Understanding the Solar Lingo

shmockey

I am a newbie here, I did the cheap and easy thing, I went to Harbour Freight and bought thier small system. I love it and want to upgrade to a bigger system to run more things in the house. The problem that I am having is trying to figure out the panels, what is the imp and vmp? From my understanding the i in imp is for current and the v in vmp is voltage, correct me if I am wrong. I dont know what the mp's are for.

If anyone could help me to understand all of this so that I can figure out what charge controler to get and how to correctly break down what I am getting.

Thanks for any help that anyone can give.

RCinFLA

Re: Understanding the Solar Lingo

Not sure how much electronics knowledge you have. A PV cell is an illumination based current source that is voltage clamped by an inherent diode of PV cell.

This means for a given sun illumination level there will be a current output that is fairly constant from zero volts on cell to point where inherent diode starts to conduct and shunts the illumination generated current to negative terminal through the inherent diode.

To get maximum power you want the load to be such to achieve the highest voltage for a given illumination current. But there is a top end where external load will allow cell voltage to rise to a point where the inherent diode starts to shunt the illumination based current away instead of delivering it to external load. If PV cell is totally unloaded (open circuit) all the illumination generated current will go down the inherent cell diode. This is called Voltage Open Circuit, or Voc. This is about 0.65 vdc per silicon cell at 25 deg C.

To get the most power out of a PV cell (panel is just a bunch of cells in series) then loading must be adjusted to a voltage point that allows just a small amount of the generated current to leak back down the inherent diode and most of the current is delivered to the external load. This occurs where about 3% of the generated current shunts down inherent diode and 97% of the current is delivered to external load. This is called Voltage maximum power point or Vmpp. It is about 0.52 vdc per silicon cell at 25 deg C. It yields the maximum V*I product, maximum power.

The inherent diode has a negative temp conduction voltage coefficient, meaning its voltage gets lower when cell is hotter. This means available output power goes down as cell gets hotter.

There are two types of charge controllers. The cheapest is known as PWM controller which regulates the battery charge voltage by just directly connecting and disconnecting the panel from the battery. Because the panel is a current source its voltage will drop to whatever the immediate battery voltage is with a charge current based on output current from panel.

The other, more costly, type of controller is called an MPPT controller. It has a switching power supply that converts a high voltage from panel to a lower voltage of battery with about 96% power conversion efficiency. This means there will be more charge current delivered to battery then the panel current, when the panel voltage is higher then the battery.

For a PWM charge controller the Vmpp is pretty much irrelevent.

Cariboocoot

Re: Understanding the Solar Lingo

Handy-dandy glossary of terms: http://forum.solar-electric.com/showthread.php?t=6136
And I can't believe we left Imp out of it!

It's all down to Ohm's Law and yes the I is current and the V is Voltage; the mp is Maximum Power. Vmp is the Voltage when the panel is putting out maximum power (as opposed to Voltage Open Circuit - no load on the panel) and Imp is the current at maximum power. The panel's ratings of these is under lab conditions and in the real world it varies according to illumination as RCinFLA explained (he's very smart, btw).

This ties in with MPPT charge controllers: Maximum Power Point Tracking.

icarus

Re: Understanding the Solar Lingo

the "mps" are max power. So VMP is voltage at maximum power point. IMP is current at maximum power point. So a panel like my Solarex 64s have a Pmax of 64 watts, a Vmp of 17.4 and a Imp of 3.66 amps. 3.66*17.4=63.684 watts.


T

PS Voc is open circuit voltage, Isc is short circuit amps.

shmockey

Re: Understanding the Solar Lingo

Thank you both for your reply, and RCinFLA, I dont know much about electric, I am a carpenter. I know that this is tricky, but I feel strongly that I have the ability, with all of your help here, to learn how to do this correctly.

So I guess I can say that I am looking at getting two Sun 205 watt 11.27 Imp 18.20 Vmp panels and am not sure on the Controller yet. I have a 5000 watt inverter and will upgrade yearly on the panels for sure.

Like I said I am just starting so right now I am trying to learn as much as I can to do this right. Again thanks for your help so far.

Cariboocoot

Re: Understanding the Solar Lingo

You should start by defining what you're trying to do. There's lots of uses for solar electric power, and not all of them are good/simple/economical.

Usually we start by determining the load you're trying to supply. It's much better to say "I need this much power, how do I get it?" rather than "I have this much power, what do I do with it?"

Or as Tony says: "Avoid ready, aim, fire!"

The load will tell you what you need for an inverter and a battery bank, and the size of the battery bank will tell you what you need for panels/controller.

If you've already got 2 205W panels (410 Watts total) then you can work backwards:
410W @ 80% efficiency = 328 Watts / 14.2 Volts (charging for a 12V system) = 23 Amps, which would charge 230 Amp/hrs of battery easily. That's two T105's in series (225 Amp/hrs @ 6V each).
Battery capacity would yield about 100 Amp/hrs (not advisable to discharge beyond 50%) @ 12V = roughly 1200 Watt hours.
Panel yield would be around 328 W * 4 hours "good sun" = 1312 Watt hours per day (very close to battery figure).

This is pretty much a good, basic 12 Volt system set-up.

Your inverter: is it a MSW or sine wave? I'm guessing MSW, probably made by AIMS ir Chicago Electric?
Be advised that 12 Volts isn't really suitable for supplying 5000 Watts. The DC current can approach 475 Amps, which is very high.

shmockey

Re: Understanding the Solar Lingo

cariboocoot, thank you, I am trying to run as much as I can to start, Lights, celing fans things of that nature for the night, and work my way up from there. as for the make of the inverter I am not sure but I think it is sine, I didnt buy it, my father stopped trucking and that was the inverter that he had bought for his condo tractor.

so I guess what I am saying is I know I have the money to buy 2 of the panels and a controller with all the wiring, brakers and such to mount and hook everything up. I just figured I would do the math from there to see what all I can run here at the house and start with that.

I am currently going around the house and pluging in stuff to the kil-o-watt meter to see what everything eats in power.

BB.

Re: Understanding the Solar Lingo

To understand Vmp and Imp (voltage/current maximum power)... Remember that Power=Voltage*Current...

Solar panels, more or less output a constant current from zero volts to Vmp--at Vmp, the current output of the solar panel begins to drop off to zero (when Voc--Voltage open circuit is reached).

Here is a nice set of graphs for a generic solar panel... You can see how voltage vs current behaves, and they have a P=V*I curve too.

Part of your confusion with Vmp/Imp is that less expensive solar charge controllers (PWM) do not pay attention to Vmp*Imp--they just past current (I) through to the battery (as long as Vmp>Vbatt-charging).

There are more expensive charge controllers (MPPT--Maximum Power Point Tracking) that are sort of the DC equivant of a variable transformer... They can take High Voltage and Low Current and "down convert" it into low voltage and high current for the battery bank. The MPPT controllers have a computer inside that keeps track of the Pmp=Vmp*Imp equation and current power output--and adjust Imp/Vmp up/down to for Pmax (Vmp changes. with temperature, load, and light levels).

In the end, it is very difficult to start with a small system and "grow" it as your $$$ allows... It is much better to define how much power you need, and then figure out how to generate it.

If you have grid power--it is almost always better to go with a Grid Tied system (no batteries, just solar panels and a GT inverter tied to your utility lines). It is the most efficient, cost effective, and least maintenance of any solar based system.

There are off-grid systems (no utility power nearby) and hybrid systems (grid tied for 95% of the time, and the other 5%, they can run off grid with solar panels and a battery bank)... But none of these types of systems is "cheap and easy".

But, before you start with solar, spend your time and money conserving power throughout your home... It is almost always better to conserve first than to spend your money on a solar power system before you have done extreme conservation.

-Bill

icarus

Re: Understanding the Solar Lingo

It always surprises me people buy huge (5kw) inverters when they only have small batteries and small panels.I seems intuitive that an inverter that power 5000 watts how does one expect to do that with 500 watts of panels, and 1200 wh of battery? Do people think there is some magic formula that a big inverter will magically make power?

Tony

No offense intended to the OP, or anyone else for that matter,, but it does seem to be a recurring theme.

niel

Re: Understanding the Solar Lingo

icarus wrote: »

It always surprises me people buy huge (5kw) inverters when they only have small batteries and small panels.I seems intuitive that an inverter that power 5000 watts how does one expect to do that with 500 watts of panels, and 1200 wh of battery? Do people think there is some magic formula that a big inverter will magically make power?

Tony

No offense intended to the OP, or anyone else for that matter,, but it does seem to be a recurring theme.

tony,
it's human nature. they know larger numbers are better. the problem is that's the cheapest of the larger numbers they can buy. it doesn't make it right as things need to be addressed for needs and to come within a proper balance of components that will fulfill those needs. it's an offshoot of the ready, fire, aim syndrome.

shmockey

Re: Understanding the Solar Lingo

Thank you BB that is helpfull so that I can continue to figure out just what I am going to need to go with what I am able to buy right now. I have looked at the MPPT's and I think that is what I am going with, I don't have a problem with spending more right now on the right things and getting fewer so that I can upgrade in the future. Am I wrong with thinking that a good MPPT with a few batteries and a few panels is a good way to start so that I could add more batteries and panels later, as I would allready have the controler to hadle it?

Now as for the whole "5000 watt inverter" thing. Once again I have it because my father no longer needs it and I was talking to him about them since he used them in his truck to run his microwave, dorm size fridge, computer (laptop) and coffee machine. I told him I was looking at maybe a 600 to 1200, and he told me that he didnt need his anymore. That is why I got it, not because I was looking for the largest thing I could get. I am on a budget here to do this, I will grow the system as I can. Its not ready, aim and fire it is what can I afford at this time, and free is a great price.

Thanks to all for the imput.

Cariboocoot

Re: Understanding the Solar Lingo

An MPPT controller to start with is a good idea, but they are expensive vs. a PWM type. From our host's site:
http://store.solar-electric.com/mochco.html
60 Amp Tristar PWM is $200. 60 Amp Morningstar MPPT is over $500.
Either of these will allow you to triple your PV size, and there is a 'harvest advantage' to the MPPT. Is it worth the money? Only you can decide that.

I understand the inverter is an inherited item. Good! Saves money and it will produce power. Just don't expect to run it at capacity. It probably isn't pure sine, though; a 12V 5kW sine wave inverter is a rarity to say the least (can't think of any manufacturer). In which case, be prepared for the MSW disadvantages: AC induction motors will not run properly on it, some CFL's won't either, as well as certain battery chargers, et cetera. If you have a lot of things like that which won't run on MSW, you might consider saving the money from the MPPT and buying an inexpensive pure sine inverter like one of these: http://store.solar-electric.com/sasiwain1.html

Keep in mind that the problem here isn't the 5kW inverter per se, it's that there won't be enough DC capacity behind it to supply its full output for any significant length of time. Your Dad's microwave, 'frige, and coffee machine all drew large amounts of power (rough guess: 1000 Watts for microwave, 900 Watts 'start up' on the 'frige, maybe 200 Watts running, probably 1200-1500 Watts for the coffee machine). But he had an engine recharged DC source with a really big battery capacity (diesel truck), right?

Getting your loads measured with the Kill-A-Watt is the best way to start!

BB.

Re: Understanding the Solar Lingo

There are 15 amp MPPT controllers to 60-80 amp MPPT controllers. A pretty wide range.

In general, solar PV power is very expensive. And so you are looking for conservation every where... Not only the traditional conservation of loads (efficiency, turn off when not used, etc.)--But you are also looking for conservation with your solar power equipment too...

PWM controllers and smaller MPPT controllers have lower operational losses than the big 60+ amp controllers... If you want to buy a 60 amp MPPT controller for eventual expansion, but only have 200 watts of solar panels--the Big Controller will probably give you less power to your battery because of the losses inside the larger controller to operate is electronics and power electronics. So, for example, you would probably want 400 watts of panels minimum if getting a larger MPPT controller.

Regarding the inverter... Even a smaller 300 watt TSW inverter may use around 6 watts just outputting 120 VAC with no load... A large 5,000 watt inverter may take 20 watts or more (depending on brand/model) before your first light is connected...

Anyway, I would suggest you decide how you want to go from here...

Say you want backup/emergency power.

Start with a battery bank, AC charge controller, inverter sized to your loads (1,200-1,500 watt minimum if you want to run a typical 17-20 cuft Energy Star Refrigerator). And get a nice generator to keep the batteries charged during an outage (say a Honda eu2000i or similar).

Note on the genset, this will allow to you charge your battery bank during the day and run some lights, fridge at night. Also, a smaller inverter/generator type (like the Honda eu family) are very fuel efficient at lighter electrical loads (down to 400 watts or less for the eu2000i). Storing/obtaining fuel during an long term outage is a huge nightmare during a wide spread power outage (say ice storms, hurricane, etc.).

Now you have your system sized and capable of fulfilling a needed function--Next, you can add solar panels + Charge Controller as funds permit.

Just an idea of how I would approach the issue--customize for your needs.

-Bill

dwh

Re: Understanding the Solar Lingo

Cariboocoot wrote: »

You should start by defining what you're trying to do.

That's the standard line around here, and for most people, who are trying to set up off-grid or grid-tie or power a boat or RV or whatnot, that's the best advice.

But I think it ignores the hobbyist.

Q: What are you trying to do?
A: Goof around with solar.

To the OP:

Take a look at this site from one of the forum members, 2manytoyz. He started out goofing around with the Harbor Freight Kit, and just keeps adding on to his system:

http://2manytoyz.com/

Cariboocoot

Re: Understanding the Solar Lingo

"Goof around with solar" is a perfectly legitimate answer to the question, dwh!

Only we'll call it "learn about solar" just so everyone feels good about themselves.

But really it does make a difference for any advice given.