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I have two solar panels, a Sanyo HIT 240HDE4 (35.5 V and 6.77 A at max power) and a Panasonic (SANYO) HIT VBHN245SJ25 (44.3 V and 5.54 A at max power).

These panels are mismatched in both voltage and current, so aren't ideally suited to being connected either in series or parallel.

Can I add a voltage regulator/converter to one of these panels to bring the voltage in line with the other, therefore allowing me to connect them in parallel?

For example, if I bring the 240 W panel up to 44.3 V.

Then I can connect both to my MPPT controller and not waste energy.

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While it is possible to transform the output of either panel to match the other, the cost of doing so and the gains made are such that it is unlikely to be worthwhile. The characteristics of your MPPT converter and your wiring length and diameter (and so resistance) may have a greater bearing. Placing them in series about halves the effective current compared to parallel connection so reduces wiring copper loss to 25% for the same wiring. Some converters are limited in voltage range they will accept and series connection may not be an option.

If you do not yet have the MPPT converter then using a per panel microinverter to produce mains AC at the panel may be the best solution. This allows combining outputs directly and the high voltage reduces wiring losses. Some microinverters will tolerate the voltage of either of your panels and others will work only with one or the other. Note that the MPPT Vin range and maximum Vin range may not match and it is possible to have an inverter operate but not offer MPPT.

If you already have the inverter the a better solution may be to change the relative mounting angles to gain a larger "spread" of power during the day or year and thereby obtain more energy overall.

The voltage mismatch is 35.5/44.3 = 80.1%
The current mismatch is 5.54/6.77 = 81.8%

These values are so close for practical purposes that series or parallel connection is about as good power wise. Voltage and power have a negative change with increasing temperature and current usually has a small positive change but this affects both panels is not going to be enough to make a useful difference.

You could use a converter to up or down scale the voltage from either panel but unless this was done with an MPPT converter you would probably lose most of the 20% difference in the converter. If this was done with a 2nd MPPT converter you would be better off using the two converters to drive the common load. Depending on what the load was the two converters may "fight" so this needs to be done with due care.

Possibly the best practical solution is to arrange the panels so that one is pointed "suboptimally" so that it receives more sun at a period in the day or season than it would when optimally pointed? If the panels are fixed in position then the angle change could either be seasonally related so the off-angle panel was optimised for e.g. Autumn rather than Summer, or daily related so it got maximum insolation either earlier or later in the say than would usually be optimum.

If skewing panels I'd expect at 1st glance that placing them in series and deoptimising the 35.5 V/6.77 A panel would work best. (Working through the options MAY produce a different result). If the 6.77 A panels was off angled so it made about 5.5 A at solar noon in optimum conditions then it would make more output at some other time. The disadvantage of this is that if you e.g. optimise for morning then you get even less in the afternoon and so on. SO some half optimised result may work best overall.

One could spend many fascinating hours trying to get this right - and its probably not very valuable. Both panels are rated at about 240 Watts. If you lose 20% of one it's about 50 Watts loss peak or 10% of total max power, and less overall. You could add an extra 50 Watt panel of suitable rating or just keep them cleaner than most people usually do, or arrange so they can be adjustably tilted slightly seasonally and change the angle a few times a year.
(Daily angle changes are much more effort intensive.)

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