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I think people might be over estimating how much power is needed to turn a starter motor. Assuming your car starts nice and cleanly, the starter load is almost not worth considering.
My supercap bank has a miniscule 90mah capacity. I was able to get four consecutive starts out of it in my Suzuki Swift (I immediately shut down the engine upon firing each time so the alt didn't have time to energize).
That's 0.3Wh per start. So a 60Ah battery is good for around 216 starts, assuming no other loads The starter is a big instantaneous load, but even with hard core EOC and city traffic it's total duty cycle is only maybe 60 seconds a day. It's actually 'always on' things like the ventilation fan that creates the majority of electrical demand.
I average 16mph, that's only a fraction faster than a NYC taxi. Over my 20km commute, I need the starter motor about a dozen times. Most of the time I bump start.
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