I use one of these:
http://www.harborfreight.com/cpi/cta...emnumber=44768

View the product manual for details.

At 1.5 watt it keeps a small motorcycle battery charged indefinately
for powing a typical instrument panel with radio and audio varios etc.

a 5 watt panel might be too much and will need a good charge reulater
so as not to cook the battery.

It is only 14" x 6"x7/8" and I mount it near the nose under the canopy
so as not to cause glare and is out of my sight lines.

Typically on sale for $12 every couple months.

It has a nice little remote flashing light to show that it is charging
and is regulated so as not to cook the battery.

Ray