Turning the electricity on for 15 minutes or running the oil for an hour?

going by my old ‘selectos burner’if i run it for one hour it will heat 30 gallon of water and heat four rads for 1/4 of a gallon of oil,cost about 50 pence,….if you run a electric shower for 15 minutes burning say 8 k.w at 12 pence = cost is 24 pence approx,but no more hot water.