Advice for what? I assume you are a smaller business than E-on? Should be easy to under cut them......
Um. If only life was that simple.
Different dynamics.
Small company, low(er) overheads, lower buying power, need to make profit quicker.
Large company, High Overheads, Massive buying power, no need to make profit in a hurry.
Large businesses are more than willing to sell at less that cost these days, and will recover the profit and any loss element of installation through "maintenance" contracts, often lasting five years minimum.
From experience, going against the big boys is never that easy or straightforward - if it comes down to a money fight, you've lost. Do whatever you can to stay away from the cost of the job, and concentrate on the undoubted extra value you'll bring to the table - quality, quicker service, better knowledge, lower cost of life (not necessarily initial cost), and so on.
Also, it pays to not ever forget that the customer isn't buying an "xyz" special - they're buying YOU, or your competitor. It really is a personality contest in the end. Whether or not what you're offering can do the job is normally sorted within minutes of any sales pitch.