i'm going to correct the math slightly.

i multiplied the f by 155% thinking it would come out in the wash, but i actually dropped a factor. see, for me, i have a rebate, so it does come out in the wash - for my bill, specifically, i was right. but, more generally, that added term is important.

so, lets say your cost is x + f, where f is the fixed rate. the increase in cost will be 1.55x + f. then, you add tax: 1.13*(1.55x + f) = 1.7515x + 1.13f. then, you can subtract the 31.8%

1.7515x + 1.13f - 0.318*(1.7515x + 1.13f) =
x*(1.7515 - 0.318*1.7515) + f*(1.13-.318*1.13) =
1.194523x + 0.77066f.

that's your new price, where x is the previous cost of electricity minus the fixed rate and f is the fixed rate.

your previous price was:

(x +f)*1.13 - (x+f)*1.13*0.08 =
(x + f)(1.0396).

so, the difference in price is then still +0.154923x (albeit f less than the previous x), but it's also -0.26894f. the reason for this is that the fixed price did not go up by 55%, but will go down by 32%. that's the term i dropped.

here, f is about $27. so, you're still looking at that 15.5% increase (on an x that is f less), but then minus a fixed amount of $7.26. that's my error. i apologize.

so, if your electrical bill was $127 last month before taxes and adjustments, then x is $100 and f is $27, so your new bill will be 15.5% higher in electrical costs (applied to the $100, not the $127), but then $7.26 less. that $7.26 is a fixed amount and will not increase or decrease with usage. so, your bill will then go from $132 to $140, rather than from $132 to $152. it's still not $2 - it's just $7.26 less than the 15.5% i previously calculated.

so, your bills will go up by 15.5%, still.

it's just that then they go down by another $7.26. and, i'll let you calculate whether that's in your interest or not.

but, x*1.155 - 7.26 < (x + 2.00) <----> x*.155<9.26 <----> x<$59.74.

so, if your bill is usually $83 or higher you're going to be paying more than $2 more, and the amount will increase as you use. if your bill is actually often in the $300 range (assume x=300, f=27), you're looking at a $39 increase, which is more than 11%. that number will approach 15.5% as costs increase, but never exceed it.

so, that's another way to look at it - that 15.5% is a limit. a maximum amount. that's what you get by dropping the "error term".