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I think it wants 9V AC, and it sounds like your adapter might be outputting 9V DC.

Thanks , yes i just tested it with a multimeter, it is dc. I do have a 12vac adapter that has a 13.45vac  output at no load (im in Australia so input is 230v +- 6%) . Will i need to do hardware mods to use this?

Not all states have done the switch to 230V yet, although they all pay lip service to it.  Here in SEQ I believe it's still 240V, and I regularly see brief excursions past 255V.  Energex blame the widespread deployment of PV for that (and mis-configured inverters that don't cut out when they should: ~256V from memory).   Do you know what your actual grid voltage was when your unloaded transformer was outputting 13.45VAC?

There's some good detail on the resistor maths here:

http://openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-an-acac-power-adapter

http://openenergymonitor.org/emon/buildingblocks/acac-component-tolerances

and the schematic for your device here:

https://github.com/openenergymonitor/Hardware/blob/master/emonTxV2/schematic.png

Looks like R13 and R14 form the divider if you decide it's best to scale things back a bit.

Didn't measure mains voltage. At this point im just preparing for the arrival of my shipment. Ill buy a 9v adapter if it means replacing the resistors, just that i have the 12v lying around. On the emonTX am i trying to keep the input voltage <3.3v or 5v?

The input on your emonTx is designed for a 9 V ac adapter, which unloaded can output anything up to 25% more. As dBC says, what we really need to know is the mains voltage when you measure the output, so that we can calculate the maximum output voltage at maximum mains voltage. Only then can I tell you if it's going to be OK.

13.4Vac @ 243Vac mains, appreciate the advice. Thanks.

Aust supply apparently is 230v nominal +10% -2%

Page 36 of the following document shows the range to be mostly 245-252V

https://www.ausgrid.com.au/~/media/Files/Network/Regulations%20and%20Rep...

Whereabouts in Aus are you awinner?  The attached plot shows a typical week at my place in SEQ.  And this  document:

https://www.aer.gov.au/system/files/Energex%20-%2029.%20Power%20Quality%20Strategic%20Plan%20-%20October%202014.pdf 

shows just how difficult voltage regulation has become where there's widespread PV deployment.

On the emonTX am i trying to keep the input voltage <3.3v or 5v?

I don't have one to measure but the schematic above says 3V.  I'm not sure if that's a typo and is meant to read 3.3V.  That voltage regulator does come in 3V and 3.3V versions, but I don't know which one is used.  Others here should be able to shed some light.

I'm in Newtown NSW, close to Sydney CBD, a fairly "green" area, so there are a lot of PV installations around.

I'll look into the details tomorrow, one question first: are you using the ac adapter also as the sole power supply for the emonTx? (The alternative is that you use the ac adapter only to measure the voltage, and you use a 5 V dc USB supply to power the emonTx.)

Thanks, I will be using a usb 5v, the emonWRT is what im waiting on.

Sorry, that question was redundant - I just realised that you have an emonTx V2, which needs the separate power supply. That makes it a lot easier.

Assuming the maximum mains voltage you'll see is 260 V, that means your transformer will output 14.34 V maximum. You want to design for 1.1 V rms at the ADC input, so keeping R14 = 10 kΩ, you need to change R13 to 120 kΩ (from 100 kΩ). If you don't want to change the resistor on the pcb, you could add a 22 kΩ in series at the transformer secondary terminals. You'll need to change your calibration constant to about 236. (The actual 'correct' value will depend on the resistor tolerances, and whether you change R13 or add a series resistor.)

Thanks, good thing about the post being slow at this time of the year is that it gives you plenty of time to plan ahead :)

Found this today:

http://www.instructables.com/id/9VAC-power-supply-for-x0xb0x/

I had a stack of dc transformers lying around. Opened up a couple of 12v dc ones and tried the trick above but unfortunately the resulting unloaded ac voltage on the both was ~15vac, so i tried the same on a 9v dc adapter. The result is a AC adapter with a unloaded 10.75Vac @ 241v, im assuming this will work without any further modifications. I calculate my calibration constant as 246 is this correct?

Yes, the 9 V one is very close to the 'shop' adapter, so it will probably be OK. The constant is also good as a starting point, but you may need to tweak it by a percent or few to account for resistor tolerances and the uncertainty in the processor's reference voltage that the ADC uses.

Thanks Robert, appreciate the help.