Nor do many comparison sites.

A variable direct debit (VDD) allows the supplier to take only the amount you owe from your account.

A Monthly or quarterly direct debit usually means the supplier takes an agreed fixed amount from your account, but can be read as a VDD.  If it is an agreed fixed amount you can be sure the will not underestimate your consumption and you will end the year with a big credit - and no interest on your money they are holding.   Try and get the money back and see how hard it is.  This happened to me until I got wise to it.

See for yourself how many sites use the term "variable direct debit" - it is not in their interest to promote it. Energy watch don't seem to have cought on - although I have pointed this out to them.

Why should I be restricted to just green tarriffs?  There are others out there.

I hope this clarifies the issue for you and many others, including suppliers, customers and Energywatch.