Solar panel voltage drops when connected to load

Question

I have a 100 W solar panel with these specifications:

• Optimum operating voltage = 18.1 V
• Optimum operating current = 5.52 A
• Voc = 22.1 V
• Isc = 5.86 A.

I connected this solar panel directly to a 100 W microinverter and connected a 33 W lamp to the inverter.

The voltage from the solar panels dropped from 19 V to 8 V.

Did something go wrong with the solar panel?

Answer 1

Your panel is very probably fine (no guarantees, though).

A solar panel is roughly a current source over most of its V/I characteristic, not a voltage source.

So, the voltage you see across it depends on the impedance of the load that is connected (or the voltage of the battery that is connected); it isn't set by the solar panel itself.

The impedance of the load you have is pulling the solar panel's voltage down to 8 V, but the solar panel still delivers about 5 A under full sun, or about 40 W, which is all the power it needs to deliver for your lamp; it just isn't sitting at its Maximum Power Point, where it could and would deliver more (and, in this case, too much) power.

Here's a (measured) example of a 3 W load (a DC/DC converter generating 5V, with a loaded output) connected to a nominally 12 V, 10 W solar panel under full sun:

^{(Image source: me)}

The areas of the coloured rectangles are V·I, so they represent power. The connected load's impedance makes the panel source about 0.6 A, delivering much less power than it would with a load that makes it source 0.57 A.

You can see how the solar panel's voltage drops to 5 V while still delivering all the power needed for this particular load.

Answer 2

I know this is old. I am writing this in sympathy to the OP, and others with similar issues.

I have a similar problem, but I think it is to be expected. Unfortunately, I was not expecting this from anything I read, so I am sharing an example of a real-life instance. (If this is NOT normal, of course, I would LOVE to hear how to fix it.)

I have a grid-tied solar array with 16 panels arranged in 2 strings with 8 panels each, using a Growatt Min 6000-TL-X string inverter. This is grid-tied with no batteries, and I can't fiddle with MPPT.

The panels are 365 W producing a theoretical total of 5840 W, well below the maximum for the inverter. I never get anything close. I get around 3800 W peak at midday.

I_mp is 10.62 A and I_sc is 11.26 A per panel. V_mp is 34.4 V and V_oc is 41.4 V per panel.

I only recently completed it and turned it on. I get good irradiance with no shadows.

When measured with no load (disconnected from the grid) I get 290 to 304 V per string to the inverter, that's 290/8 = 36.25 V. This is higher than V_mp but lower than V_oc per panel.

When measured under load, the current increases with the expected curve but never gets to 10.62 or 11.26 A. I expected current to vary, and accept that the current is probably appropriate for my angle and time of year.

I expected the voltage to behave differently than it does. When measured under load the voltage varies throughout the day. It never gets to the no-load measurement. It actually goes DOWN when irradiance and current increases. It does not follow the current curve, but starts high, dips low, rises again, and more or less levels out, slowly decreasing until the end of the day when it rises again and bobs around a bit. I assume this is a function of grid load and current affecting voltage.

Although both strings should be equal, one gets to 272 V during morning hours with little current, and then sags to 250 V, sometimes as low as 224 V. The second string stays below 250 V and sometimes goes below 200 V.

I have a "shine" monitor that produces nice graphic outputs. The current and power curves look reasonable (although lower than expected). The voltage is not a nice curve, and the odd shape was not expected, but I have never seen other people's curves, so I was NOT expecting this. If you are also new to this, and not expecting such odd looking results from your voltage measurements, at least now you know you are not alone, and this is probably normal.

Answer 3

No, that just means the lamp is using more power than the panels are providing. You'll only get 100W out of the panels at "optimum" conditions.

Answer 4

1. Your solar panels are rated to provide 100 watts under optimum conditions.
2. Under optimum conditions and no load, your panels will have a voltage of 22.1 volts.
3. With no load, you say the voltage is 19 volts - that means your solar panels are not getting full sunlight to produce 100 watts.
4. The inverter will waste a good bit of power in converting the DC from the solar panels to AC. It would not be surprising if the inverter wasted as much power as it puts out - your 33 watt lamp would then require 66 watts from the solar panels.

Solar panels do not provide a fixed voltage and current. They convert a certain percentage of light to power. Less light means less power. Less power means lower voltage and current.

Your solar panels aren't getting enough light, and your load needs more power than you think it does.