I've never had any money to invest, so maybe I'm way off base here.....
But I get the impression that it would make more financial sense to invest that $5k in the stock market.
Eight years of compounding savings versus just getting your money back (Does this equipment retain its value on the second-hand market or would people be silly to buy used?) then hoping you can turn a profit near the end or after the warranty has expired.
Dunno. Just thinking out loud.
You are thinking well. It's called opportunity cost, an important part of every serious investment analysis. 'the value of my house went up' doesn't necessarily mean that it was an efficient investment.
http://en.wikipedia.org/wiki/Opportunity_cost
So I did a bit of research. Please point out any mistakes because this interests me..
As Dave mentions, due to Australia's disparity in generated/imported energy costs, his ROI could have significantly shorter if he simply installed fewer panels. Two years is possible! The excess panels (over what he consumes) are probably a really really poor investment.
Although I think this highlights the fact that, to be fair, his investment was only a good one due to Australia's high cost of electricity, low installation costs, and high insolation levels.
My payback period for a similar system would be >20 years.
Installed cost ~$10k. $13k if I want a ten year inverter warranty. (Sunnyboy 3000TL factory warranty is five?). In my area specifically, 2MWh appears to be a very generous annual generation estimate for a 3KW array. At $0.118 per KWh (same price generated/imported), that's ~$275 worth of electric savings and a payback period of thirty years (assuming I go with the cheapest inverter).
The price drops substantially if I install the system myself (my time has no value), but the payback period would probably still be outside of the warranty period.
Due to the equal pay/charge structure, scaling up/down wouldn't affect the ROI either.
I'm better off investing that money (pre-tax) in a very low risk index and seeing >$7k in my bank around the same point Dave's system has paid for itself (net zero). After twenty years, Dave's system would have generated <$9k while my investment would sit North of $13k. All of that assumes no maintenance costs as well.
My scenario is not an outlier either. Most countries have a lower cost of electricity than Australia. Reasons for those high(er) costs are not interminable either. Some of those countries/states heavily tax non-green energy producers and funnel those funds to prop green projects. Directly or through carbon credits, etc. Others simply have an antiquated energy generating/transporting systems. Australia's high prices are attributed to their temporary coal dependency (decreasing in favor of solar).
The chance of those things changing is more likely than the global market crashing IHMO.
Not to mention Australia's notably greater solar insulation than most of the developed world.
So I think solar is great in some geographical areas. Although it appears to be a bad investment in most areas.... or only a good investment due to temporary government assistance.
As resources become scarce it will surely become more a more sound investment in more areas.