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Summary of Electrical Voltage Measurements

What follows is a brief summary of the thousands of measurements I have made of the electrical voltage supplied to my home by Ontario's Hydro One power utility, beginning in December 2015.  After some general observations, a chart and a table show how much voltage my appliances have received on average each month since then.  In the past, Hydro One has often supplied too much voltage, that is, well in excess of an acceptable voltage (an average of 120 VAC).  A few words about methodology can be found here

Five General Observations about the Voltage

  1. the voltage tends to be higher at night than during daylight hours or early evening;
  2. during Dec 2015 through March 2016, the voltages delivered to my home were much too high, and that is what I am complaining about;
  3. during July, August and September, 2016, the average voltage at my home was near normal, that is, near the nominal voltage of 120 VAC, and I have no complaint about that;
  4. however, since then the average voltage has gradually increased, and is perhaps cause for concern (see chart and table below);
  5. my measurements did not find any significant relationship between the day of the week and the average voltage on that day.

Comparing the Months

Graph of average monthly voltages

The graph above and the table below will give you a good idea of what voltages we get from our wall outlets at my home south of Ottawa, especially the difference between winter and summer months.  If your electricity is supplied by Hydro One, maybe you suffer from the same excess voltage during the winter months as I did during the winter of 2015/16.  Keep in mind that the average voltage should be near 120.0 VAC, although at any given time it might be anywhere in the range between 118 and 122 VAC.  That would be "normal" and acceptable, but voltages as high as 127 VAC (as I have measured) are definitely not acceptable, although Hydro One might try to pass them off as "normal". 

Summary Table Of Monthly Voltage Measurements
Month Average Voltage Range of Voltages No. of Measurements
Dec 2015 124.25 VAC 121.0 - 127.2  135
Jan 2016 124.22 VAC 121.1 - 127.7  238
Feb 2016 124.17 VAC 121.1 - 127.4  283
Mar 2016 123.70 VAC  119.8 - 127.3  347
Apr 2016 122.68 VAC  119.7 - 125.7 272
May 2016 122.27 VAC 119.8 - 124.2 283
Jun 2016 122.51 VAC 120.5 - 124.4 246
Jul 2016 119.43 VAC 115.2 - 124.1 393
Aug 2016 119.44 VAC 114.9 - 123.1 317
Sep 2016 120.67 VAC  117.1 - 123.8 301
Oct 2016 121.12 VAC 117.6 - 124.3 344
Nov 2016 121.55 VAC 117.5 - 124.8 342
Dec 2016 121.91 VAC 118.7 - 124.7 373
Jan 2017 121.30 VAC 117.3 - 124.4 295
Feb 2017 121.86 VAC 118.9 - 124.7 224
Mar 2017 121.51 VAC 117.3 - 124.8 351
Apr 2017 121.44 VAC 118.4 - 124.6 316
May 2017 * 121.64 VAC * 118.2 - 124.8 * 213 *
Jun 2017 122.52 VAC 118.1 - 125.7 222
Jul 2017 122.15 VAC 117.6 - 125.5 366
Aug 2017 ** 122.4 VAC ** 119.2 - 125.0 ** 59 **

* Final results based on measurements for the first 22 days of May 2017.
** Interim results based on limited data for the first 14 days of August 2017.

Average voltages within the range 118-122 VAC are acceptable and are highlighted in green.  High and low voltages within the range 118-122 VAC are acceptable and are also highlighted in green.  Voltages over 127 VAC are considered dangerous ("extreme" as specified by the Canadian Standards Association), are therefore not acceptable and are highlighted in red.

More detailed information about the voltage measurements at my house can be found here with separate graphs for each month beginning December 2015 and continuing up to the end of last month (updated monthly). 

Finally, if you have questions or comments about any of the information here, please contact me


  • all measurements are taken manually;
  • measurements are taken at more or less random times during the day and night (as a light sleeper, it is easy for me to take a few measurements each night, though not as often as during the day);
  • measurements are generally taken less often than once per hour;
  • all measurements are taken at the same electrical wall outlet;
  • all measurements are taken using the same digital voltmeter (DVM);
  • this DVM has been checked for consistency against my other voltmeters (I have several);
  • voltages are measured to the nearest one-tenth of a volt;
  • all monthly data is compiled in a QuattroPro spreadsheet and treated the same as for other months;
  • averages (means) for each month are calculated to the nearest one-hundredth of a volt, given that data sets for each month generally contain more than two hundred independent measurements;
  • please see Got Data? below if you would like to share and compare raw data and analysis with me.

Got Data?

If you have voltage measurements from your own home and would like to share them, please contact me and I would be glad to exchange raw data, analytic results and other observations with you.