Sunday, September 02, 2012

Solar Power Update - Annual Costs

I've blogged before about our solar power installation and we just completed our first full year where we didn't change anything, so we have a clear baseline of how much we generated and used, which I will describe in this post.

Our system is a grid tied net metering setup. This means that we generate more electricity than we use during the day and run the meter backwards in the summer, then use (cheaper) electricity at night and in the winter from the grid. Once a year our PG&E bill is netted out and we pay the difference. Last year our total electric bill was about $500 but during the year we changed everything by adding a second solar array, a Nissan Leaf electric car and a heat pump for heating and cooling. This year, with a consistent setup for the entire period, our bill was negative, so we didn't owe PG&E anything. However, this didn't mean that PG&E paid us the difference, because the payback rule is based on how many KWh you generate rather than how much you would have paid for it. In our case, although our net bill was -$495, we used 610 KWh more than we generated for the whole year. The details of our PG&E bill are shown at the end of this post.

In effect, we could have used another $495 worth of electricity for free. Going into next year, we will set the heat pump to work harder at lower temperatures before it switches over to propane, which will reduce our carbon footprint and save us some propane costs. For some of last winter we used propane when the outside temperature fell below 45F, and at some point we reduced that to 40F. Unfortunately the controller setting we have is in increments of 5F, and I'm doubtful that we can pump heat out of air at 35F, but it's worth a try. We already converted our other appliances from propane to electric, and other than the heat pump our major consumers are a large tank electric water heater, induction range, clothes dryer, hot tub and well pump.

We did around 11,000 miles in the Nissan Leaf in the last year, but estimate that about a third of our charging was at work, as Laurel's commute is far enough to be an easy one way run, but requires a charge to get home again up the hill (we live at 2400ft, and she works at sea level!). She can take the car pool lane with the "white sticker" that pure electric cars get, which saves a lot of time, but running at high speed on the freeway uses a lot more power than around town. When I get to use the Leaf my commute is much closer and I don't bother charging it at work. Our long term average consumption is 3.7 miles/KWh, which is worse than most Leaf owners because of the freeway miles, hill climbing and "having fun". We don't pay for charging at work, and the marginal cost of electricity at home is zero because we are generating a negative net bill for the year, so we only use the "ECO" driving style if we are pushing the limits of its range on an unusual trip. The Leaf is the first car we pick for drives in its range, it's entertaining to drive as well as having extremely low running costs. All it needs for maintenance is tires, there is no engine oil to change, and the brakes wear very slowly due to electric regeneration. Here's the year so far as recorded by the Leaf's "Carwings" system.

The PG&E tariff we are on is called E6 Net Energy Metering, it has three rates for the summer, and two rates in the winter. The rules and rates are described in these documents at the PG&E web site: Rules and Rates
The rate that PG&E will pay us for any extra KWh we generate is about 3 cents/KWh as described in this document
Our base overnight rate that we pay is around 10 cents/KWh, the afternoon rate is around 28 cents/KWh, this is why we can generate hundreds of dollars in a negative bill while using hundreds of KWh.

Our solar setup is in two arrays that I described on my blog before, the total summer peak output is about 10KW, and we get up to 70KWh per day. In the winter this drops to about 40KWh per day, and clouds and rainy days greatly reduce the output. We're in a sunny spot on top of the mountains, and the monthly output from the 6.5KW array is shown in the plots below since we turned it on in April 2011. The other array adds about half this amount. Our annual true-up period runs from the beginning of September to the end of August.

PG&E send us a bill every month for about $11 of connection fees and taxes, and the current running total. Our final bill for the last year is shown below. Solar costs were described in a previous post.


  1. Considering the constantly growing needs of the human society, the solar power system seems to be the most effective source of renewable energy available.

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  2. Agreed. The solar power is indeed the suitable renewable energy for us to use. Although during the first time we will purchase it it will cost us in the long run having a home solar energy will save us lots of money.