's panels, and have 2 choices:

135w, 18V, 36 cell panel.
Peak power (Wp) 135
MPP voltage (V) 17,60
MPP current (A) 7,67 A
Two of these in series, will give 270w; 7.67A and 35,2V

Or a single panel of 72 cells, 290wp
Peak power (Wp) 290
MPP voltage (V) 35,8
MPP current (A) 8,10

Pricewise, there isn't much difference.
Which is the better choice? Surely 2 smaller panels will be easier to transport and install, but are there any other differences?

Rybren

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

If it were me, I think that I'd go with the 290W panel. You get a touch more power and you don't have to join the outputs of the two panels - both of these points are fairly minor.

The biggest issue that would make me take the 290W panel is expansion down the road. Those smaller panels are getting harder and harder to come by; so if you want to expand later on, you're kind of stuck. I think that you'd be much more likely to be able to match the 290W panel. Of course, in 5 or 6 years time, even those panels may be hard to match.

BB.

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Assuming that is a "true" MPPT charge controller which can take the solar panel output voltage range (hot Vmpp, cold Voc).

There have been a lot of inexpensive imports which are PWM controllers with a MPPT sticker glued on.

-Bill

Cariboocoot

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

You've got one 180 Amp hour 12 Volt battery?
If that's what you're sticking with I'd go with the two smaller panels and a PWM type controller. The reason being cost: a PWM capable of handling 20 Amps (the two panels in parallel) is far less money than an MPPT type.

20 Amp Sunsaver $84
30 Amp MidNite Kid $285

Why spend an extra $200?

westbranch

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Please keep in mind Icarus's #1 Rule of solar: "Loads always grow with time"... ie plan on adding more panel(s) in a few years or possibly months

Cariboocoot

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

westbranch wrote: »

Please keep in mind Icarus's #1 Rule of solar: "Loads always grow with time"... ie plan on adding more panel(s) in a few years or possibly months

Also keep in mind 'Coots dictum of not using a 12 Volt system unless you have no other choice.

(This is why I said "if that's what you're sticking with".)

samppa

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

This is the controller I've been planning to get:
http://www.ev-power.eu/Solar-GridFree/MPPT-Solar-Regulator-12-24-V-30-A-Input-150V-TR-3215RN.html?cur=1
I saw a review somewhere saying it is a true mppt controller.

This is an off-grid cottage on an island and is only used from around late april - october.. so pretty much only summer use. Wood heater, propane fridge.. cooking is with propane or wood grill outside. Midsummer we may be there for a few weeks in a row, other than that it's mostly weekend use, even then not every weekend.

So loads are 12v led lights, laptop/cellphone charging, radio, that's pretty much it. No tv. But perhaps I want to expand in the coming years.
Actually I really would like to get this for spring/fall when weather is cold:
http://www.wallas.fi/index.php?id=64
It needs <10Ah/24h@12v of power.

Cariboocoot

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

That "perhaps I want to expand" is the whole problem. If you start out small you stay small or else you replace everything. These systems do not expand easily, even if you plan for it. Example: mine is set up so that I can add more panels. Sharp stopped making those panels. Whoops! Now it's complicated to add more panels.

You could put in a small 12 Volt system and expect to install a larger 24 Volt system alongside once you need it. In that case you may as well go with the least expensive batteries (GC2's) and charging to accommodate them. It may even run that nasty diesel heater as-is.

Then you have to look at an MPPT controller as it will probably be more economical to get GT style panels. You need about 400 Watts worth for a set of GC2's and that's right at the "tipping point" to make the MPPT worth the extra money. How far it will be between array and controller is another factor, as the MPPT will allow running higher Voltage array for the 12 Volt system; good for reducing V-drop over distance.

Avoid that controller you listed. Search the forum for Tracer (use Google as site:solar-electric.com Tracer) and you will see not-too-favourable reviews.

samppa

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Yeah, I get it.. I think i'll just install one 135w panel with a pwm controller and a smaller 80-100 Ah battery for now.. it's enough for the led lights I have, and if I eventually get that heater it will be enough to run it sometimes.
I know, diesel sounds nasty.. you can also use biodiesel with this (slight improvement).. but there aren't many options if I want something that would heat the cottage before I go there and keep a constant temp. Problem with wood burners is always the same: you have to heat it really hot in the evening, and in cold weather the cottage will be quite cool by morning..
Cold weather here means around 0 degrees celcius..

Cariboocoot

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Zero degrees Celsius isn't cold. We've just about warmed up to that for overnight not. You should experience -40C sometime. Hopefully very, very briefly.

samppa

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Yeah, winter's get down to -30c here regularly
I just don't go to this cottage after it gets below freezing - it's on an island, and after the sea starts to freeze it's impossible to get there for a while. And when ice is thick enough, it's too cold to use it.. not insulated properly for winter.

BB.

Re: 1 big panel or 2 smaller ones in series - which is better to buy?

Just make sure that the charge controller limits the battery voltage (float) correctly (a nicer controller will let you adjust the float voltage).

AGM do not like over voltage during charging/floating. But Flooded Cells over charged for months at a time will "boil dry" too if there is nobody to add distilled water... The batteries will be toast in either case.

Good thing is self discharge is low in cold weather, and lead acid batteries "like cold" when they are not being cycled (for every 10C drop below 25C, their aging life is extended by ~2x)--Assuming you don't freeze them (fully charged lead acid battery will take -40C just fine--A fully discharge battery will freeze at ~0C).

-Bill