The short version
You have the right tariff version when every charge on the bill can be rebuilt from it. Until then, nothing you calculate means anything.
It is tempting to skip this. You have the bill, you have the tariff, the rate codes match, so on with the comparison. Skipping it is how people end up presenting a customer with savings that do not exist, from a rate that expired, against charges they misread. Doing it takes twenty minutes and makes everything after it defensible.
Step 1
Take three things off the bill.
Step 2
The book has more than one version.
Step 3
Pick the version that covers those dates.
Step 4
Give every charge line a home.
Step 5
Rebuild the total, then flag the difference.
Why small gaps happen
When your rebuilt total is close but not exact, work through these in order. Rounding is the last thing you check, not the first.
Silent pro-rating. The tariff shows a monthly charge. The utility actually bills it by days: $16.27 × 29 ÷ 30 = $15.73. Tariffs very often do not say this in writing — it is simply how they bill. This is the single most common cause.
Tax already inside the rate. Some states publish rates with certain taxes baked in. Adding the tax again double-charges it.
A mid-period rate change. If a new version took effect inside the service period, the bill splits into two blocks at two prices.
A rider you missed. A small unnamed line is usually a rider, and riders are easy to overlook because they live in a separate part of the tariff.
Then, and only then, rounding. Real rounding differences are pennies, not dollars.
Big gaps are a different animal. They mean the wrong tariff version, the wrong rate schedule, a meter multiplier you did not apply, or a genuine billing error by the utility. All four are worth finding.
The one thing to remember
Flag every gap, however small, and let someone decide what it means. Never quietly decide a difference is acceptable. A tiny error that scales across many meters and many months is exactly the kind of finding we exist to catch.
Check yourself
How do you know you have the right tariff version?
When you can rebuild every charge line on the bill from it and land on the printed total, give or take a difference you can explain.
Your rebuilt total is $12 under the bill on a 29-day period. First suspect?
Pro-rating. Check whether the fixed monthly charges were billed by day count rather than as a full month.
You find a charge line that matches nothing in the tariff. What are the three possibilities?
You have the wrong version, there is a rider you have not found, or the utility is charging something it should not.