I have been building LED lights for a number of years. An early example here:
http://www.fieldlines.com/index.php?topic=131303.0a couple of those were installed in Fiji and were still working to the best of my knowledge up until cyclone Winston took the ceilings they were attached to away this year. An extra one I built still lights my basement kitchen tied directly to a solar panel to this day.
They are driven by a circuit I learned from one of my favorite people, Commanda, which was part of the Master Class Faq posted in the LED lighting section here.
http://www.fieldlines.com/index.php/topic,131217.0.htmlIn this way the current can be limited even with significant changes to supply voltage. A buck converter could be used to feed it if optimum efficiency is a concern.
I tend to drive LED's on lights I have at half of their rated current or less. Heat degrades the phosphor's used in white LED's and changes the color output overtime. Heat is the enemy of longevity in LED's. So if I build something to last I want it to run cool. The case on these is a giant passive heat sink as well so there is nothing to break and no power requirement.
Here is a current smaller iteration from a batch I finished a couple days ago intended as part of a batch I tend to send out as relief supplies.
It uses Cree LED's mounted on what have become standard 20mm Star aluminum bases and is driven at approximately 400mA. At 9-12V supply voltage it runs cool, with current increasing to full at around 12V where it consumes about 4.8W. The light becomes warm to the touch after running for extended time at higher supply voltages. It is intended to be run from a 12V battery, however, it can be connected directly to 12V panels, running at up to 20V, it just will run a bit warmer as the driver will essentially be wasting 3.2W to heat which is dissipated by the case. These are not built to optimize efficiency at higher voltages, but to survive.
Heat is the enemy of longevity in these. Rich