Is a PV panel equally efficient during all sun hours?

[Feisal]

When a PV array is set up to true south and sloped correctly at solar noon so the sun irradiation is perpendicular at it, does that mean the array will give out equal voltage during the presumed sun hours (5 hrs in my case)?

[solarix]

There are several factors that go into figuring what a PV panel will produce. You can think of a PV panel as a photon (packet of light energy) to electron (bit of electrical energy) converter. Photons in = electrons out, with most panels at typical efficiency give you 15watts of electrical energy for every watt of light energy that hits it. This is affected by heat - with the panel becoming less efficient the hotter it gets (your panel spec sheet should show a temp coefficient). This is all happens at roughly a consistent voltage with primarily the current (amps) changing as the quantity of photons hitting the panel changes. So yes, the voltage stays fairly equal during your 5 hours, but the current varies greatly. As the sun angle to the panel varies from perpendicular, the effective surface area hit by the sunshine decreases proportional to the cosine of the sun angle. If you are not up on trigonometry, that means since the sun moves 15 degrees through the sky per hour, 2 hours past noon the cosine of 30 degrees is 87%, 3 hours past noon is 45 degrees or 71%, 4 hours past noon is 60degrees or 50% and it goes down fast after that. So yes, the efficiency doesn't change with sun angle, but the productivity does as the panel is getting less sun.

Just because the PV panel is illuminated, some people mistakenly think that it should be putting out full power. PV panel output is a function of how much sun is hitting it. When the sun is straight on from the panel you get max power, when the sun is at an angle you get proportionately less. I see some (too many) solar installations where the array is fine for the middle of the day, but positioned where obstructions will shade it at other times. (Sorry if I'm over emphasizing this, but I find many people just don't get this)

[Feisal]

..So yes, the voltage stays fairly equal during your 5 hours, but the current varies greatly.....)

Thanks solarix for your detailed explanation.
Let me see if I understood you: battery charging wise, since W=V*I, watts & volts are fairly constant but current is fluctuating, so the panels don't charge the battery equally during my 5 sun hours, am I right?

[solarix]

yup, the charging curve (on a clear day) is pretty close to a parabola. you will only see max power for an hour or so around noon and the 6 hours between 9am and 3pm produce the great bulk of your energy - about 81%. And that is not counting the effect of sunlight glancing off the PV glass as the sun gets lower. PV panels all have anti-reflective coatings and surface textures to try and minimize that problem though. W=V * I so if the voltage (V) is constant, as the current (I) changes so does the power (W).

[Feisal]

yup, the charging curve (on a clear day) is pretty close to a parabola. you will only see max power for an hour or so around noon and the 6 hours between 9am and 3pm produce the great bulk of your energy - about 81%. And that is not counting the effect of sunlight glancing off the PV glass as the sun gets lower. PV panels all have anti-reflective coatings and surface textures to try and minimize that problem though. W=V * I so if the voltage (V) is constant, as the current (I) changes so does the power (W).

So many thanks solarix.

[J.P.M.]

There are several factors that go into figuring what a PV panel will produce. You can think of a PV panel as a photon (packet of light energy) to electron (bit of electrical energy) converter. Photons in = electrons out, with most panels at typical efficiency give you 15watts of electrical energy for every watt of light energy that hits it. This is affected by heat - with the panel becoming less efficient the hotter it gets (your panel spec sheet should show a temp coefficient). This is all happens at roughly a consistent voltage with primarily the current (amps) changing as the quantity of photons hitting the panel changes. So yes, the voltage stays fairly equal during your 5 hours, but the current varies greatly. As the sun angle to the panel varies from perpendicular, the effective surface area hit by the sunshine decreases proportional to the cosine of the sun angle. If you are not up on trigonometry, that means since the sun moves 15 degrees through the sky per hour, 2 hours past noon the cosine of 30 degrees is 87%, 3 hours past noon is 45 degrees or 71%, 4 hours past noon is 60degrees or 50% and it goes down fast after that. So yes, the efficiency doesn't change with sun angle, but the productivity does as the panel is getting less sun.

Just because the PV panel is illuminated, some people mistakenly think that it should be putting out full power. PV panel output is a function of how much sun is hitting it. When the sun is straight on from the panel you get max power, when the sun is at an angle you get proportionately less. I see some (too many) solar installations where the array is fine for the middle of the day, but positioned where obstructions will shade it at other times. (Sorry if I'm over emphasizing this, but I find many people just don't get this)

I'd only add to Solarix' mention of panel voltage and panel temp., and not trying to complicate things: Provided environmental conditions of temp. and wind don't change, the panel voltage at off vertical conditions will change slightly - probably higher due to the panels being slightly cooler because they are not exposed to as much solar radiation. An energy balance on the panel at two different angles of incidence of irradiance is one way to estimate panel temp. and thus estimate a panel voltage at different levels of P.O.A. irradiance.

[sensij]

Adding to what the others have said... panel voltage does vary throughout the day as the panels warm up and cool down, typically a range of 10%-20%. However, during the peak production midday hours the voltage tends to be steady, and the power output is dominated by the current, mostly a function of plane of array irradiance. Here are some charts of a 60 cell 260 W panel on a clear winter day earlier in the week (Amps and Volts in the first chart, Watts and Amps in the second). amps_volts.JPG
power_current.JPG

[Feisal]

Thanks J.P.M. for your elaboration.

[Feisal]

Adding to what the others have said... panel voltage does vary throughout ...

Thanks sensij for the details.

[J.P.M.]

To a first approx., if the other important parameters besides P.O.A. irradiance that affect panel temp. stay constant/mostly unchanged (temp., wind vector), the voltage will usually fluctuate mostly linearly, and inversely, as f(change of P.O.A. irradiance) and will change the least when the change in the P.O.A. irradiance ( dP.O.A/dt) is closest to zero.

Under clear skies, for mostly south facing arrays, that will be around the time of min. incidence angle, +/- something like 45 min to an hour or so on either side of the time of min. incidence angle as Sensij's graphs seem to indicate.

This voltage dependence on temp. is a consequence of increased temp. due increased P.O.A. - more irradiance causes things to get hotter in the sun as explained by a heat balance on the panel or the array. This is not to be confused with increased current as P.O.A. irradiance increases.

[J.P.M.]

Thanks J.P.M. for your elaboration.

You're welcome.

[inetdog]

One more comment on the standard meaning of words used in PV:

You are asking about the behavior during all of the hours that the sun is shining. We can also call that during "daylight hours" in colloquial English.

When calculating the total output of a PV array over the course of the day and relating it to the power (rate of energy production) during the peak time, usually at solar noon, we can create a fictional concept.
We can say that if the panel produces 200W at peak and produces 800Wh (watt-hours) over the ten hour period of daylight then the production is equal to what we would get from 4 hours of peak production. We may use the shorthand of saying that your location gets 4 solar hours or sun hours per day. Totally separate from the ten hours of daylight from sunrise to sunset.

[Feisal]

One more comment on the standard meaning of words used in PV:

You are asking about the behavior during all ......Totally separate from the ten hours of daylight from sunrise to sunset.

Thanks inetdog for the elobration.

[SunEagle]

One more comment on the standard meaning of words used in PV:

You are asking about the behavior during all of the hours that the sun is shining. We can also call that during "daylight hours" in colloquial English.

When calculating the total output of a PV array over the course of the day and relating it to the power (rate of energy production) during the peak time, usually at solar noon, we can create a fictional concept.
We can say that if the panel produces 200W at peak and produces 800Wh (watt-hours) over the ten hour period of daylight then the production is equal to what we would get from 4 hours of peak production. We may use the shorthand of saying that your location gets 4 solar hours or sun hours per day. Totally separate from the ten hours of daylight from sunrise to sunset.

That is a very good and simple way to explain it.