How to read your gas bill (MJ, tiers and supply charges explained)

Gas bills are shorter than electricity bills and somehow more confusing. The units are different, the meter doesn't even measure the thing you're billed for, and the pricing comes in tiers that nobody explains. Fifteen minutes here fixes all of that permanently.

Bill out. Let's go.

Megajoules, and why your meter is lying to you (legally)

Electricity is billed in kilowatt hours. Gas is billed in megajoules (MJ). But here's the genuinely odd part: your gas meter doesn't measure megajoules. It measures volume, in cubic metres, of whatever came through the pipe.

The energy in a cubic metre of gas varies with its composition and with pressure. So your bill converts volume to energy using two published factors: a heating value (how much energy this network's gas carries per cubic metre) and a pressure correction factor. Meter reading, times heating value, times pressure factor, equals your MJ. The factors are printed on the bill, usually in small type near the meter data. You're not expected to audit them, but knowing the chain exists demystifies the number: the MJ figure is a calculation, not a reading.

For scale: one kWh is 3.6 MJ. A household with gas heating, hot water and cooktop can easily run through tens of thousands of MJ over winter, which is why the numbers on a gas bill look enormous compared to an electricity bill. Different unit, same money.

The MIRN: your gas address

Where electricity has the NMI, gas has the MIRN (Meter Installation Registration Number), sometimes shown as a DPI in some markets. It identifies your gas connection point, stays with the property, and travels with you through any retailer switch. It's on every bill, usually near the meter details. Our upload flow reads it, because switching processes need it.

The rates table: tiers are the whole game

Daily supply charge. Same concept as electricity: a fixed cents-per-day charge for being connected, payable regardless of usage. For low-usage gas households (say, cooktop only), the supply charge can be the majority of the bill, at which point the per-MJ rate you're on barely matters and the supply charge is the number to compare. Some households in that position reasonably ask whether the connection is worth keeping at all; that's a bigger decision than a plan switch, but the bill is where the evidence lives.

Tiered usage rates. Gas plans typically price usage in blocks: the first X megajoules per day at one rate, the next block at a lower (occasionally higher) rate, and so on. The tier boundaries are commonly expressed as "first 50 MJ per day", "next 100 MJ per day", averaged across the billing period. Two consequences follow. First, your effective rate per MJ depends on how much you use: a heavy winter quarter reaches deeper tiers than a summer quarter, so your average price per MJ actually shifts with the season. Second, comparing two gas plans by their first-tier rate alone is meaningless; a plan with a sharp first tier and steep later tiers can lose badly for a heating household. The only honest comparison runs your actual usage through each plan's full tier structure. (You can guess where this is going.)

The winter cliff

Gas usage is the most seasonal number in household energy. If you heat with gas, your winter quarter can be several times your summer quarter, in a way electricity rarely matches. Two practical implications:

A single gas bill is a terrible basis for an annual estimate. A summer bill makes any plan look cheap; a winter bill makes every plan look brutal. Our engine's seasonal projection exists precisely for this, and uploading two or more bills (ideally from different seasons) tightens the estimate considerably.

Winter is when tier structures bite. Deep-winter usage reaches into tiers a summer comparison never touches. A plan that wins your summer loses your winter, and the year is what you actually pay for.

The rest of the bill

The remaining furniture matches the electricity bill: billing period and day count (check it before panicking at a jump), meter read type (actual versus estimated, and estimated winter reads are worth challenging), any concessions or rebates as credits, and conditional discounts with the usual fine print. Our electricity bill guide covers those mechanics in detail, and everything there about conditional discounts applies to gas identically: when we price a plan, a discount only counts if it's guaranteed.

Put the bill to work

We compare gas plans the same way we compare electricity: thousands of published plans drawn from the government's Consumer Data Right energy register, projected onto your actual usage through each plan's full tier structure, with seasonal swing accounted for. Upload your gas bill (or gas and electricity together, since running both at once is the full household picture) and see your projected annual cost, ranked. Estimates, not quotes, workings shown.

For the broader diagnostic, start at why is my bill so high. For the tariff mechanics on the electricity side, see electricity tariffs explained, and for benchmarks, the average electricity bill in Australia.