In Iraq and Afghanistan, more than half the supply convoys are carrying fuel, which is a huge diversion of resources. It's more than just the cost of the fuel, although that is problem in itself. Although the U.S. Department of Defense buys fuel for about 70 cents a liter ($2.85 a gallon), it price increases five times after you've paid all the transportation costs to get it to a remote base in peacetime. In wartime, as in Afghanistan, that will cost you over $25 a liter if you truck it in, and more than four times that if you deliver it by helicopter. The high cost of ground transport in a combat zone is due to the additional security.
Combat commanders have done the math, and with more combat units out and about these days, they are demanding more solar panels and fuel cells, in order to lower the demand for generator fuel. With all those computers and electronic gadgets out there, the demand for electricity, especially by units in combat, is huge. If each combat battalion can get by with one, or more, fewer fuel convoys a month, that means more troop support from helicopter gunships and UAVs. Moreover, the cost of protecting those fuel convoys makes solar-panel juice cost about the same as the conventional generator electricity. Commanders also found that water purification equipment quickly paid for itself by cutting the amount of water that had to be trucked in.
This has become more of an issue in Afghanistan, where there are more small FOBs (Forward Operating Bases) spread over a larger area. That's because more of the operations in Afghanistan are in rural areas, as opposed to the more urbanized Iraq battle zones. There's plenty of sunlight, and even Special Forces troops on long range patrols will carry flexible solar panels that can be draped on the back of their packs, recharging batteries for night-vision goggles, gun sights and many other electronic items troops carry these days. The Special Forces also pioneered the use of water bottles containing purification capabilities. In Summer time, such devices are reassuring, and make it easier to fly in essential supplies like food and ammo.
Using solar panels and water purification systems (that don't require electrical power) makes it possible to maintain reserves of fuel and drinkable water, in case you are attacked and the panels shot up and access to local water supplies blocked. The panels and water purification also make it possible to establish and maintain more FOBs with existing aircraft and helicopters. Thus the renewable energy gear becomes a weapon, making more air transportation available for direct combat support.