Any recommendations for soft brush for cleaning panels?

As I've written about here over the years, I've done what I believe is some rather extensive and what I also believe is interesting and informed experiments on my array including rates of panel fouling and what works best to effectively clean an array and keep it clean in a practical and workable sense.

Some observations that may be useful with respect to your post:

1.) Hard water spots on a panel are pretty transparent to solar irradiance of the frequencies that PV panels use to generate power.
If it's just the hard water spots (mostly from Group 2A chemicals from the periodic table), they seem to not affect performance in any way I've been able to measure and document by comparing instantaneous array performance over several years both before and after cleaning several hundred times on clear days at times when the solar incidence angle on the array is < 5 degrees.
The human eye is a biased and so poor indicator of solar panel performance as f(panel cleanliness).
I've found that what looks like white, semi-translucent stuff from dried on hard water residue on an array may not matter much in terms of reduced system performance.
2.) Tap water simply hosed over an array at a rate of maybe about 3l/panel without scrubbing/brushing seems to restore about 3/4 or so of the performance lost to fouling that's usually encountered by my array (semi-rural location in zip 92026).
As far as I've been able to measure and document, plain old tap water works as well as anything including distilled or deionized water for routine cleaning on a weekly or monthly basis.
3.) For longer periods between cleaning or for dried on guano or owl skrocks, mild dishwashing detergent ("Dawn", etc.) and tap water with a soft brush seems to get it done for me. Procedure I use is hose with water/soap up with soft brush/rinse and then drip dry.
4.) Squeegeeing after rinsing or washing does not improve my array's performance in any way I've been able to measure.
5.) Not a plug, but the best brushing tool I've found for my array (4 X 4 Sunpower 327's in landscape) when I do soap it down is an 18" wide professional window scrubber from Unger. Washable and replaceable. Big box stores have them. I use the brush with 3 ea. 6' extension sections also from Unger. The brush elements are soft and don't scratch.
6.) During dry season around here, I hose my array with tap water only as described above about every 4 weeks or so or if it doesn't rain significantly, and that method and frequency seems to hold the fouling penalty to an average of maybe 2 or 3% of the clean(unfouled) performance.

I believe tap water works as well as distilled or D.I. water and I believe I've got data to back up that statement.

Take what you want of the above. Scrap the rest.

Thanks for the thoughtful and informative reply. I landed on one of your posts on this topic earlier, and that is why I was not too worried about hard water or squeeging. I was thinking of that exact same Unger pro window scrubber, btw; I have something similar, but mine is only about 14" wide, and 18" would make the job go quicker. Do you have the model with the "integrated Scrub Zone non-abrasive pad" on one side that is alleged to provide extra cleaning power? I would worry that that "non-abrasive pad" may in fact be just abrasive enough to scratch glass, so may not use the feature if I get that brush. Maybe they have an 18" model that is microfiber all the way around rather than the one I am looking at now that reserves one side for the "Scrub Zone." By the way, I live in a rural area, with a lot of wind and dust, so the panels would really appreciate an occasional wash, but in terms of energy generation, I really haven't noticed any fluctuation at all due to fouling (tho I haven't made a careful study of it).

Otherchuck

You're welcome. Just take it FWIW while remembering you get what you pay for (or less).

Which post ? some of them are real epistles.
If I'd have had more time, I'd have written less.

Again, not meant as a plug but the Unger stuff works best for me: What I've used for going on 12 years is Unger model # 961720. I started with the 14" size and quickly shifted to the 18" for the array. I now use the 14" for windows. I don't use the squeegee side on the panels, just the microfiber - which cloth BTW is about as soft as my cat's fur, and also easy to remove from the bar and toss in with the laundry when necessary.
Just use cold water to wash and air dry - the sleeve tends to shrink a bit with heat and because it's a tight fit over the bar it becomes a chore to get back onto the bar - a real PITA.

One other comment: I don't extend the poles to their full distance - they tend to be less rigid allowing the head to flop sideways more than is useful.
I use 4ft. of the 6ft. pole and I use 3 poles, Unger # 972920.

IMO only, the microfiber cloth sleeve is fit for purpose for panel cleaning by itself without the abrasive pad.
I also avoid products such as Unger model # 975520 with the abrasive pad embedded in the microfiber sleeve for this application (however tempting it may be), specifically because of the potential damage that such a pad might do to the anti-reflective coating on most or probably now all PV panel surfaces, including those on my array.

Also, and not that I'm too worried about getting busted by Sunpower or the abrasive police for doing so, but using any abrasive on a panel's surface would give them ample and immediate reason to disallow any claim made against their product. I'm sure any abrasive action would be visible under close inspection. Besides and in the final analysis, most any and all abrasives are unnecessary. Just a little elbow grease 1X/awhile.

Rough (or soft ?) analogy: Treat, or at least clean your panels' glazing like you treat your skin.