You have to define your clear objectives in the different cases and I wrote the guideline:
The principle is to calculate the total energy E to be stored in the buffer by clearly defining its function (power multiplied by time), stove or solar, which makes it possible to calculate the volume of water required by knowing its storage temperature and that the water stores 4,18J / ° Cg is: E = (Tf-Ti) x4,18xV with E in KJ, V in liters, T in ° C Tf, final hot T and T of Ti cold water inlet .
But it is necessary to define the energy needs, duration of hot water heating with stove turned off, or without sun, multiplied by the power.
For the sun, you need to know your sunshine in average winter, the typical duration without sun (therefore the local climate), the efficiency of the collectors (depending on their nature and the T of the heating water or DHW (very different)) which fixes the energy to receive on sunny days and therefore the collector area.
See the site and read while assimilating as well as econology:
http://www.apper-solaire.org/?Calculshttp://www.apper-solaire.org/http://www.apper-solaire.org/?FaqBe wary of formulas that are not well understood, in particular the assumptions must be clear, otherwise errors guaranteed.
So try to understand well instead of looking for a saving formula used with your eyes closed without making the effort to understand.
So it all depends on your requirements and the insulation of the ball.
It's like the tens of thousands of pages of nuclear safety that have been planted in Japan !!
Because they forgot correlations of events of low probabilities which with the tsunami, realized all together (theoretical probability multiplied by independent events, of 1 in a billion) suppressed all cooling, even of emergency, on plants which cannot not do without it at all !!
Keeping the heat of summer to heat winter requires a balloon of more than 300m3 (or 1000m3 of mini earth much cheaper with drilling) perfectly insulated with 2m to 3m of well designed insulation !!
You make 25 boreholes at 15m depth on 100m2 of land (volume of 1000m3) with exchanger pipes and adequate solar thermal collectors at low cost and you will no longer need a CO2 stove and pollution in perpetuity !!
