Insufficient data... However remember that 1 BTU is the amount of heat needed to raise 1 pound of water 1 degree F. Therefore you have to figure how many degrees of temperature difference you need between your incoming water feed and the desired temp. going into the boiler, and divide that into the 150kBTU number. This will tell you how many pounds of water, which when divided by the weight of the water (approx 8 lbs / gallon) will then give you the number of gallons needed.
It might be helpful if you could give more details about what you are trying to accomplish - is this a DHW heater? Are you simply trying to prevent it from burning oil?
A couple of other things to consider -
1. If you EVER get freezing temps in your location, you MUST have some sort of freeze protection in the solar system - either by using a drain-back type setup, or using a seperate loop w/ a glycol antifreeze mix and a heat exchanger in the tank. The latter is probably more efficient as it will need less energy to circulate the heated water.
2. It is probably better to have a tank that you heat with a seperate heat exchanger fed by a panel with its own fluid circulation system. This will get the heat from your roof into the system much more efficiently than trying to run your cold feed through the panel as needed to replenish the hot water supply as it is used. It also allows a relatively low output panel to supply heat over a long period of time rather than attempting to instantly heat water as it is being used.
3. Often it is good to "overheat" the water in the storage tank, as this greatly increases your effective supply volume. Standard DHW systems run at around 120*F, and codes say that you should NEVER get hotter than that out of the faucets to avoid scalding dangers.
The way to do this is set up your tank to let it get heated up to 180-190*F if the panel will push it that hard, and then use an automatic "mixing valve" on the tank output to mix whatever temperature is in the tank w/ added cold water to bring it back to 120*F.
Set the oil burner to fire at 120*F, so as to maintain your guaranteed minimum temperature in the tank. If the solar can get the tank to, or above that temp, it will keep the burner from firing at all, if not, the burner will fire less as it only has to make up the difference between what the solar does and the set temperature.
ART
