While I wouldn't regard environmentalists as fools, the strictly financial viability of solar panels depends on a number of factors;
Energy Inflation What price trends you expect will apply to other energy over the life of the system. If you believe in
peak oil, then all bets are off on what might happen when this recession bottoms out. The last time demand exceeded supply, oil prices went up by 700%, or by $120. Many expect the prices to go to $200 or $300 per barrel very quickly.
Panel Prices It could be argued that installation prices are falling (and they have done in recent years because solar moved from being a niche product, into the mainstream), but we are also reaching peak copper, and energy prices are starting to have a serious bearing on the price of other components of a panel, such as aluminium. Margins in the industry are screwed to the floor and I would expect prices to rise in coming years.
How much hot water you use - If you only take frugal showers, there are only two people in the house, and you use a dishwasher and washing machine that can't be filled with hot water, etc., then you may find your hot water useage is reduced
Your Heating System If you already have a very efficient hot water system such as a gas fired combi boiler, then the savings will be reduced. But most houses have the heating turned off for six months of the summer. During that time, the hot water comes from either an immersion heater, or by switching on a boiler, heating boiler, flue and miles of pipework just to heat a cylinder in the hotpress
Your existing cylinder - this will be replaced with a solar one. If the existing cylinder is wasting heat, then there will be year-round savings on this. Changing the cylinder is a substantial chunk of the cost of a solar installation.
At current energy prices, a solar water heater can give a payback period of about 12 years in situations where the the existing heating system is as above. Thats a return of over 8%. I think the new government solidarity bond is offering about half that?
In my own situation, our central heating is off for 8 months of the year. I fill our washing machine with hot water from the tap (using a short hose on the tap in the utility room) and we like baths. Its a no-brainer for someone in an energy efficient house whose heating is off all the time. A bit more difficult to say for an old and cold house with a really efficient pellet boiler that is on a lot of the time.