Summer of 2018
The summer of 2018, especially July and August, were unusually warm and humid in Ontario. The extra use of air conditioning equipment in homes, stores, other businesses, and factories placed a heavy load on the power generating system of Hydro One and their partners, Ontario's electricity provider. Measurements of the voltage at my home in eastern Ontario show that, as the temperature went up, the house voltage tended to go down, as would be expected in the case of an increasing load on the electricity system.
Regulating the voltage is a complicated business and the variable load presented by customers' appliances and machinery is just one of the many factors involved. The decrease in voltage shown in the chart above was not large, attesting to the ability of Hydro One and its partners to continue to provide a reasonable voltage in spite of the increased load on the generating system, but it was large enough to measure, as shown in the chart.
My study did not take account of day of the week, which might be an additional variable affecting the delivered voltage. Also the data for each day (usually 144 measurements) were lumped together and reduced to a mean voltage for the entire day and a mean temperature for the entire day. For the three-month June-July-August period, data is plotted for 91 days. Still, the dependence of delivered voltage on ambient temperature is fairly clear.
Temperature data for Ottawa International Airport, not far from where I live, came from Environment Canada. I must point out that Environment Canada's "mean" temperature for each day is not a mean of 24 hourly temperature readings, as might be reasonably expected, but is instead the simple average of the maximum and minimum temperatures for that day. This is how Environment Canada defines "mean temperature", a definition probably disputed by most physicists and statisticians.
An Equation of Voltage
Graduates of high school algebra will be relieved to know that the somewhat scattered data points in the chart can be reduced to a least-squares-fit linear equation, in this case
V = -0.1162T + 124.1
where V is the mean house voltage (VAC) for the day and T is the mean temperature for the day in degrees Celsius. See the sloping red line in the graph. So for every Celsius degree of increase in temperature during those three months, the voltage at my home decreased, on average, by 0.116 VAC.
Please don't hesitate to contact me if you have comments or questions.