Re: In terms of watts per dollar...
Just to add a UK perspective where it's a little bit different:
Here hot water is predominantly generated using natural gas. Over half of UK boilers are condensing boilers heating water at somewhere around 80% to 85% efficiency and that proportion is growing all the time.
Hot water only represents only around a fifth of total domestic heat requirements so if solar water heating generated all of your hot water (impossible in the UK) you would save about £150 per year. A more likely figure is about £100 per year. A new solar water heating installation costs about £3,000 to £5,000.
A 4kW PV system on the other hand costs somewhere around £6,000 to £8,000, generates about 3,600kWh a year which is about the average electricity demand. Assuming you use half and export half then you'll save £180 on avoided electricity costs alone (ignoring feed in tariff and export rates). Also the PV installation is avoiding electricity generated on average at about 50% efficiency at best.
So there's very little in it in terms of payback times. I love solar water heating though and it would be great if the industry could get the costs down. It's a good fit with solar PV as evacuated tube installations only takes up a small proportion of a house's roof so you can fit PV panels around it as well and get the best of both worlds.
SEWTHA is a great book but it didn't have all the answers and did slip gradually out of date in some key areas (it was published all the way back in 2008 I think). It was a very important contribution to the energy debate though.