How do you work out how much your bill is going to increase so you can decide whether to move to a fixed rate or stay on the standard variable?
That's a bit of an impossible question! A variable tariff is just that: variable. The amount you pay will fluctuate as the wholesale price (the price at which your supplier buys their energy in) fluctuates. If it falls you're quids in; if it rises your bills will go up accordingly.
The only thing you can really is to firstly look at what you're paying at the moment on your variable rate. This will be a combination of the standing charge, which is an amount you pay per day regardless of whether you use any energy or not, and the unit charge which is the amount they charge you per unit of energy used.
Then compare this with the fixed deals you can find and see whether they're cheaper or more expensive. If you can, find a Winter bill and work out how it compares with the deals on offer. You'd calculate this as standing charge x no days in the month = total monthly standing charge, and units used in a month x unit charge = total monthly unit charge, then add total monthly standing charge and total monthly unit charge together.
If a fixed deal would work out cheaper then great but you need to be aware that if energy prices fall you won't get the lower price. You can end your fixed term period early to take advantage of a better offer but there might be an early-exit penalty (many don't have this, however, so it's worth checking). If a fixed deal would be more expensive then you need to decide which risk you prefer: higher but fixed bills, or hoping that variable rates won't go up too much but knowing that they might go up a lot.
I always prefer to know what my costs will be, even if it costs a bit more in the long run but it's an individual thing 