The weight of your 120 gallons of water is 1000.8 pounds
Therefore, if your storage starts out at 180°F and ends up at 140°F.
You will have removed 40,032 BTUs from it in total.
As an example, this amount of heat might keep a well insulated 2300 sq ft house in New England warm for 1 hours in colder weather or 2 in the shoulder season.
If you planned well, you now can build the perfect system to suit your needs and get even more comfort from your renewable biomass fuel.
Since the weather and heat load differ daily, it is virtually impossible to accurately size a wood boiler for all winter conditions. At many times the boiler will be too big - and at some times it may even struggle to keep up with a very heavy heating and DHW load. These are cases where large water storage systems can help.
A water storage system takes any excess output of the boiler and stores it in water tanks. Then, when this heat is needed, it can be drawn from those tanks instead of directly from the boiler. The benefits to such a system are many, but include:
Storage is best when a boiler is properly sized or oversized. It can tame a oversized boiler, but cannot help much with one that is undersized, since such a boiler will always struggle to keep up with load and have little left for charging the storage.
There are two very important variables in waters storage to pay attention to - that of storage size and that of the particular type of radiation (heat delivery) system you have in your home. As to storage size, bigger is usually better!
Storage can be either pressurized or non-pressurized - the non-pressurized systems usually use heat exchangers to transfer the heat into and out of storage. A third possibility is a very low pressurized system, so called "open" systems which have only enough pressure to allow the water to get to the highest area of the home.
As to heat delivery systems, the key to getting the most out of storage is to have radiation (in-floor, baseboard, cast-iron, etc.) which is capable of still producing usable heat at lower water temperatures, say 110 to 140 degrees. The reason is simple - the amount of BTUs which can be stored in a given amount of water is based on what the starting temperature is and how much the temp of that water can be lowered and still be of use. An example would be this - if your system requires water at 150 degrees or warmer, the same size of storage would only be able to hold 1/2 as many BTU's as if your system could use 120 degree water. That's a big difference!
If you are building new or doing massive remodeling, consider radiant floor systems which can often use water as low as 100 degrees. If you already have radiators or another system installed, have it inspected and rated by a pro to determine at what temperature it can still heat the house. Another hint is that some so-called "kickspace" heaters, which can be easily retrofit installed, can use water as low as 120 degrees. Such a heater may provide a nice addition to your office or a particular area that you want to be able to heat often with the storage water.