I did this exercise last month for elecy.
I get or ask for the daily service cost and unit rate.
I then, based in my last 12 months use, i'm aware consumption can go up or down but you need a base figure to compare them all, calculate the cost for each supplier.
This is important if the daily cost is not the same.
If it is then only the unit cost is needed to be compared.
This is in my view the only way to compare the different tarrifs.
Whilst doing research i had the tarrif details of a supplier and had worked out a figure.
I rang them to get some further info as not clear on site.
They calculated i would save more than i had calculated.
when i questioned the figure, guess what, they had used a random consumption figure even tho' i had give my figure.
I think they used mine, realised the saving did not look to good so pluck a figure to make the £££££ look good.