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Hi Zen,

Thanks your interest in EmonTx, it is still in development and yet to be officially launched on the site.

Your correct, EmonTx only measures current (using a CT) and not voltage. It calculates power P=VI by assuming V=240v (UK grid AC voltage) and PF=1. This is not as accurate as measuring voltage and power since the voltage and power factor can fluctuate. In other worlds EmonTx measures apparent power, not real power.

EmonTx has been designed to be a battery powered wireless node which is easy to install. Often we found when installing energy monitoring systems that there is no power point close to the electricity utility meter to plug in a AC-AC voltage adapter which is needed for voltage measurement. 

I am considering in future versions of EmonTx including the option to plug in an AC-AC adapter to measure voltage. 

Hope this makes sense.

All the best, Glyn. 

Hi Glyn,

   Thank you very much.  I got it.  

   I have been planning to build something like power  box  ( http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2008/cj72_xg37/cj72_xg37/index.html ) and EmonTx.

     I found some product such as efergy e2 http://www.efergy.com/Products/efergy-Shop-Energy-Monitors/EFERGY/Efergy-E2/pid-184320.aspx that can show watt information but using only CT sensor. I think they would estimate other values  like your answer.

    May you have any ideas what is the best way to estimate to get most accurate watt information ?

Sincerely,

zen

Hi Zen,

The two estimates which are required are grid voltage and power factor. Grid voltage can be estimated to be 240V as this is the UK grid voltage (+-10%). The power factor can be assumed to be close to 1 as for resistive loads. AC power thory is explained well by Trsytan on this page: http://openenergymonitor.org/emon/node/61

CT only power measurement is a trade in towards convenience and ease of installation and accuracy. If higher accuracy is required see 'Mains AC non-invasive 3.0' in the input buid section. This design includes voltage measurment.

Glyn. 

Hi Glyn

    Thanks for your help.  May I use your design (EmoxTx) be some part of our product ?   Is it open source project that i can do that ?

Best regards,

zen

 Hi Zen,

The emonTx hardware designs and software (to be released soon!) are made available under the GNU GPL V3 open-source License and follows the terms of the OSHW (Open-source hardware) Statement of Principles 1.0 .

What this basically means is that you are free to modify, re-distribute and even market the design as you wish. As long as you attribute OpenEnergyMonitor and make any resulting designs available under the same open-source license. 

I look forward to hearing about what you do with the designs. We have some PCB's which we hope to make available to buy through the openenergymonitor website in the near future. 

Hope this answers your questions. 

All the best, Glyn. 

Hi Glyn.

    I will look forward to hearing your progress of EmonTx. 
  

Thanks for your advice.

zen