Actual charging watts - compared to- Panel rated Watts

[Jim Mac]

Hi,

I want to hear peoples responses to what their actual charging watts are in full sun light compared to advertised panel capabilities..

I finally have shunts and monitoring equipment hooked up, and here is what I am seeing..

I have 4 panels advertised at 120 watts each. Total of 480 watts according to the manufacturer.. When they are in full sunlight charging my 24V battery bank, I see about 380 watts...

I suppose I need to do some more math in respect to wire resistance. But does anyone have other data to share?

[Rob Beckers]

Hi Jim,

What kind of charge controller? Is this an MPPT (maximum power point tracking) or just regular series controller? Reason I'm asking is that a regular controller (non-MPPT) will essentially just connect those panels directly to the batteries. That is inherently inefficient, since the panel voltage will be tied to the battery voltage (which is not the same as the panel voltage for maximum power output of a panel). How efficient this works depends on the modules, how well they fit the battery voltage, and the state of charge of the batteries, since the latter determines the voltage.

Having said that, even for an MPPT charge controller it's relatively rare to see panels reach their rated power. About the only times that happens is in winter, when it's really cold, with snow on the ground to reflect the light, and you have a really clear sky. Even on days like today, around 2C outside, the modules on my roof are already above 25C. Their voltage shows the cells are around 30C inside those modules (sun is not that strong today either). To give you an idea of inefficiencies; I use a derating factor of 0.75 to account for module losses due to temperature, dust, wiring losses, inverter losses etc. On an annual basis that seems to be just about the right number.

For battery based systems without an MPPT controller I use 0.60x as the derating factor for system sizing. This would be to calculate energy stored by the batteries, so it takes battery inefficiency into account too. How close to reality that factor is I don't know (yet); I have far less experience with off-grid systems vs. grid-tie.

-RoB-

[Joe Blake]

Hi Jim,

There are so many factors involved it's difficult to know where to start.

I've got a "grid-feed" array of 8 panels which is supposed to give a maximum of 1650 watts, and usually the inverter power reading tells me that they're delivering (roughly) about 12-1300 watts on a bright day. Starting in October (2010) they were generating 8-9 kWh/day (on average) then come end of December it hits 10 kWh/day and stays there barring overcast or rainy days. Then around about mid Feb it starts going down again and in Mid March it's averaging about 8 kWh/day. This is purely a function of the height of the sun in the sky and angle the panels meet the sun. If I was able to adjust this angle day by day I'm sure I'd get much better figures, but I can't do it, so I just live with it.

However, I DO have a separate, stand-alone system which charges 150 Amp Hour of batteries, with two fixed panels of 64 watts each (ie fixed in the sense that I only move them twice a year to change their angle, but also a smaller portable unit (120 watts) which I can manoeuvre during the day. At midday on a clear day the ammeter show 15 Amps which, assuming the batteries are at 12.5 volts (which the volt meter shows) that gives a tad under 190 watts. However during the day the sun moves and the Amps drop down. I've seen just by changing the smaller panel orientation the Amps go from about 5 Amps up to 9.

Lastly, what many people are surprised to learn is that a clear sky is not necessarily going to give the best generation. A light cloud cover has seen my roof top panels of 1650 watts go up to 1700 watts when a thin cloud moved over the face of the sun.

Most figures printed tend to give the most optimistic view on power generated, but if you read the fine print the guarantee may say that the panel will generate a maximum of X watts for Y years. But X will not be the maximum.

The guy who installed my rooftop system told me that when the panels are new they can be expected to generate MORE than their rated maximum, but this drops off after moderate use, so they can generate the rated max (under optimal conditions - for optimal read "rare").

Nothing's simple, is it? :blink1:

Joe

[Julian Egelstaff]

Yeah, this isn't simple is it.

We have a 3.15 kW array (DC). And a 2.8 kW inverter (AC). But the inverter can accept some "overpower" up to about 3.03 kW DC. The max the inverter has ever output is 2.87 kW AC.

OK, but when/how do you get the max ratings? Well, you need 1000 W/sq.m of sunlight to get the nameplate capacity, and that's if you're at a perfect angle and orientation, and no shade, etc, etc.

But then there's line loss, and there's also the efficiency of any conversions. If you're charging batteries, then you don't need an inverter I believe? Because it's DC to DC? So your main sources of loss will be related to orientation towards the sun I would guess, and secondarily, related to wiring and connections.

In real life for our grid-tie system, our array, which faces 160 degrees south, and is tilted at 30 degrees, and has no shade, typically produces in the 1.5 to 2.5 kW range, for the middle of the day, in decent weather, depending on the time of year.

On a daily basis, that equals 10.3 kWh a day, averaged over the entire year. I know that for a fact now, since it's been turned on for a whole year.

We have complete production logs and graphs at our website: http://www.yourturn.ca/solar/

So, what it all boils down to, to put it in terms that are comparable across different sized systems...we get about 3,600 kWh a year, from a 3.15 DC array. So, 3600/3.15 = 1193. Therefore, we're getting 1193 kWh per year, for each installed kW of nameplate capacity. This is high for Toronto. Our Power (www.ourpower.ca) will quote you a lower average figure as what is likely in Toronto. The federal government also has some general figures that shed some light (ha ha) on the situation: https://glfc.cfsnet.nfis.org/mapserver/pv/municip.php?n=1752&NEK=e

Our number will fluctuate and generally trend down as the array ages.

As for what this says about "nameplate" capacity vs. real world production...well, obviously panels fluctuate in how much power they will give you, and the trick is knowing how much direct sun you're likely to get in your area, and also making sure your array is oriented as well as possible. Everyone's mileage is going to vary on this question by a fair bit.

--Julian

[Dale Sheler]

It sounds to me like the most direct and simple answer to the question "What should my panels put out?" Is simply to read what the meter says at the end of a year, that number is what your panels make in your particular installation in your particular area, and even then that number will almost certainly differ from year to year, so, basically, what your getting is what your getting.

[Rob Beckers]

It sounds to me like the most direct and simple answer to the question "What should my panels put out?" Is simply to read what the meter says at the end of a year, that number is what your panels make in your particular installation in your particular area, and even then that number will almost certainly differ from year to year, so, basically, what your getting is what your getting.

Well, yeah, but that doesn't help people who are thinking of installing PV and need to know what it will produce in kWh's and revenue. Nor does it help those that want to find out if their array is working as it should.

It also discounts the fact that solar PV is very predictable over the long term.

Easiest way to find out is to run PVWatts (click this link) (http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/). Select Canada, and the nearest city on the list. Change the settings to reflect the array size, orientation, and tilt. Then make sure to set the efficiency to 75%. Let it rip, and that number should be pretty close to what you'll get on an average year. The first few years will likely produce a bit more, the last few years of the 20-year contract will likely be a little less, overall it should even out pretty well. This is for an unshaded array. For shading I can also tell you what it will do, but that takes quite a bit more work.

-RoB-

[Jim Mac]

Some great advice here...

Rob, I do not think it is an MPPT controller. It uses 4 relays that tie together in parallel and the solar hot wire connects right to the relay leads. It's a cheap ColemanAir that they say can handle 3000 watts

After I wrote the post we had a little snow with sun and I saw the watts just about reach manufacture ratings. (20 watts under).

It seems it can get very confusing and very expensive to add all the bells and whistles. So I went ahead and purchased 8 more panels @ 120 watts each, giving the solar system a max rating of 1440 watts.