Thursday, December 24, 2009

Solar Power - Winter Solstice and New Ducting Update

I've previously shared the first few months of output from our solar system, we turned it on at the start of September, and on a clear day we were getting around 24KWh. As the days got shorter and the Sun got lower in the sky, the clear day output dropped off, and earlier this week we hit the winter solstice, the shortest day. Wednesday was a clear day, and we generated about 16KWh, as shown in the plot below.

We have also seen the benefit of our E6-time of use rate plan. We generate electricity during the week while we are mostly out of the house at the highest rate, and use it at the cheapest rate. So we continue to be net consumers of electricity, generating between 50% and 70% of what we use, but our electricity bill is basically zero at this point. In addition, PG&E just gave us a rebate of $67 due to lower energy costs, so the standing charge of $11.50/month is zeroed out for almost six months.

We converted our gas appliances to electric, for heating water, drying clothes and cooking, and currently only use gas for the furnace that heats the house. We looked into replacing the furnace with a ground source heat pump (GSHP) but got wildly varying designs and costs from everyone we talked to so have postponed that idea. The one thing everyone agreed on was that our ducting was sub-standard, with insufficient flow for A/C and barely enough for the furnace, and it was poorly constructed using the lowest quality materials.

So we have just replaced the ducts throughout the house, changing the configuration to include separate zones for upstairs and downstairs, adding a new return path from upstairs and one extra outlet. The ducts are insulated to R8, rather than R4.2, so they don't lose heat, and they have fewer leaks and a properly designed balance of air flow into each room. This is done using something called a Manual-J calculation, and there is also an independent analysis service that verifies that everything is working correctly. The main effect of this is that we will use less propane, and have much better control over temperature upstairs. We often want to heat the bedrooms without wanting to heat downstairs to the same level, and we now have a thermostat in the main bedroom for upstairs, as well as one downstairs.

I'm planning to blog some more about GSHP options, its quite complicated....

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