The morning of the first day of each month is when I read the meters and download another month's worth of circuit data from the SiteSage monitor. I have 39 months of data now. There are some items so consistent that I could move the current transformers to other unmonitored circuits, for example the clothes washer - 0.4 kWh/month, month after month. We apparently aren't substantially dirtier season to season. Then there are the predictable seasonally variable loads - the refrigerator and freezer use more energy in the summer (low 40s each per month) and less in winter (high 20s). Similarly, the heat pump water uses more in the winter when the basement is cooler (low 60Fs) and less in summer as it warms up, while our usage of hot water is remarkably consistent at 14 gallons daily.
Once I gather the data each month, one of the first things I look at is the net energy flow - did we need to import energy from the grid, or did we have a surplus of generation and therefore exported energy? This can vary substantially. Let's look at March, the month just past.Here are the net kWh for each of the four months of March 2014 - 2017:
March 2014 - 29 kWh imported
March 2015 - 48 kWh exported
March 2016 - 216 kWh exported
March 2017 - 53 kWh exported
Quite a variation - what's happening? Well, the big variations are in the weather - how cold and how cloudy was each month? In the table below, all values are kWh except for heating degree days base 65 (HDD65) and the ratio of energy used by the heat pump to the HDD65.
| Used | Generated | Net | Heat pump | Used except heat pump | HDD65 | HP kWh/HDD65 | |
| Mar-14 | 565 | 536 | 29 | 343 | 222 | 1043 | 0.33 |
| Mar-15 | 446 | 494 | -48 | 247 | 199 | 1023 | 0.24 |
| Mar-16 | 358 | 574 | -216 | 150 | 208 | 789 | 0.19 |
| Mar-17 | 461 | 514 | -53 | 209 | 252 | 1013 | 0.21 |
In March 2014, I was still working out the control settings on the heat pump which caused it to cycle to full speed then stop, so the high usage that month is primarily the heat pump. In March 2016, the weather was sunnier than normal, and much warmer - presto, a large net export of energy. Our usage in March 2015 and March 2017 was very similar, as was the solar generation, so the net export numbers were almost the same. The heat pump used less in 2017 for a similar amount of heating demand, and I'm pretty sure this is partially accounted for by Jill building a few fires in the woodstove on cold evenings. Our usage of energy besides the heat pump was up in 2017 over 2015 - no one item stands out, but slightly more use of the range, lights and plug loads, HPWH, and ERV add up.
Over the long haul it tends to average out, but we don't draw too many conclusions from one year's worth of data!
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