Archive for April, 2013

72% of our usage is grid supplied, even though we produced more than we used!

Supply Mix

In the last year (April 1, 2012 to March 31, 2013), we used 6,046 kWh of electricity and produced 8,576 kWh for a net surplus of 2,530 kWh. That’s awesome right?

But 72% of the electricity we used was supplied by the grid when solar could not supply enough power to cover the need at that moment, like at night or on a cloudy day*.

Or another way to look at it, although we produce more than we use, most of what we produce we don’t really use directly. We only use 28% of what we produce. The rest goes back to the grid to pay back what we used when the sun wasn’t shining and to build up a surplus for a rainy day.

So even with all that sun, we still draw a lot of power from the grid that requires coal and other nasties to be burnt to serve our electricity needs.

It makes sense, most of our heavy use, hot water for showers, cooking, washing dishes, all occurs early in the morning or evening when the sun is not at it’s brightest or best angle. The more we time our usage to occur when the sun is shining, the less we demand of the grid.

There has to be a common industry term for this? Anyone know? Percentage of power supplied by the grid as compared to total usage when solar or other renewable is in the mix?

To me this seems like a much more important number to track if you have solar and are concerned with your direct carbon producing footprint.

* In order to find how much energy we used from the grid I added up all the usage values on an hourly basis that were greater than what was being produced by solar. For example from 5-6am, total demand was 1000Wh. The sun was just coming up and the system was only producing 200Wh. That means the grid was supplied 800Wh during that time. 80% is grid supplied for this hour. Now do that for every hour in a year. Hint, it helps if all your energy values are stored in a database.

Estimating heat energy for 2012 – Revised

Now that we have January – March 2013 circuit-level usage values, I thought I’d go back and revisit my original 2012 heat estimate using a different method.

I had estimated January – March 2012 heat energy based on a linear regression analysis of our April – December heat values. There are a number of problems with this approach. Mainly that heat pumps use more energy the colder it gets outside, and secondly the amount of passive heat we gain from the sun can significantly reduce the amount of energy required for heat.

This time I used a less formulaic approach to estimate heat energy usage. I simply calculated the kWh/HDD per month for 2012 and 2013, and compared the values.

First lets look at the first 3 months of 2013. We recorded 3,239 HDD, a 20% increase from 2012 to 2013. We used 746 kWh for those 3 months. If we divide 746 kWh by 3,239 HDD we get 0.230 kWh per HDD.

Now let’s try the same for the first 3 months of 2012. We recorded 2,107 HDD and I estimated 327 kWh for heat energy. 327 / 2,107 = 0.121 kWh per HDD. That is a 128% difference from 2013. Something is clearly off.

Since 2013 was colder and less sunny, I would expect our 2013 efficiency to be less because heat pumps become less efficient at lower temperatures. So I manually adjusted the 2012 kWh values so that the kWh/HDD percentage was similar to the 2013 values, then I lowered it a bit to take into account the warmer temperatures and increase sun in 2012. Did I mention this wasn’t very scientific?

What we get is closer to 620 kWh for heat energy for the first 3 months of 2012. This is roughly a 90% increase from my earlier estimate. It also means that a 20% increase in colder weather roughly equals 20% more heat energy usage.

Q1 2012-2013 heat energy comparison

Looking at Q1 performance again, that means out of the 445 kWh increase in 2013, 65% of that increase was due to heat energy, 33% was water heating and everything else was 2%. That sounds a little more realistic.

First quarter 2013 performance

Q1 2013 summary: 20% more cold, 22% more usage and 15% less sun.

Now that we are starting to collect our second year of performance data, I’ll be comparing our new data to the same period last year. Year-over-year comparisons should be more illustrative than comparing to previous months. I’m also switching to a quarterly reporting period to look for larger trends, but I will continue to post data monthly at

**See April 7 Update at end of post.

In Q1 last year our net usage was 228 kWh. This year is was 953 kWh, a 300% increase! In March 2012 we actually generated a surplus, but not this year.

Overall, here’s how the first quarter compares to 2012.


2013 Usage Solar PV Net usage
or (surplus)
daily usage
Total 2,499 1,546 953 28.1 3,238
881 478 403 28.4 1,189
812 449 362 29.0 1,067
806 618 188 26.0 983
2012 Usage Solar PV Net usage
or (surplus)
daily usage
Total 2,054 1,824 228 22.6 2,701
873 369 504 28.2 1,125
666 597 69 23.0 957
515 860 (345) 16.6 619
All values in kWh (except HDD which is base 65°F).

Seeing all the increases, I wanted to understand if our 22% increased usage (445 kWh) was due mainly to the 20% increase in colder weather (HDD), or if other factors were leading to our increased electricity usage.

Broadly speaking, hot water, heat and all other circuits (mainly plugs loads and lighting) each accounted for about 30% of the total energy for the 1st quarter. Out of the three, hot water is the only one we can accurately compare because we still lack circuit-level data for Q1 2012.

For the last three months we’ve averaged 74.8 gallons of water per day, up 46% from first quarter 2012. That’s an extra 600 to 800 gallons of water per month, or nearly 24 extra gallons per day. This includes a 25% increase in hot water usage. Although we don’t have circuit-level data on the hot water heater, we do have monthly hot water usage data. We can use this to estimate the energy required to make hot water in Q1 2012.

Our hot water heater used an average of 275.8 watts to heat one gallon of water in Q1 2013 (water heaters have to work harder in the winter). We used an extra 533 gallons of hot water in Q1 2013. 533 * 275.8 = 147 kWh. That’s 33% of the overall usage increase right there.

As for the cause of the increase? To remain happily married I will only say two words, goats and mother-in-law. I don’t think I should say anything more on that topic.

Now let’s look at heating.

Determining the increase in heat energy is not easy to answer because we only have estimated usage values for heat in Q1 2012. Estimated values for heating have a much higher degree of uncertainty. Using this quarter’s values to check the accuracy of my 2012 estimate shows that my estimate may have been off quite a bit. It’s hard to know how much it may be off because I don’t know an easy way to factor in heat gain directly from the sun shining through the windows. Below are the heat pump electric usage values for Q1 2012 and 2013.

Heat (kWh) 2012* 2013 change
Total 327 746 128%
Jan 149 282 89%
Feb 119 270 127%
Mar 59 194 229%
* 2012 values estimated (see Estimating heat energy for 2012).
All values in kWh.

Based on the values in this table, we used an additional 422 kWh or a 128% increase in energy use for heating in 2013. This would almost completely use up the 445 kWh overall usage increase! So I know this can’t be correct. For example, if I use the HDD formula, 0.2261 x 1900 HDD(base 50) + 0.756, I get 430 kWh, which is far below the actual recorded 746 kWh. Which means my estimated heat values for 2012 are off. But we know that heating has increased some amount due to the colder temperatures and cloudier skies. We just can’t determine how much it increased.

So let’s look at all other circuits. Again we can’t really compare apples-to-apples yet, but there are a number of changes that have occurred this year. As I mentioned, we now have a barn full of goats, chickens and other critters. On the coldest days we used heated water buckets to keep the water from freezing. In March we had an egg incubator running for 23 days and and we’re using grow lamps to start our garden seedlings. As of March 30 we’re also now using a 250W heat lamp in the chic brooder. This will last for a few weeks before we switch to a smaller wattage, pushing our energy usage up 4 to 5 times it’s usual base load for about 5-6 weeks. Soon the incubator will be in use again. We’re going to try hatching and raising some turkeys this year.

In summary, Q1 usage is up compared to last year. Some of the reasons for this are fairly obvious, others remain difficult to tease out of the data. Hopefully next quarter will offer better comparisons and we’ll finally be able to compare apples-to-apples with circuit-level data. It should be easier to see where usage has changed and why. If we understand why, then we have a better chance of improving performance over time.

April 7 Update: Using a different method I estimated  that heat energy most likely did increase 20%. That means out of the 445 kWh increase in 2013, 65% of that increase was due to heat energy, 33% was water heating and everything else was 2%. That sounds a little more realistic.

You can see heat maps and detailed charts of temperature and electrical usage at View hourly, daily and monthly values for solar, usage, net usage, temperatures and HDD.

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