As an estimate, a single Bitcoin transaction will use roughly 100 kWh. Bitcoin is the hardest and most processor intensive currency to mine.
However, energy consumption varies hugely depending on the competition for the coin, its scarcity and how complex the blockchain is.
In real world terms, electricity costs around 11.62 pence (UK) per KWh, so you would only be spending a few pence per computer per day to generate significant revenue.