Well for starters, 'pengarna' would be THE MONEY, not just money. But this one is a little different because it only has a plural form, sort of the opposite to English where we only have a singular form (there is no 'moneys'.)
The Swedes will have to correct me if I'm wrong here, but I think 'pengarna' has extremely little use, except perhaps if you were referring to different LOTS of money - for example, the money from the Danish investors, the money from the Chinese investors, and the bank's loan - they might collectively be referred to as 'pengarna'...?
I'm going to have to differ with you on pengarna. I use it quite frequently: Satte du in pengarna på kontot? = Did you put the money into the account? Stoppa pengarna i den här bruna påsen = Stick/put the money in this brown bag (ok, I don't really use that one that much)
There actually is a singular form of "pengar". It is is peng. It means a piece of money or coin, depending on the situation. It is more on the archaic side nowadays, though.