If you are happy with a return on investment of 35%, please invest £100,000 with me, and I will give you back £35,000.
What you meant to say was that the rate of return was 35%, but unless you know how long you will be getting a return, it's a totally meaningless figure.
If the array lasts two years, your return will be £700, or 70%, so you will have made a loss. If it lasts 20 years and needs no maintenance your return will be £6,000 or 600%.
Might be wise to reckon on ten years, in which case it will prove to have been a good deal. Furthermore you omitted to count the cost of generator repairs or replacement, not to mention the hassle, noise, danger, and inconvenience.