Staywarm is a tarriff designed fr peace of mind not financial economy. What this means as Jalexa pointed out that your entire year's usage (whatever it maybe) is paid for at a set contractual rate, paid monthly. This means you can use as much or as little as you like. If you use twice the amount your payment is worth then e.on cannot ask you to pay for it, equally if you use less you cannot ask for the difference. this generally gives people peace of mind as no matter the usage they know excactly where they will pay upto 12 months in advance.
As the contractual term is 12 months, each year you will recieve a renewal, e.on will look at your usage over the past 12 months and calculate your usage. E.on have 3 seperate bands for staywarm energy customer's low, medium and high, and each has they're own rates they'll take into consideation the size of the property (in bedrooms) and the number of occupants and set the following 12 month's rates. As will standard direct debits they will have sent you a letter regarding this.
If you have used a lot of energy in the last 12 months you might have moved bands from a lower to a higher banding.With very high usage there is the potential to withdraw the product from you. (there is a clause in the contract regarding what type of user the product is designed for, and what types of usage allows them to cancel the contract). Undoubtedly at the start your usage profile met the conditions of the tarriff, and your usage may possibly have changed, since being on the tarriff with such a "security blanket" for lack of a better term you may not be as vigilant with your consumption as you may have been in the past.
There's two options available really. Firstly swallow the higher payments and work to reduce your consumption to what is required to maintain your property and your own well being and see if this draws a reduced premium next time around, or ask to switch from StayWarm to a regular tariff where you pay for each unit you have used if this will work out cheaper for you.
If you decide to switch to a tarriff where you pay for your actual consumption you might want to enquire into the social tarriff I beleive e.on operate called WarmAssist, as long as certain criteria are present these may help reduce your energy bills.
Another issue I see reguarly with the retired energy customer's is that they recieve an energy grant from the government to help pay for the winter consumption. Did you recieve this and did you send this to E.on as this would have helped reeuce your monthly payments? or did you keep this and use it for some other purpose?