How to Compare Two Bills With Different Billing Days

Why this matters

"My bill went up 50 percent and the only thing that changed is your heat pump." Before anyone opens an air handler, look at the top of the two bills, where the service dates are. A billing period is not a calendar month. Meter reads drift with routes, weekends and holidays, so one bill can cover 28 days and the next 35, and the longer one shows 25 percent more use with nothing in the house having changed at all. A shop that answers the 50 percent instead of the per-day figure either chases a fault that is not there or concedes one it did not cause. The fix is a division and two more comparisons, in a fixed order: days, then rate, then weather.

Step 1: Read the service period and the billing days on each bill

Find the "service from / to" dates or the read dates, and the number of days the bill covers. Many bills print the days outright; use that figure rather than counting yourself, because utilities differ on whether both read dates are counted. Copy the kWh (or therms, or gallons) used in the period, from the meter-read section, not from the summary.

Also note whether each read was actual or estimated. An estimated read followed by an actual one moves use between bills regardless of the days. See related: Estimated Versus Actual Meter Reads and the Catch-Up Bill.

What breaks if you skip it: everything after this step. A per-day figure computed on the wrong day count is wrong by the same proportion.

Step 2: Divide use by days, and split the increase into its two parts

Use per day = units used / billing days.

Then the ratio of the two totals is the product of two ratios:

(use B / use A) = (days B / days A) x (use per day B / use per day A)

That one line tells the customer how much of the jump is calendar and how much is the house. Many bills print an "average daily use" figure; it is the same division, and checking it against your own catches a misread day count.

Step 3: Then look at the price, because a long bill can cost more per unit too

Once the days are out, check whether each unit cost the same on both bills.

• A rate change mid-period. The bill shows it as two lines with prorated days. Note the price per kWh in each.
• Tiers. On a tiered rate, the cheaper first block (the allowance) may be prorated to the billing days or fixed per bill, and the bill's rate section or the tariff says which. If it is prorated, a longer bill gets a proportionally bigger first block. If it is fixed, a longer bill pushes more of its use into the dearer tier. See related: Tiered and Seasonal Rates: Why the Last Kilowatt-Hour Costs More.
• The fixed charge. The monthly customer charge is usually a flat amount per bill (some tariffs prorate it by days), so on most bills it does not grow with a long period. Compare energy charges per day and per unit with the fixed charge set aside, then add it back for the total.

The reason this comes second and not first: price explains the bill's amount, not the use. A customer who used the same per day but paid more per unit has a rate question for the utility, not an equipment question for you.

Step 4: Then compare against the same period last year

Even with days and rate equalized, December and January are not the same weather, so last month is not a fair comparison for a heating or cooling house. The fair comparison is the same billing period a year earlier, which most bills print as a bar or a line. Convert that one to per day too, because last year's read dates were different.

Then add degree days for the exact read dates of each period: heating degree days (HDD) in winter, cooling degree days in summer. Use per degree day is a rough check on whether the house used more for the weather it had. How to Weather-Normalize Two Winters of Bills owns the method, including the base use a per-degree-day figure must remove before it can be trusted; here it is only a direction check.

Worked example: the 50 percent bill

An all-electric house with a heat pump installed in October. The customer's January bill is 50 percent above December's.

                     DEC BILL     JAN BILL     JAN LAST YR
Read dates           11/28-12/26  12/26-1/30   12/27-1/29
Billing days         28           35           33
kWh used             980          1,470        1,353
kWh per day          35.0         42.0         41.0
HDD in period        672          1,155        1,040
HDD per day          24.0         33.0         31.5
kWh per HDD          1.46         1.27         1.30
Read                 actual       actual       actual

Days. 1,470 / 980 = 1.50, the customer's 50 percent. But 35 / 28 = 1.25, and per day 42.0 / 35.0 = 1.20. Check the split: 1.25 x 1.20 = 1.50. So seven extra days on the calendar account for a 25 percent rise on their own, the house's use per day rose 20 percent, and the two compound to the 50. Both reads were actual, so no catch-up is hiding in either.

Price. The rate has a first block of 20 kWh per day, prorated to the billing days, and a second block priced at 1.3 times the first. Call the first block's price R. December's 28 days give a 560 kWh first block; 980 minus 560 is 420 kWh in the second. January's 35 days give 700; 1,470 minus 700 is 770 in the second.

• December energy: 560 x R + 420 x 1.3 R = 560 R + 546 R = 1,106 R
• January energy: 700 x R + 770 x 1.3 R = 700 R + 1,001 R = 1,701 R

The share of kWh in the dearer block went from 420 / 980 = 43 percent to 770 / 1,470 = 52 percent, so January's energy cost per kWh is 1,701 / 1,470 = 1.157 R against 1,106 / 980 = 1.129 R, about 2.5 percent dearer per unit. Energy cost per day went from 1,106 / 28 = 39.5 R to 1,701 / 35 = 48.6 R, up 23 percent: the 20 percent more use per day, made slightly dearer by the tier. Adding a fixed charge of 100 R per bill (illustrative) to each, the totals are 1,206 R and 1,801 R, a rise of 49 percent on the bill, but per day it is 43.1 R against 51.5 R, a rise of 19 percent. The fixed charge pulls the per-day rise below the energy figure because it is spread over more days on the long bill.

Weather. Last January's bill covered 33 days at 41.0 kWh a day. This January ran 42.0, which is 42.0 / 41.0 = 1.024, about 2 percent more per day. This January was also colder per day, 33.0 HDD against 31.5, about 5 percent. Crudely, kWh per HDD fell from 1.30 to 1.27. December's 1.46 is the highest of the three because base use (the water heater, the refrigerator, lighting) is a bigger share of a mild month, which is exactly why that crude figure is a direction check and not a measurement.

What the 50 percent turns out to be. A quarter more days, a December-to-January weather change that raised use per day by a fifth, and a tier that made the extra kWh slightly dearer. Against last January, same house, similar weather, the new heat pump is using about the same per day in a slightly colder month. Nothing in this points at a fault.

A second case: when the days explain almost none of it

The same three comparisons, run on a gas-heated house, can point the other way, and that is the point of running them in order rather than deciding in advance what the answer will be.

A customer's February gas bill covers 29 days and 182 therms. Last February's covered 28 days and 120 therms. The total rose 182 / 120 = 1.52 times. Days: 29 / 28 = 1.036, so the calendar accounts for under 4 percent. Per day: 182 / 29 = 6.28 therms against 120 / 28 = 4.29, a ratio of 1.46. Check: 1.036 x 1.46 = 1.51, which matches the totals to rounding. The price per therm on both bills is within a percent, so the rate step changes nothing. Weather: 30.1 HDD a day this February against 29.6 last, under 2 percent colder.

So a 46 percent rise in use per day, against a winter less than 2 percent colder, on the same rate. That is a real change in the house, and the call moves from explaining the bill to finding the cause. How to Diagnose a High-Bill Call, in Order takes it from there. The per-day comparison has done its job either way: in the first case it cleared the equipment, in this one it earned the visit.

Saying it to the customer

Lead with the per-day figure, because it is the one they can check on their own bill: "This bill covers 35 days and the last one covered 28. Per day you used 42 kWh against 35." Then last year: "Last January you used 41 a day." Then stop. If they want the tier explanation, it is there; most do not need it. If the per-day figure against last year had risen sharply, that is when the call becomes a diagnosis. How to Diagnose a High-Bill Call, in Order owns that sequence, and How to Take a High-Bill Call at the Office owns the call itself.

Do not tell the customer the bill "should" go back down next month. The next read date is the utility's, and the next weather is nobody's.

What changes the answer

• The tier allowance is fixed per bill, not prorated. Then a long bill pushes a larger share of its use into the dearer block than it would on a prorated rate, and the price step accounts for more of the increase. The days step is unchanged, which is why it always comes first.
• One of the reads was estimated. The per-day comparison is meaningless until an actual read lands; compare across the pair of bills that spans the estimate.
• The house changed for real. A new load (a hot tub, an EV, a space heater in a cold bedroom) shows as a per-day rise against last year that the degree days do not explain. See related: The Loads a Customer Forgets They Own.

How to know you compared fairly

• Every bill on your sheet has its days, its kWh and whether the read was actual.
• The ratio of totals equals days ratio times per-day ratio, to rounding.
• Price per unit was checked with the fixed charge set aside.
• The comparison that carries the conclusion is against the same period last year, per day.
• The degree-day figure is called a direction check unless base use was removed.

References

• See related: Anatomy of a Residential Electric Bill, Line by Line
• See related: How to Weather-Normalize Two Winters of Bills
• See related: Tiered and Seasonal Rates: Why the Last Kilowatt-Hour Costs More
• See related: Estimated Versus Actual Meter Reads and the Catch-Up Bill
• The utility's published tariff, which states whether tier allowances are prorated by billing days