how is contribution based ESA calculated?

I receive PIP and contribution based ESA. I have been receiving a work pension since I turned 60 in 2018. I have savings in excess of £16,000. Every year I get a letter from the DWP to tell me what my ESA will be and how I can appeal their decision if I don't agree with it. HOWEVER, the letter gives NO explanation as to how they come up with the figure. I get £14.96 ESA a week. Can anyone explain how the DWP calculates this? I assume that it's related to my pension income, which makes it "means tested", although obviously not the right kind of means testing to qualify for the energy support!

Thank you Gary. I have asked the DWP every year, in writing, how they calculated their figure and have never had a reply! How on earth is anyone expected to be able to appeal their calculations, without knowing how those calculations were made? (This is of course a rhetorical question ). Only 17 months to go until I finally get my State Pension and can forget about ESA forever; it can't come soon enough...

This year, armed with the knowledge of how to calculate my ESA after my annual private pension increase, I checked the amount of ESA support group on the government website ( £128.85), did my calculations and came up with a weekly figure of £21.60. As the DWP hadn't acted on my letter telling them about my income increase and are paying me £26.86 p.w. I phoned them to say that I am being paid too much. While I was on hold (1hr 40mins) I was told that my next ESA payment will be £89.40. That is 2 weeks of £44.70 which is what their letter says I am entitled to before they reduce it down to £26.86. When I finally got through to a call handler, he told me that the actual ESA support group figure is £129.50; the government's website shows the wrong amount! He couldn't explain why they were going to pay me £89.40. He was extremely helpful and told me that my weekly payment going forward will be £22.25.
The moral of the story is: even when you know how to calculate the amount of contribution based ESA you are entitled to when your income goes up, unless the government website gives you the right figure to start with, you stand no chance of getting it right.