question

longbedbob avatar image
longbedbob asked

PWM reality vs expected MPPT production for RV.

Hi, gang.


I'm excited to report that the first array on my fifth wheel camper is finished. But I am not pleased with the results.

The system is a "name brand" product with four 100w x 12v panels in series parallel with their 30A PWM charge controller. I'm using the Victron battery shunt for monitoring battery output, but using the controller's built in bluetooth to access the actual charging data from the panels.


Please follow along and double check my math.


The panels are mounted flat. The sky was clear and all four panels were in max sun at mid day, which reached an elevation of 62 degrees.


The system generated only 255 watts. That's just 64% efficient. To say that I'm disappointed would be an understatement.


I read that PWM charge controllers are 75-80% efficient. But I wasn't satisfied with the answer. There's still some watts missing.


Taking the sine of 62 degrees (or the cosine of 28) and multiplying by 100, I arrive at 88 watts. So, my theoretical maximum production as they lay on the roof is a combined 352 watts. Assuming a 75% efficient conversion for the charge controller, I'm only 10 watts away from my actual production.


Using the 80% figure and factoring in the temperature coefficient and some cabling loses and this satisfies my OCD.


Are my assumptions in the right direction? If not, I might have some other issues to address. Perhaps the charge controller is sensing the lead acid battery's charging inefficiencies and it is effectively lowering the displayed power??


Moving forward, I want to install two and possibly three 24v panels at 200 watts each and also arranged in parallel. They will be mounted on tiltable racks. I'm eyeing the Victron SmartSolar 100/50. I also plan on running AWG 8 from beginning to end to minimize wiring loss.


Assuming the new array is tilted towards the sun, what sort of overall efficiency/production can I realistically expect with the new MPPT controller?





MPPT SmartSolar
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6 Answers
wkirby avatar image
wkirby answered ·

I can tell you my results for today. Sun angle, for my location, at mid day, is about 46° at this time of year. Today's mid day temperature 23°C. Partly cloudy conditions, mostly sunny.

Array 1: 45° tilt, SSW direction, 1000W, 8 years old, Victron MPPT 150/70.
Today's maximum recorded output was 1097W. 4.40KWh

Array 2: 45° tilt, SSW direction, 1100W, 4years old, Victron MPPT 150/70.
Today's maximum recorded output was 1165W. Yield: 4.52KWh

Array 3: 25° tilt, East direction, 500W, 8 years old, Victron MPPT 150/35.
Today's maximum recorded output was 471W. Yield: 2.16KWh

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Nickus de Vos avatar image
Nickus de Vos answered ·

How full was your batteries and was there any load on your system?

Depending on this and with panels lying flat, I don't think you were doing too badly.


Maybe someone else can chime in on what difference it will make if you leave the panels as is, but upgrade to a MPPT controller, I never had a PWM, so I don't know.

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wkirby avatar image wkirby ♦♦ commented ·

I have an ESS installation. When battery is full then energy is diverted into the grid and so PV panels produce their maximum power that conditions allow for the entire day.

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spirou avatar image
spirou answered ·

State of batteries will play a big role here. You won't see max until your loads are bigger than what the panels can produce.

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wkirby avatar image wkirby ♦♦ commented ·

Panels produce maximum that they can all day. When battery is full then backfeed to grid.

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Joshua Warren avatar image Joshua Warren wkirby ♦♦ commented ·

Fairly certain he's not backfeeding anything off a fifth wheel camper.

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spirou avatar image spirou wkirby ♦♦ commented ·

Yep, replying to original post :)

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klim8skeptic avatar image
klim8skeptic answered ·

I dont see any mention of battery (system) voltage, or pv array voltage.

The PWM or Mppt whitepaper explains the differences between charger topologies.

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Charles Gomes avatar image Charles Gomes commented ·

The whitepaper really helps understanding panel limits.

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longbedbob avatar image
longbedbob answered ·

Thanks for the responses.

I think that I have determined why the system is so inefficient. It essentially boils down to the limitations of PWM technology.

Assuming that my batteries were mostly charged at 12.5 Volts, the panels will drop from Vmp (17.9V) to match the battery voltage. Assuming an optimal current of 5.72 amps, the panels will only charge at a theoretical maximum of ~72 watts. Is this correct?

An MPPT controller would utilize those missing 5 volts and create more charge current and, therefore, yield more power and efficiency.

These panels are already semi permanently installed. Also, shading prevents a series arrangement. The only modification that I think I can reasonably make is to switch types of charge controllers.

Would replacing the PWM controller with, say, the Victron 100/50 see any gains? I ask because I understand that they only start working at ~ 5V above battery voltage. They make sense for 24V or more, but I don't know how well it would function in a 12V system.

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Charles Gomes avatar image
Charles Gomes answered ·

Wondering if you got better numbers. I'm designing a similar system and wanted to ensure I'm not making it sub-optimal when putting 5 panels in parallel.

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