Gas station without pumps

2016 July 30

Average annual power use

Filed under: Uncategorized — gasstationwithoutpumps @ 00:47
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I just got my “True Up” bill from PG&E—it has been about a year since the solar panels were installed. During that time, the panels generated about 2.63MWh of electricity (7.2kWh/day): 77kWh more than we used during the year. PG&E reimbursed me $2.11 for the extra electricity, but wiped out the $106 of Net Energy Metering credits that we had accrued from generating electricity during peak time and using electricity during off-peak times.

Next year, we’ll be facing a minimum delivery charge of about $120 for the year. If we follow the same peak/off-peak usage, that means that we could use about another $226 worth of electricity without increasing our bill (other than losing the $2.11 credit). That would be about 1.5MWh off-peak, or only 660kWh peak consumption. What that translates to for us is that I will be buying a dehumidifier for our house, to reduce the condensation on the walls. Current Energy Start rated dehumidifiers remove about 1.85 liters of water per kWh used, and I don’t think we need to remove 2775 liters of water a year (7.6 l/day) from our house, so the dehumidfier will add nothing to our electricity bill.  Based on reviews (in Consumer Reports and on Amazon), we’re looking at the 30-pint Whynter RPD-321EW Energy Star Portable Dehumidifier, is it has good performance in cool rooms (our house gets quite cool in winter, especially when we’re both at work) and is relatively easy to empty (we don’t have a convenient way to rig up a drain hose).

We are fairly light users of electricity by US standards. We used about 2.63MWh a year, but the US average is 10.932 MWh/year, and the California average is 6.744MWh/year [].  PG&E also reports what people in our area use: similar houses use 6.042MWh/year, and efficient similar houses use 3.262MWh/year [].

Part of the reason we use so little electricity is that we rely on natural gas for heating, hot water, cooking, and clothes drying, using about 433 therms a year.  Here we are not particularly efficient: PG&E reports that similar houses use 548 therms/year, but efficient similar houses use only 293 therms/year []. Shorter showers and setting up a clothes line would probably reduce our usage, but heating is the biggest chunk, and our house is already as cool as we are willing to live in.  We’ve invested in insulation over the years, but there is only so much you can do with a poured-concrete house for sane amounts of money.

A therm is about 29.3001 kWh, so our natural gas use is about equivalent to 12.7MWh—much more energy usage than our electricity!

We’ve been planning to buy carbon offset credits for our energy usage this year (see previous estimates in Solar lies).  Nothing for electricity of course, since we had a slight surplus there.  According to PG&E, natural gas produces about 6.1 kg CO2 per therm (and their electricity generation is about 238 g/kWh, only slightly more than the 208g for the same amount of energy from natural gas) [].

I calculate approximately the following CO2 production from our various uses this year:

My wife and I have considered taking another trip this year, to Boston, which would add another 4.9MT. Note that flying is by far the most energy intensive thing we do—reducing travel is probably the only way we could significantly reduce our carbon footprint.  As carbon offsets, we’re considering projects like, which cost $6–$10 per MT.  Do any of my readers know of good carbon offsets that aren’t scams or just enabling polluters?


2016 January 3

Solar lies

Filed under: Uncategorized — gasstationwithoutpumps @ 18:27
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Enphase, who make the microinverters I use and monitor their output, reported to me

Enphase Energy maximizes your solar energy production and keeps you informed about your system. Your monthly energy report shows how your system performed and how much you contributed to offsetting the global carbon footprint.

Week Peak Power Energy Produced
12/01/2015 – 12/07/2015 925 W 24.7 kWh
12/08/2015 – 12/14/2015 887 W 23.3 kWh
12/15/2015 – 12/21/2015 928 W 22.5 kWh
12/22/2015 – 12/28/2015 1.05 kW 26.1 kWh
12/29/2015 – 12/31/2015 873 W 13.4 kWh
December 2015 Total: 110 kWh
Previous Month Total: 140 kWh
Year to Date: 1.00 MWh

For more details on these production results, please visit your Enphase® system.

Your Carbon Offset for this month: 168 lbs

You have offset the equivalent of: 2 Trees

The total energy generated seems correct, but the carbon offset is clearly wrong.  PG&E reports emission factors in the range 0.391–0.641 lbs CO2/kWh (or 0.177–0.291 g/Wh) for their electricity generation, depending on the year.  []
That means that my generating 110kWh reduces CO2 emissions by 43–70.5 lbs (19.5–32kg), not 168 lbs.

Enphase’s computation using 1.53 lbs CO2/kWh is quite high—not worst case (lignite coal is 2.17 lbs CO2/kWh []), but much higher than the US average of 1.222 lbs CO2/kWh used by [].

Of course, household solar energy production in the summer mainly affects peaking, not baseload power generation, and that averages about 0.689551 g/Wh across the US [].  With that number, 110kWh would be 75.85kg CO2 (167 lb), so that is what Enphase is reporting—the largest number that they could conceivably justify.

But in the winter, mid-day is not peak power usage—early evening is in California, so it really isn’t justifiable to use the national, year-round average peaking generator CO2 emissions for California winter solar generation.  The annual non-CO2 emissions for non-baseload power in California is only 1.01887 lbs/kWh  (0.462 g/Wh) [], which is the largest number that Enphase could honestly claim in the summer (probably still high for PG&E, which uses a smaller proportion of fossil fuel generation than most California utilities).

Enphase is not the worst offender in inflating carbon offsets—the “Cool Campus Challenge” at UC had some really ludicrous estimates of how much CO2 would be reduced by various changes in habit. For example, they seemed to assume that turning off a computer monitor when not in use would save 200lbs of CO2 a year, which for PG&E electricity would be 300–500kWh/year, or 36–58W.  But most monitors drop into a power-save mode after being idle for half an hour, taking less than 1W, so a more honest estimate of how much CO2 would be saved is around 15kWh, or 6–10 lbs CO2.  Their estimates of 50lbs of CO2 reduction for unplugging vampire loads was similarly ludicrous—most of the power supplies in my house meet at least Level IV efficiency standards, with “vampire” loads under 0.1W. Unplugging all of them would save maybe 1w, or 3–6 lbs CO2.

I wish that people who provided carbon offset calculations would be more honest about them. It does no one any good to  think that small symbolic gestures do much to reduce their carbon footprint.

One thing that is a little more than symbolic—closing my laptop so that it sleeps reduces its power consumption by 20W, so closing it for 12 hours a day saves 88kWh/year, or about 34–56 lbs of CO2.

But almost anything I do with electricity reduction is mainly symbolic, as my biggest carbon footprint is usually for heating my house (and hot water) with natural gas: about 433 therm/year or 2300 kg CO2 (5100 lbs) [].

I expect this year to make a couple of family trips, for a total flight distance of around 14000km, adding another 2500 kg CO2 this year [] I may do another 2000 km of Amtrak train rides (about 70–90 g/km [European figures for diesel traction from]) for another 140–180 kg CO2.

Even as a bicyclist without a car, transportation contributes a lot to my carbon footprint—more than I expected.

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