Hello mate,
I think for Single, say 5kg batches, 70ltr Esky is way to big IMO.. I think you would be better off(for singles) to go with your smaller esky, it will hold temp much better even if the sides are thinner as the Mash will hold temp very well by retaining the heat and not letting it escape into deadspace of a much larger esky..
If you consider doing double batches, go with the big one but I would plan on the smaller one for single batches... I used a 38ltr esky and it did the job very well, it was quite old and busted up but held heat well, maybe only dropping 1 degree over a 90 min mash..
I now have 57ltr Igloo esky and still for singles, it lost 5 degrees over a 90 min mash, so ill now be doing 1.5 batches or maybe doubles as the esky is really big enough for that size and its only 57ltrs.. Singles just seem to get lost in there..... Check out this link for my ESKY:
http://www.aussiehomebrewer.com/forum/index.php?showtopic=33455&hl=
To be updated shortly....
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