A ‘green’ tariff can be one of two things. Either the supplier will promise to match your usage with generation from renewable sources of energy, or it will contribute towards environmental schemes on your behalf. They are good if you are concerned about the environmental impact of your gas and electricity use.
You should also consider how regularly you pay for your energy and by what method for example, you can pay by: cash/ cheque, direct debit, standing order, prepayments and so on. And to save money you should consider switching supplier fairly regularly, which is simple to do. You can get help comparing tariffs at the Citizen’s Advice’s website here.