Logging energy usage (per day) against external temperatures.

To the creators of this project and site... well done, this is really good.

I live in the US and have an interest in monitoring my home electrical power consumption and heating oil use.  I have been manually recording oil usage data for a couple of years using an elapsed hours meter connected across the vaporising blower motor of both the oil boiler and the oil-fired water heater. I wanted a way to see how my oil usage was reduced by improved insulation, boiler adjustments, thermostat tweaking, etc, but  for this info to be useful it needs to be correlated against the temperatures outside over the same logging interval.  To do this I have been using the published values of "Heating Degree Days"; these I get weekly from a local airport which posts data on weather underground, complete with an almanac of past weather data.  I believe the concept of Heating-Degree-Days is not so familiar outside the USA (see http://en.wikipedia.org/wiki/Heating_degree_day), but the assumption is that homes are heated to 65F (this is 18.3C).  If the outside temperature is 60F for 24 hours, this would be 5 (heating) degree days.  If it stayed at this temperature for a week it would be 35 degree days; similarly, if the temperature outside was 30F for 24 hours, then this would also be 35 degree days.  If the outside temperature is above 65F then no credit is taken for this (ie the degree days total does not reduce)!  In short, this is a great measure of how cold its been outside over a defined period of time.  Just outside NY city, we experience 5500 heating degree days per winter.

Useful graphs can be made plotting energy (boiler oil consumption, in my case) usage vs degree days (I normally do this over a week). Of course, there is a little variation (noise) in the data due to unmeasured solar gain through the windows, but this is still an incredibly useful tool - last year I saw that a $300 investment in insulation material (DIY) yielded a saving (when corrected for how cold the winter was) of about $170 in a single year.....  With a number of small changes my annual oil consumption is now less that 50% of what the previous owner used for about 20 years!

Now, where am I going with this?  This concept lends itself nicely to automation, and daily plotting of energy use vs degree days - and this is where the OpenEnergyMonitor could come in...  The hardware is mainly there (a 1-wire temperature sensor located outdoors in a shaded area), the addition of a simple monitor to show when the boiler runs (eg an LDR looking at the boiler run-light, or thermostat on the flue pipe)? and a (small, hopefully)! software addition would be required.

Simply taking external temperature data, say, at hourly intervals, calculating the difference in outside temperature below 65F, summing and dividing by 24, and resetting daily at midnight would give the degree days for each day. Summing appropriately could give the weekly value, monthly, and for the season (defined at 1 July to 30 June the following year).  Data could also be taken showing the boiler run hours for each day, so the two variables can be cross-correlated by plotting in a simple graph.

Sorry, I am not a programmer, but maybe someone might like to take this on???



seannation's picture

Re: Logging energy usage (per day) against external temperatures.

This is probably simply a case of making a small modification to the emonCMS software. There might already be functionality there for doing some custom plots... I don't know, because I've not used it. In principle you can just connect up your hardware monitors already in place to the emonTx, add a temperature sensor, and then use the software to make the plots you're interested in.

Not a definitive answer, but I don't think it'll be too tricky.

TrystanLea's picture

Re: Logging energy usage (per day) against external temperatures.

Hello AndyB, we are thinking along similar lines!! I've been doing a lot of work over the last month on buidling thermodynamic/heating models. Taking house dimentions, insulation, draught levels, heat input, outside/inside temperatures to create a model that yes could be combined with live monitoring to create some really interesting analysis.

With accurate monitoring of temperature inside and out and monitoring of heat input if you have gas or electric heatpump, one could use the model to reach a better value for the U-value of the buildings and therefore check for instance if an insulation or draught proofing measure is having the effect that was calculated it should give.


AndyB's picture

Re: Logging energy usage (per day) against external temperatures.

Hi Trystan,

Thanks for your note.  Since my original posting, I stumbled upon this webpage; http://www.energylens.com/articles/degree-days

This is a little negative on the use of degree days, but the conclusions are interesting and rather more encouraging.  Personally I believe in the use of this data, my two and a half years of monitoring seems sensible, and it does allow me to draw conclusions that I believe to be useful (it should be noted that these guys are trying to sell some software, and convincing people that this is all more complicated than necessary is probably a good tactic, for them)!  An Arduino-based automated system giving degree-day information with both US and UK formats (degree (farenheit) days with 65F base, and degree (centigrade) with 15.5C base, respectively), on a daily, weekly, monthly and seasonal basis would be incredibly useful.  I find that there are many weeks per year where the degree days are below 10 (degree (F) days for each day when averaged over a week - ie an average temperature above 55F (or ~13C), and the fuel consumption is at, or close to, zero - the reasons for this are discussed in the webpage above, but will include heat from people, electrical appliances, solar gain and insulation levels.  Above this threshold the consumption seems to be reasonably linear with degree days (non-linearities will occur due to variations in solar gain - which could be plotted on a third axis as solar panel production for the week (or day).  Like you, I would like to be able to make a decent thermal model of the house, and be very confident that small changes (improvements) can be resolved accurately and over a short timeframe without waiting for annual statistics.  Something like this could be extremely beneficial in changing people's behaviour with respect to energy usage.....



aideen's picture

Re: Logging energy usage (per day) against external temperatures.

Re: Degree days:

Oxford University's Environmental Change Institute are interested in degree days and have some information and links here:


For outdoor temps, you might be able to use data from a Pachube feed near you. e.g. for Cambridge, UK see:




DarrenEvans's picture

Re: Logging energy usage (per day) against external temperatures.

Thanks, that's sent me in a new direction ie monitor the outside temperature yourself rather than rely on third party data. I'd like to come up with an overall U value for the house as a whole.

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