You'll need to do some calculations to work out which is cheaper. Let's assume that your PHEV is a Mitsubishi Outlander, it can cover 20 miles off a charge and your electricity provider charges 34p per kWh. With its 13.8kWh battery, you'd pay £4.69 to fully charge it - which works out at around 23p per mile. With petrol at 167.22 per litre, assuming a real-world fuel consumption (with a flat battery) of 40mpg, you'd pay around 19p per mile to run it on petrol. Obviously these figures can be substituted based on your own car and electricity tariff, and it's worth bearing in mind that many companies charge less for electricity at off-peak times (such as overnight).