How effective are your panels?

[rogher]

I have been trying to discover if solar panels 'max-out' or not.
By 'max-out' I mean that they reach a maximum output before the sun is perpendicular to the panels.
As the newer inverters can provide a detailed history of power harvested, I wondered if those who have them could share their findings.

I'd like to know what your maximum rate of harvesting power is, for what capacity array, and for how long on a 'good' day that has been achieved.
Does your rate of input flatten-out for much of the day or does it rise, then fall (never maxing-out)?

I am trying to determine over what angle of incidence a set of panels can produce full power and, if there is a maximum output, how the output compares with the claimed performance of the panels.
So a 4Kw set of panels may be able to produce 3.6Kw for 2hrs on a day when conditions are most favourable (high Summer), showing that the rays do not need to be perpendicular to the panels to get maximum performance and the max measured performance is 90% of the specification.

 

[DThames]

PVWatts is remarkably accurate based on my system's production. Put in the total panel watts, the angle, azimuth and boom, you have a yearly number. You can play with the angle and see the results. As far as solar angle at any given moment in the day, the output is very subject to sky, ambient temp, and wind conditions. The sort answer is I can get good power about 3 hours per day with a fixed array. I am in Arkansas. By "good power" I mean a fairly high percent of total system capacity.

Use PVWatts to size your system and don't spend too much time in the details of the panel's rating.

edit..I might add, my statement above about PVWatts, my raw production numbers line up with predictions. I have some losses getting into the grid meter. You can increase the PVWatts losses estimate, if you know you have specific additional losses. I have a 250 foot run back to my main breaker panel and I size the wire on the min limit, so I have 2% or so lost going back to my main meter.

 

[rogher]

Thanks for getting back so quickly.
I've not looked at PVWatts but assume that this gives theoretical power available for your latitude and angle of panel.
I have read claims that panels are capable of 'miraculous' things (like achieving maximum power when the sun is not perpendicular to the panels or even behind cloud or partly shaded.
I find that difficult to believe but suddenly thought that, if you could record the power harvested real-time, you might be able to do as I described: see if the power cuts-off/maxes-out for the best part of the day and also compare the measured output with claimed potential.
I'm not too bothered about power loss but am interested to know if maximum power output is achievable before the sun is perpendicular to the panels.
I rather suspect that graphs of power vs time will rise to a peak (below max theoretical) and fall again in a bell-shape.

 

[DThames]

Thanks for getting back so quickly.
I've not looked at PVWatts but assume that this gives theoretical power available for your latitude and angle of panel.
I have read claims that panels are capable of 'miraculous' things (like achieving maximum power when the sun is not perpendicular to the panels or even behind cloud or partly shaded.
I find that difficult to believe but suddenly thought that, if you could record the power harvested real-time, you might be able to do as I described: see if the power cuts-off/maxes-out for the best part of the day and also compare the measured output with claimed potential.
I'm not too bothered about power loss but am interested to know if maximum power output is achievable before the sun is perpendicular to the panels.
I rather suspect that graphs of power vs time will rise to a peak (below max theoretical) and fall again in a bell-shape.

My observations is that my peak power is possible about 90 minutes before and after solar noon. It takes cool and windy conditions for that to happen. I would state that my observed "peak" is still less than the panel's rated power. I have 340watt panels and I see 325w from time to time if the panels are cool enough.

 

[rogher]

That does suggest that panels 'max-out' if you get 3hr max performance (90min + 90min).

In the past I was interested in tracking systems but, if panels max-out for a significant arc of radiation, the need to follow the sun reduces significantly.

 

[DThames]

This is what a good day looks like

 

[Slvrsky07]

It should be a nice bell curve like the previous post. At the solar max, when the sun is at its highest point, my panels are within 2-3 watts of their max advertised value. Now they are really new but as they get older it will be less and less until its lowest rated value which is usually 80-90% of the new wattage per the spec sheet. This is around 25 years it says.

 

[Ampster]

I have owned four or five systems over the years and my focus has been to maximize to power that is harvested each day. To do that the orientation of my panels will be in several directions, so some arrays reach their own peak at different times of the day.

 

[Delmar]

Once my 4 x 100 watt HQST panels actually output 400 watts. Was a Texas cold morning in the mid-40sF, full sun and a decent breeze to chill my golf cart panels.

In the usual Texas heat I never saw more than 300-320 watts.

 

[rogher]

This is what a good day looks like

Is that a real-life measurement?
- or the result of a theoretical calculation?

It supports my view but, if maxing-out can happen (maybe with newer technology panels?), the curve should be capped at the max power that the panel(s) can provide.

 

[Ampster]

the curve should be capped at the max power that the panel(s) can provide

Many systems are designed around a DC to AC ratio of 1.25 to 1, so that the inverter is often the device that maxes out. As I mentioned earlier, that in many circumstances results in optimum kWh production per day. The panels are limited by their orientation and tilt whereas a system can be optimized by combining other components to optimize kWhs harvested from a given array.

 

[rogher]

That's a good point about the inverter maxing-out. Maybe that's what was being referred to when I first heard the term.

When I was considering a tracking system I concluded that it might be easier to simply increase the panel area and not attempt 'perpendicularity'.
And that's what inspired my interest in yield vs angle of incidence.

 

[GLC]

Best sun, best angle, best temp-panels at 74% efficiency.

 

[DThames]

That's a good point about the inverter maxing-out. Maybe that's what was being referred to when I first heard the term.

When I was considering a tracking system I concluded that it might be easier to simply increase the panel area and not attempt 'perpendicularity'.
And that's what inspired my interest in yield vs angle of incidence.

Tilt adjustment over the year will get you from something like 71-72% up to about 74%. Two axis real time tracking will approach 100% (for given sky/sun conditions). If you have the space, an extra panel normally trumps changing the tilt 2 or 4 times a year. As previously stated, having more DC PV watts rated than AC inverter rated watts is important if you want production to be up. You almost never hit the panels rating and if you do, it is for only a short time of the day. If you are "over paneled" you can get good production on less than ideal days.

 

[Ampster]

As previously stated, having more DC PV watts rated than AC inverter rated watts is important if you want production to be up. You almost never hit the panels rating and if you do, it is for only a short time of the day. If you are "over paneled" you can get good production on less than ideal days.

Yes that is the reason so many systems are configured with DC to AC ratios greater than 1 to 1.

 

[rogher]

Surely the underrating of the inverter only benefits the capital investment?
An inverter with greater capacity might have slightly higher losses but should harvest slightly more overall when the smaller one maxes-out.

 

[Ampster]

Surely the underrating of the inverter only benefits the capital investment?

My point was that we are talking about a system who purpose is to harvest energy from the sun. All the components should work together to create a result. Optimum results are obtained by designing the component mix appropriately.

 

[Slvrsky07]

One thing I was told, not sure how true or not, is that a string inverter needs to almost maxed out for peak efficiency. Now is that worth clipping? Not sure if I believe that.

 

[Ampster]

Now is that worth clipping? Not sure if I believe that.

Clipping is mis understood. It often looks worse than it calculates out. Anybody can configure a larger inverter and never see the wasted capacity.

 

[Slvrsky07]

Can you help me understand why it would be better to clip than have a similar size inverter to array? As I see it, for example if you have a 7.5kw array with either a 8kw inverter you would get more energy output than having the same array with a smaller inverter. Attached is an example of how I understand it works with the same solar bell curve and 2 sizes of inverter. If I am wrong in thinking this way, please correct me.