You need to know the power rating of the pump, which is usually given in watts. If for example the rating is 1500w, divide by 1000 to give 1.5kw multiply this by the time the pump is running, in your case 24hrs to give 36 Kilowatt hours.(Kwh) The Kwh is the standard unit the power companies charge for so look on your last bill and some were on it it will give you the cost per Kwh (eg 10p per unit) multiply this by 36 and this gives you the cost in this case �3.60p per day. I've got a pond and rarely leave the pump on 24/7