Many of us are severely inconvenienced if our local power grid goes down for even a few hours, and may be in a serious crisis if the grid were to be out for a day or longer. In this blog entry we will explore some questions based on measured and estimated values of our electrical consumption.

Q) How much energy is consumed by a house each day?
A) For an average October day, my home consumption was 17.8 kWh in 2012, and 16.2 kWh/day in October of 2011.

Q) Of that 17.8 kWh per day in October, how much of it was “essential items” that you would not want to be without power?
A) 4.2 kWh per day is what I would like to use in the event of a power outage.  This can be broken down as:
1.3 kWh to run the electrics on a natural gas furnace
2.0 kWh to run a 14 year-old side-by-side fridge freezer
0.8 kWh to run a new small chest style freezer
0.1 kWh to make a pot of coffee and a minimal amount for occasional use of phone chargers, lights, radios, computer, etc.

Some notes related to this:
While the daily coffee maker, fridge & freezer usage is relatively consistent throughout the year,  average furnace consumption ranges from 0.2 kWh per day in August to 3.3 kWh per day in February.  The furnace consumption could be reduced by setting the temperature considerably lower than the 21°C it is currently set to during the day.
Also, in the winter months when furnace usage is higher, the fridge/freezer usage could be reduced or eliminated by re-locating perishables to an unheated garage or shed.

Q) If your home solar install had battery back-up, could you survive all October with solar power alone running your essential loads listed above?
A) Yes, the 9.5 kWh daily average output produced from my rooftop solar install in October would have exceeded the 4.2 kWh daily minimum requirement. Most days I could have also made some use of lights, computers, entertainment systems etc.

Q) Could you run critical loads on solar alone year-round?
A) No, even with a sizable battery backup system (which I do not have) the increased furnace load and decreasing solar output realized in November, December and January means I could not meet the listed critical demand with my existing solar setup.  Here are how the historic daily average consumption & production numbers look:
October – require 4.2 kWh/day and produce 9.5 kWh (self sufficient)
November – require 5.4 kWh/day but only produce 3.9 kWh (not enough)
December – require 5.7 kWh/day but only produce  3.1 kWh (not enough)
January – require 5.8 kWh/day but only produce 3.8 kWh (not enough)
February – require 6.0 kWh/day and produce 8.7 kWh (self sufficient again!)

Note: if the existing solar array was twice as large – or could otherwise produce twice as much energy in the months with the shortest solar days (located in a location with less shading, or panels tilted at a steeper angle to take advantage of the lower position of the sun in the sky, for example) and if the system had a sufficiently large battery bank, it could power the critical loads year-round.  If this was desired, it would likely make sense to first upgrade the fridge to a more energy friendly modern model. As things are, we need to hope that any power outages occurring between November and January are short enough to allow the battery backup system to power the critical loads. At the current time I do not have a battery backup system fully installed, however, a “reasonable” system may provide 6 kWh to 12 kWh of backup power – or enough to run for one to two days in December without any added solar input. It should run for two to four days if “average” solar irradiance is also received for those December days.

Q) Where do you get your numbers?
A) Solar values are daily averages based on the recorded output from my solar electric install.  You can find current and historic output from that system here.

Energy consumption numbers are either measured directly with a plug-in power measuring device, or averaged based on historical values collected using a TED 5000 energy monitor.  This device is currently monitoring whole home usage, as well as electrical consumption by the natural gas furnace.

I look forward to, and enjoy, any: corrections, questions, comments, etc.