When the sun finally starts to shine in this country, it will let people who have solar panels installed know if their solar is actually working. It's always hard to tell if your solar is contributing to your hot water or if your supplementary heating source (gas, oil, stove, immersion etc) is covering all of your hot water demands. Only when you turn off your supplementary heating source do you realize how much your solar is contributing to your hot water.
Solar installations are usually very basic, heat is transfered from a hot exchanger (solar panels on roof) to a cooler heat exchanger (coil in hot water tank). Connecting these are your flow and return pipes that contains a heat transfer fluid (antifreeze or water). A circulation pump will push the heat transfer fluid from the hot to the cold and therefore heat up your hot water.
As long as your solar panel system is getting circulation from hot to cold then you will get heat transfer. If however your link is broken from hot to cold, by a faulty pump, lack of pressure or air lock then you will not have any heat transfer and therefore get nothing out of your solar panels.
Solar Panel Problems
Failed expansion vessel leading to a drop in pressure and air lock
Safety valve blow off due to incorrect expansion vessel pressure
Incorrect sensor location
Tubes turned the wrong way (once saw a system in 5 years that had tubes the wrong way up)