Thursday, November 12, 2009

finalizing specifications

Choosing a grid-tied system has its costs. The main one is that when the utility power goes down, so does the solar electricity. This is for several reasons. First, "grid tied" means what it says, tied to the grid, not to the house. It serves to mitigate some or all of the use of grid power, but the house still runs off the grid. Second, the house cannot run off power from the panels alone (inverted, of course). This is because of the fact of brownouts caused by clouds passing in front of the sun, or surges in demand. So a storage system is required as a buffer. This consists of a battery bank, which I decided against for reasons detailed below.

Our contractor visited with the solar energy engineer on Tuesday. They measured the estimated average daily production based on the hours of sun that can be expected per day. This measurement consists of first, a device that records the sky exposure of the site. Based on the annually integrated solar track, shadowing from trees, adjacent buildings, even our TV aerial, cloud-cover data for our location, and other parameters, it computes a number that they claim is quite reliable. In our case, we will get an average of 5.4 h/day of production. This comes to 300 kW-h/month, which is just over half of our energy usage.

We will purchase 2 kW in panels, but with a 4 kW inverter, so if we choose to add capacity, it means only additional panels. The panels are 200 W models from Canadian Solar. The inverter is a Sunny Boy 4000US. The reason to forego the batteries that would have given us a standalone system is mostly cost. The batteries are expensive and short-lived. In fact, I calculated that whatever we save in electricity costs, we will spend in replacing battery cells, which have roughly 5-year lifetimes.

Our contractor is PR Green Tech Corp. Julio Correa, the owner, is friendly and attentive, and appears to be quite devoted to his avocation. I hope this will serve us into the future as technologies evolve.

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