Find your load, then your needed operating time.
    Then you have your size from the formulae t/R = C.
            :
    Fairly basic stuff, but it's nice to have it on the list.

It's not quite so simple.  The RC time constant is how long it takes the
capacitor voltage to decay to 1/e of it's original value, but the
interesting number is how long it takes to decay to a voltage where your
circuit stops working (which is a somewhat ambiguous number in itself.)

I thought cooper electric had a little excel spreadsheet you could download
that would figure out the needed capacitance based on assorted "normal"
circuit criteria, ie:

http://www.cooperet.com/documents/PowerStor_Aerogel_Capacitor_Calculator.xls

Enjoy
BillW

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