641 Therms: The Assumption Behind Every Georgia Gas Rate Comparison
When you compare natural gas plans in Georgia, the per-therm rate is the number marketers advertise. It is not the number that decides what you pay. That is the annual cost, and an annual cost only means something if every plan is measured against the same amount of gas.
The Georgia Public Service Commission handles this in its monthly Marketers Pricing Index. Each chart assumes one typical customer’s usage, currently 641 therms a year, and applies it to every marketer and every plan on that chart. That one assumption is what makes the PSC’s “apples-to-apples” price possible.
A rate is a price, not a cost
The annual cost of a plan has three parts:
- The rate per therm, times the therms you use.
- The marketer’s monthly customer service charge, times 12.
- The Atlanta Gas Light base charge for delivery, which you pay no matter which marketer you choose.
Change the rate or the service charge and the ranking can flip. The PSC’s September 2026 fixed-rate chart has three examples:
- Same rate, different cost. Xoom Energy and Stream Energy both filed $0.659 per therm. Stream’s service charge is $6.95 a month against Xoom’s $5.85, so Stream costs about $13 more a year.
- Higher rate, lower cost. Walton EMC Natural Gas filed $0.729, above SCANA’s $0.719. Walton’s lower service charge makes its year about $6 cheaper: $1,119.22 against $1,124.81.
- Lower rate, higher cost. Gas South filed $0.690, a tenth of a cent below True Natural Gas at $0.691. Gas South’s year still costs more once service charges are included.
Half the bill is the same for everyone
On the September 2026 chart the Atlanta Gas Light base charge comes to $580.53 a year. The full annual cost of a standard 12-month fixed plan runs from about $1,073 to $1,227. So roughly half of every bill is the same whichever marketer you pick.
That is why a rate that looks much lower moves the total less than you might expect. At 641 therms, a one-cent difference in rate is worth $6.41 a year. A $1 difference in the monthly service charge is worth $12.
The assumption has changed
641 therms is the current standard, not a permanent one. The PSC has revised its typical customer twice since January 2019:
| PSC charts from | Therms per year | Annual AGL base charge |
|---|---|---|
| January 2019 | 717 | $348.16 |
| December 2023 | 613 | $460.63 |
| May 2026 | 641 | $582.18 |
Within one month’s chart, every plan is measured the same way. Across months, it is not. An annual cost from a 2023 chart and one from a 2026 chart were calculated on different usage and a different base charge, so a change in “annual cost” over time is partly a change in the assumption, not only in prices. Compare per-therm rates when looking across years, and annual costs when choosing between plans in the same month.
How Open Natural Gas uses it
Every annual cost on this site uses the usage and base charge from that month’s PSC chart. We did not invent the formula; we publish the PSC’s own inputs in a table you can sort.
There is one deliberate difference, and it is on the methodology page. Some marketers file a service charge as a range. The PSC’s published bill uses the high end. Where the marketer’s own filing says which end a new customer pays, we use that one. For Gas South in September 2026, that is $6.95 rather than $9.95, so our estimate is $1,106.22 where the PSC’s is $1,142.22. Both ends of every range are in rates.json.
What to ask before you sign up
When you see a low gas rate advertised anywhere, ask whether it includes the service charge and the base delivery charge. If it does not, it is a price, not a cost. The number to compare is the annual cost at the same usage, in the same month.
The Georgia Public Service Commission does not rank or endorse marketers.