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Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) asked

Solar power too low?

Hi,

I'm looking to solve a general problem with a considerably low solar output from my 3 Solara DCsolar E680M48 170Wp (510Wp in total).

Currently they are wired in series, but all tests have been made also wiring them in parallel. From the modules there goes one 6mm2 wire (3m long) down to a circuit breaker. From the circuit breaker another 6m long 16mm2 wire to a junction box and from there about 1m long 6mm2 into the Smart MPPT 150/35 PV input. All connections have been checked numerous times, cables are checked as well as the circuit breaker for continuity. No significant voltage drop except for what to expect, especially when modules are wired in parallel. Checked all terminals and connections for temperature.

I also checked the modules for Voc and ISC in what I call perfect conditions for my location. 23.44V and 7.81A for all 3 modules, which are even a bit higher than the rated output. Measurement were taken with an ambient temperature of 24°C, cell temperature max. 46°C. Temp.coef. is -0.32% / C°.

However I only can get about 355W out of them when wired in parallel and 370W when wired in series. This is far less (26-27%) than what I would expect, even if I account for more voltage drop or an even worse temp.coef than advertised. Modules are perfectly aligned and tilted for my location.

When I cool the modules down with cold water, output increases, which makes up for temp.coef, but still missing around 100W.

The funny thing is that with low ambient temperatures like we had in March and April, I had no problem to get full output. But there seems to be a point in temperature where everything changes all of the sudden with nothing in between.

I'm not sure everything is still within limits, but it feels like it is not. I've been in contact with Solara and they say that with my Voc and ISC measurements everything is fine and they are out of the game.


Maybe there's someone out here who has an idea where or what to look for.

MPPT ControllersMPPT SmartSolar
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Mike Dorsett avatar image
Mike Dorsett answered ·

If you had full output when the weather was cold, then it looks like the system is fine. power does drop of noticeably when the modules are hot, you have to use the cell temperature and the max power co-efficient to work out what the power under ambient conditions is. Data sheets will give STC - where the cells are operating at 25C, sometimes they also give NOC (normal operating condition) where the cells are hot. In the tropics, cell temperatures tend to run at 55 -60C so we get a lot of power loss from temperature. Also note that high level cloud, haze or other factors can reduce the insolation so you may not be getting the 1000W/m^2 that is the reference power. This changes through the year, and also has an effect on peak power. My system moves from just over 1kW/kW in the peak season to about 650 - 700W/kW at this time of year.

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ commented ·

Thanks for your input @MikeD,

there are no NOTC given in the data sheet.

I have a pretty cheap 'solar meter' which is uncalibrated and most probably showing more irradiance than physically possible on Earth's surface. But I took care that I made all measurements on a very clear day. Of course, not knowing how it looks like in Earth's atmosphere at that time.

I took the math for cell temperatures and added another 10C° (I measured 46C°) to get to 56°C, which I consider hot and everything on the theoretical side:

(56°C - 25°C) x (-0,32 %/ K) x 170 Wp = 31 K x (-0,32 % / K) x 170 Wp corresponds to -9,92 % x 170 Wp = -16,86 Wp for a single module or -50.58 Wp for all three. The power loss I'm experiencing is about 27%. Considerably higher than the 9.92%. I can add another 3 or 4% for cable losses, but this still does not make 27%.

Well, the good things is that we have have around 16 hours of sun during this time of the year and this makes up for 2.5 up to 3.0kW/day. Something I cannot achieve in March or April even with max output.


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Mike Dorsett avatar image
Mike Dorsett answered ·

Hi Stefanie,

You should be able to look up both your solar resource and seasonal variation on line. For the latter, you may need to go into on of the PV system modeling tools e.g. SMA or other. I could do this for you if you can't find one, but would need your lat and long, inclination and azimuth of the array. the seasonal variation in power can be over 50% between peak and low season. Clear day insolation would vary between 700W/m^2 to 1300W/m^2 depending on location and time of year.

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ commented ·

Thanks for the offer, Mike.

I don't have such tool, but can give you details on my location and the array.

Inclination: 35°
Azimuth: 195°
Lat/Lon: N 54.51° / E 9.58°

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ commented ·
@MikeD For some reason you were not tagged about my message below.
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kiwiken answered ·

I had a similar question when I first connected my solar panels. I have 2 strings of 3 panels, 990W per string . I switched the first string on and could see 300 watts coming in to the MPPT. I switched the second string on only to see 320 Watts, thinking something was wrong I checked all cables for polarity and connectivity. Nothing was wrong. The Mppt had throttled back the charging as the batteries were almost full. The next day I saw 1730 Watts coming off the panels.

Maybe your panels are fine and your Mppt is just doing its job

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ commented ·

Hi @kiwiken,

thanks for your thoughts.

I have a lithium system. Nothing is limiting the charge current and I made sure batteries are at a low enough charge state and voltage to allow for basically unlimited charge current.
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Mike Dorsett answered ·

yes, if you don't use the power in the batteries, the yield is down on the maximum. With off grid PV, there is always some curtailment of output, otherwise the proportion of load served by the pv tends to be low. Ive probably got 60% excess pv, and am using about 4.5kWh/day. some days it's as low as 2.5kWh, and the battery stores 6kWh. We are running about 95% on soar, with a small generator to make up the shortfall on cloudy days.

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