You are overestimating the amount of rain here. That's OK, everyone does it. We are famous for rain, and it doesn't occur to folks that we *accumulate* less than a lot of other places. Seattle gets no more on average than New York City-- it's the number of rainy and cloudy days that does it, not the amount.
It only rains 35-50 inches in the Puget Sound area, depending on where you live (in the rain shadow of the Olympic mountains or not), and most of that is Oct-February, especially November (more as you push up against the mountains). We get negligible rain in summer, and when it does rain, well, it's a good time to turn on the sprinklers! June has been cool and wet these last 2 years, but you can expect an inch of rain or less June through September.
We have 6000 gallon cisterns connected to a small roof. To keep up with our household water use, we would need to get a larger catchment--house size, but it needs to be metal because it is connected with our drinking water. Our system would need to be much, much larger to cover our small garden usage, plus house usage without turning on the well. Nope, can't do without the well, but we are a family of 4.
Two neighbors on our road with good solar exposure (zero trees) still have to run a generator in the winter. However, they still love the self-sufficiency of it. They do have a small wind generator to back it up. We chose to hook up to power because we have less of a solar site and we would have had to cut down every tree to the south of us, plus still using generator power.
Urban areas, you can hook up to the grid. It doesn't help when the power goes out (the power company can't have you generating power when they need to work on the lines) and in the winter you would be paying utilities. Seattle has had a program recently to defray some of the cost of installation for this type of system.
