Solar Panel Ratings

Re: Solar Panel Ratings

Basically, it is a matter of cost vs efficiency with mixing panels (and, to a degree, it depends on the solar charge controller)...

A solar panel puts out (roughly) a fixed voltage when the sun is shining and a variable amount of current directly porportional to the amount of solar radiation striking the panel (brighter sun, more current).

Looking that the basic electrical equations:

Power = Voltage * Current

So, if you have two panels in parallel, the one with the lowest voltage (at maximum power) will pretty much set the voltage point, and the two currents will simply add:

12 volt 5 amp panel + 24 volt 2.5 amp panel (60 watts + 60 watts = 120 watts) when connected in parallel become:

12 volts * (5 amps + 2.5 amps) = 90 watts -- or you have lost about 30 watts from the 24 volt panel because of this...

And, if you connected them in series, the current supplied will be limited by the smaller panel:

(12 volts + 24 volts) * (2.5 amps) = 90 watts -- again, a loss of 30 watts

Of course, nobody would normally connect a 12 volt with a 24 volt panel together--usually the differences in voltages and currents are much less (and the "losses" are less too)... I just used the above as an example.

In the case of mixed 12 and 24 volt panels, you could (depending on controller and battery voltages) connect two 12 volt panels in series (for 24 volts) and connect those in parallel with one 24 volt panel:

(12+12v)*5a + 24v*2.5a = 120 watts... Or full power from all panels.

There are more details to discuss with MPPT controllers (maximum controller voltages and currents) and such--how much detail do you wish to go into?

-Bill

Re: Solar Panel Ratings

Dan,

Actually, it was Niel that typed about the 24 volt panels... But I can probably answer the question.

Think of the solar panel as a battery with a certain load and voltage profile (IV curve). And you have the load, typically a lead acid type battery. Between the battery and the solar panel is the solar charge controller.

Working backwards, the Lead Acid battery needs certain voltages (with supporting current) to charge quickly and fully. Also, flooded cell lead acid batteries can use a function called equalization which is a higher voltage that causes hydrogen and oxygen gas to form on the plates which ensures that each cell is fully charged, and the movement of the bubbling gases mix the electrolite which can stratify (acid is heavier than water and tends to sink in flooded cell batteries--particularly tall batteries).

If you look up the tables for lead acid batteries, at a particular voltage they will list a required voltage for that mode (bulk charging to quickly recharge, float charging at a lower voltage to keep a battery charged without outgassing, and equalize, the highest voltage)... Also, as a lead acid battery gets warmer, these voltages drop, and as it gets colder, these voltage requirements rise.

So, for example, to equalize one brand of 12volt battery, they ask for 15.2 VDC at 80F... If it is colder, that voltage will have to be higher, if it is warmer, the voltage will be lower.

Now, there are several types of solar controllers out there. And they usually will need some voltage drop across them to cause current to flow and for them to function correctly. Say that 2 volts DC drop is a good minimum requirement. Now, you are looking at needed a solar panel that can output 17.2 volts DC. And that has to be 17.2 VDC at maximum power point--if that was 17.2 VDC open circuit, that is with zero current flow, so that does not help us.

Also, as I said before, solar panel voltages drop with with increasing panel temperature. Those ratings are for 77F (25C?) ratings, and if you ever left anything black out in the sun for an hour, the temperature of those panels will increase dramatically.

For example, a Kyocera KC80-02 (80 watt 12 volt panel) has a Vmaxpower = 16.9 Vdc... At 95F, that panel will only about about 13.2 volts... Not near enough voltage to bulk charge (say 14.2 vdc) or equalize a room temperature battery--let alone even run the solar charger properly.

For a 24 Vdc battery system, all of the voltages need to double (except the controller which still only need a couple volts minimum)--So we are looking at needing 30.4+2=32.4 Vdc to equalize and 28.4+2=30.4 Vdc to charge a room temp battery.

The KC 200 GT panel at 32F Vmaxpower is ~36 volts, but at 95F, it is 20.7 volts--way to low to charge a 24 volt battery with any standard solar charge controller...

By the way, a great place to get the basic solar panel specs for almost any manufacturer is from Xantrex's solar planning site for their GT inverter:

http://www.xantrex.com/support/gtsizing/index.asp?lang=eng#calculator

You can also plug in temperatures to find out operating conditions and it give PTC (closer to actual ratings) specs.

It is not that the KC 200 is a bad panel, it was just not designed, by itself, to charge a 24 VDC battery bank with a simple solar charge controller.

There other charge controllers out there that are MPPT type controllers... These can take solar panel strings with much higher voltages and efficiently convert them down to the needed battery voltage.

For example, use two KC 120's in series and you can charge either a 12 or a 24 volt battery bank with a MPPT type controller. For larger systems the MX-60 is a very popular (and not cheap) model.

Also, Wind-Sun has a bunch of other MPPT controllers you can read up on (buying and shipping to UK may not make much sense).

http://store.solar-electric.com/chco.html

I will stop here for now--the post is long enough and I have to do some other things.

-Bill

Re: Solar Panel Ratings

Dan,

Just to make it clear, just putting two "12vdc" panels together is not always the answer either... If you are using a "simple" solar charge controller, finding/wiring solar panels whose Vmaximumpowerpoint voltage matches the sum of the "worst case voltages over temperature and current" sum of battery voltage, controller drop, and wiring voltage drop.

That is why when you look at 12 VDC panels, it is important to look at their Vmp and see that for battery charging designed panels, you may be looking at 19-20 vdc (not the 12 vdc on the panel name plate).

Also, as you have seen in other threads, you have to look at the maximum voltage of the solar panel (Vopencircuit) at the coldest temperature you would expect in your area--connecting two 12 VDC panels together on a cold/sunny day may exceed the controller's input voltage range.

Regarding inverters... I know that Europe has many manufacturers (and requirements) that would make more sense for you to purchase locally sold equipment that what is typically available in the US--So I will suggest a unit for you to read about (functions, user manual, etc.) so that you can better see what options are available as you look around... Also, there are others that would have better experience with inverters than I (by brand and model)...

So, Xantrex and Outback are a couple of major manufacturers here in the US that make some interesting products.

The SW series has about all of the options possible for an inverter (I am sure that Outback and other vendors also have similar products)... It can use AC line, has a battery charger, generator controller, can switch from AC input to its own "pure sine wave" inverter (like a UPS), it can sell power back to the utility, and lots of programmable options.. There are also 230 VAC @ 50 Hz models too. Its idle power is ~16 watts, but it also can scan for loads and have a standby draw of 1 watt...

The only thing it does not have is an integrated wind/solar charge controller (you have to buy and install that separately).

http://www.xantrex.com/web/id/47/p/143/pt/25/product.asp

Of course, our host here sells many types of inverters here... Here is a long talk about inverters, the good and not so good, and the details to help you decide what to buy and install for your needs.

http://www.solar-electric.com/solar_inverters/inverters_for_solar_electric.htm

-Bill