We use baseboard heat and hot water that comes off an oil furnace: can we supplement with solar water heating?
We're looking to make a hopefully not-too-pricey investment go a long way towards cutting back on the financial and ecological negatives of using oil. What would be the most short and long-term cost effective (and simple) solution for using both solar and oil in a cold state like Maine? Can you directly preheat water with the solar panels to cut back on the time the oil furnace has to fire to further heat the water, or is it more complicated than that?
Yes, you certainly can do this, but it is more complicated than what you proposed purely for the reason that most solar water heating systems just don't get that much heat out of the sun in the winter.
This is largely because of the little solar exposure that they get in the winter months, when the daylight hours are short and the sun is low in the sky. This time of year is largely when you need the most heat from the system because you are trying to use it for space heating instead of burning oil or natural gas.
During the summer months, the solar thermal panels work great and should largely or totally offset your water heating usage for many months. In California, it is about 9 months of free hot water. The best way to get heat from the winter sun is to use evacuated tube solar thermal array. This type of system collects the energy from even low-angle sun exposure, earlier in the day and later into the evening than flat panel arrays and, because of the vacuum around the heated element, loses very little heat to the outside during cold days.
This is in contrast to the flat panel systems, when the amount of heat lost by the panel is greater than the amount of heat that the panel gains from the sun. All in all, solar thermal is great for spring, summer and fall DHW heating, but don't expect great things during the winter months.