Well being electrically challenged, you may be at the mercy of a local electrical contractor. There are many ways to accomplish this, some are safe and will be to code, some will be strictly prohibited by code. Messing this up can be hazardous to your health and home.
Probably the easiest way to do this is to add a second electrical panel with an isolation type knife switch. Works as described above. It has 2 inputs and one output. When you pull the lever, one input is disconnected from the output before the other input is connected. A fairly common electrical component and Probably less expensive than a interlocked circuit breaker.
The first input to this A/B switch would be from a large circuit breaker in your existing panel. The other input would be from your secondary power source. The output would feed the secondary panel.
Into the secondary panel, you move the wired circuits that you wish to be able to power from either the genset or comm power(physically move wires from main panel to the second panel). Under normal operations, the main panel feeds the second panel to power these devices. To operate, on genset, you connect up the generator with a plug to the A/B switch. You get it running and to correct volt and freq then move the A/B switch to the position that draws power from the generator. Depending upon the situation you may need to open circuit breakers in the secondary panel before doing this to keep from overloading the genset. This A/B switch and second panel is pretty much what you are getting when you purchase one of those genset power dist panels. It is just combined into a single neat package that has a specific purpose so it is more expensive.
What are the output specifications of yout 15KW genset? Unless it is capable of feeding 2 phases(240VAC) the powering of stove, water heater and furnace may be out of the question all together.
AC electrics 101: Standard houshold AC power is usually a combination of 2 or 3 electrical phases(A B C) referenced to neutral or each other to get the desired voltages. An electrical wall outlet is 1 phase to neutral(2 flat prongs on the plug) which gets you 120VAC. the third wire is to ground which provides a safety path for the 120VAC in case the equipment shorts out(instead of passing through the person operating the equipment). The current fed to say a hot water heater, kitchen oven/range or dryer is a combination of phases, either A to B, B to C or C to A without neutral. When connected this way 240VAC is provided. These phases are fed to electrical buss barrs in the power panel via a main breaker or cutoff switch. Onto these buss barrs are connected the circuit breakers that power the various circuits. Some of these breakers only connect to a single buss bar(single phase) and are used to power 120VAC circuits(second wire to complete the circuit comes from a neutral buss in the panel). Some are connected to 2 buss bars(dual phase) and have 2 outputs that feed 240 VAC circuits. That is why most of the large breakers in your panel feed a single item such as the hot water heater where the smaller single phase breakers may feed multiple circuits such as living room light or ground floor electrical outlets up to the wire and circuit breaker rating. Another job performed by the electrician is to balance the load on the phases in the panel by connecting the breakers to the proper buss bars.
You can feed a secondary panel with a large 2 phase circuit breaker which will feed 2 buss bars in the secondary panel. From these 2 buss bars you can connect either 120 or 240 VAC breakers to power the necessary devices. Here is the rub. Unless your generator is capable of feeding that secondary panel with the same 2 phase 240VAC, this won't work.
