Lookup and Sum

Hi Everyone,
I am trying to do a look up against a different table... but then i want to sum up the value against each line of the orginal value.
Is this possible please??

Example Info (there is more columns then shown below):

Description Possible Days
200 KVA 36
300 KVA 54
150 KVA 56
200 KVA 72
150 KVA 22
20 KVA 16
300 KVA 31

I need to look up each of the Possible days against each Description and then return the summed total. I have already done a count summary against the description so i know how many of each description there is.

i hoope this makes sense! Please help me out :-)