Typically 55 Ltr/ Person   Or 1.5 times daily demand see sizing Chart Below

You may have seen numerous 'recommended' cylinder sizes.
Be aware that (within reason) there is not really a right or wrong size where your system 'will or wont work'
There are conflicting pro's and con's for choosing a small or large cylinder but in most cases - larger is better.

The starting point is to understand that the solar collector conversion from light to heat is most efficient (80-90%) when heated water at the collector is not much above the outdoor 'ambient' air temperature, say 20'C
This efficiency drops progressively to say 20% at a 70-80'C temperature difference.

So heating a big warm cylinder will collect more 'energy' than a small hot cylinder.

That said, a small cylinder of hot water is more 'usefull' than a large cylinder of tepid/ warm.
This is the reason why many suggest a smaller cylinder - Higher temperatures are usefull and 'impressive'!

However, providing you have a modern boiler to provide the final few degree lift, if as and when needed,
Bigger is better!
Your nett fossil energy consumption will actually be less.

A larger cylinder (solar volume) will: Collect a greater % of the kw energy available.Opporate at lower stress, risk of overheating reduced.Store energy from good days through rainy daysIncrease the annual average solar contribution.

The downside is that your neighbor may tell you his system is 'more efficient' - simply because the temperature readings are higher! - You may or may not feel obliged to inform him!

Note that heat loss from cylinder is less at the lower temperatures.
Heat loss with 50mm foam is circa 60 watts per 100 litre, per hour (water 55/ room 20'c)

DESIGN RULES OF THUMB - The 55 rule.

Years of experience have concluded the 55 Optimisation Rule

With a fixed or given amount of solar radiation or 'energy', we could design a system to heat for example
  10 Litres by 100'C - Industrial use (from say 10 to 110'c), or
100 Litres by 10'C - Swimming pool (from say 10 to 20'c) or
  50 Litres by 50'C - Domestic (from say 10 to 60'c)

Whist solar technology can produce steam, The solar collector technology we are interested in and refered to here is the glazed collector for domestic hot water.

For domestic use we try to optimise design averages to 55'C 55'c - System design average output.55% - Average system contribution - 55% (of annual hot water)55Lt - Litres/ per person - Solar storage volume within cylinder.55% - Average 'system' efficiency - 55% conversion of solar irradiation available to 'usefull' heat.
  (75'c in collector when 20'c Ambient)

The 55 Liters per person applies to the 'typical' 3-4 person domestic household.
i.e. 1.4 working adults, 2.1 kids.

This may be progessively reduced for 5 and more, progressivly increased for 3 and less.
More for power shower, Less for frugal use.
Your particular hot water consumption may also be more/ less than the above average.

To be accurate, measure your own typical/ average (Spring-Autaum) hot water requirements in
Litres daily at 55'C and multiply by:

In addition to this total, you should add the boiler heated volume - This will depend on factors such as the KW rating or 'recovery time' of the coil efficiency chosen - see 'coil sizing' and coil/volume ratios within chart below.
This may seem complicated and hard work, but will ensure that you get the maximum optimised efficiency
from the 'system' as a whole and not just the solar collector. (sorry - this bit to follow - please ask if interested)

Efficiency gains without spending cash!

Do not be overly concerned as most systems are installed without having been 'optimised' and owners are still happy with performance.

If you take note of the above, You can make a significant efficiency improvement at very little additional cost - Sometimes none!

Insulate!!

End.

1.5 - Typical daily domestic consumption pattern -
1.8 - Greater early morning use - before sunrise
1.3 - Greater early evening use - after school.
1.1 - Greater mid day use - Lunch Schools Catering