I read my bord gais meter weekly, work out the kwh and every bill, I'm usually a few Euro ahead, this is how i do it, just an example of my latest reading, frommlast week,
Meter reading 8243 minus 8053 (reading from last bill)
= 190 x 10.9894 = 2088 kwh
2088 kwh x 0.0037 (carbon tax) = 773 (round off to €7.73)
2088 kwh x 0.05132 (unit rate) = 10716 (€107.16)
number of days since last bill 63 x .2266 = €14.26
add all those, then divide by 100 x 13.5 to get 13.5% tax, add that and you have what should be the bill you'll be getting, works for me every time