I was going to post this in the xantrex part of the solar guppy forum but it is more a general mppt question rather than xantrex.

My battery bank will be 48volts I will be getting Evergreen 190watt panels.
Specs are http://www.evergreensolar.com/upload/pdf/us/S190_US_010407.pdf

My question is what is the best voltage for my array to be wired. I am basically thinking it comes down to efficiency, but I would think a high voltage low amp is more efficient almost always but maybe there is a sweet spot.

It is better to go with max voltage or just a little bit above battery charging voltages. For example on 48v battery bank, for the panels should I aim for mid 60's volts for my panels, 90's or maybe 120's.
I know I really have to check the charge controller the xantrex xw in this situation, well atleast for the 120's voltage. As it would be a close call on the Voc numbers. My guess the 90's voltage range would be best in this case.
I would imagine there are a fair number of things varying with the changing voltage array. Just basically which is the most appropriate value to run.
Keep in Mind my array will not start the days off lower than 60degrees F. With most of common day temps. 80's-mid 90's. SO I am never going to out perform specs. But hopefully I do not fall to short of them either.

although these are 24v pvs, they are not optimal for charging at 24v so you just couldn't say 2 in series will do it because most lead acid batteries will need charge voltages higher than the vmp will go for 2 seriesed 190s for use in a 48v battery system. because of that you will need minimumly 3 in series yielding a total vmp of 80.1v. now you can wire more pvs in series if you would like to any mppt controller like the xw60 or mx60 types, but you must observe that the open circuit voltages of the string do not exceed the limits the controller was meant for and temperature does affect the voltage output of any pv so that has to be accounted for as well. efficiency in the controller goes down slightly with higher voltages, but wire losses decrease with higher voltages. my choice would be strings of 3 190s in series.

All charge controller are less efficient as the voltages incease, its a technical thing called switching losses.

The Evergreens are 18V nomial panels, not 24V ... so in moderate climate you need a minimum of three in series to charge a 48V battery bank.

Since your installation is a warm/hot climate, I would recommend 4 in series, which which will be a vmp of about 84 volts in hot weather. I would NOT go any higher ( more than 4 panels in a series string.

just so there's no confusion here i was stating from a vmp standpoint and not a norm of operation standpoint for it being 24v. sg is correct that it is designed for 18v operations much like a pv with a vmp of 17v is a 12v pv. the vmp for the record on the 190 is 26.7v and does not change or dissagree with our determinations in the numbers of pvs in a string for you.