[This post is part of a series about our net-zero residential solar project – see a list of links to the full series here, a list of frequently asked questions here or click here to bring up all Green-related posts. Next Post in Series / Previous Post in Series.]
Once you’ve determined your site is viable, the next set of questions surround what your system will cost. The first step in assessing this is looking at your historical energy consumption from past power bills (or your account summary on the electric company’s website) to get a sense of what your annual consumption and expenditure is. Next you look at the amount of roof space available for panels and the expected production of those panels given the site orientation. Now you have the target, and a sense of the resources available to meet it.
Is a true net-zero project possible on your site? In other words, can you make as much energy as you consume in a year? If so, do you even want to take on a project that large, or would you be happy with a modest system designed merely to defray some of your electricity costs and usage (say for example, the electricity you use on one major energy-hogging luxury such as a pool pump or hot tub if you have either of those.)
If a modest system is your goal, you have a pretty easy set of choices: you can optimize your system for convenience, appearance, budget, or whatever else is important to you. For example, you have the option of choosing any one of a variety of less efficient panels on a dollars-per-watt basis or based on the aesthetics of the system. For example, SunPower makes some panels that are all black and very sleek-looking. You give up some efficiency with the all-black design, but some prefer the look. Or you could choose cheaper less efficient panels and just put a few more up than you would have otherwise and still come out ahead on a dollars per watt basis.
If you are like us, however, and you really want to push for a net-zero installation, then you are going to be focusing on panel efficiency and maximum power output per square meter above all else. You are less of a dollars-per-watt buyer and more of a watts-per-square-meter buyer who puts efficiency ahead of other considerations. Of course, with more efficiency comes more power, but also more cost. There are two main reasons for that: first, super-high efficiency panels simply cost more because they are the state of the art and in greater demand for cramped installations; and second, a higher power output from the roof means more wiring and inverter capacity and associated equipment is needed in the basement to convert the output into household current.
So that first design sit-down conversation is where you need to set your level of ambition. In our case, our level of ambition was high – we were going to do every thing we could to get to net-zero. Fortunately, we had enough well-oriented roof area to get there with high-efficiency panels. We ultimately calculated that we would need 44 of Sunpower’s highest efficiency panels.
These E19 series panels have an industry-leading efficiency of 19.5% and they put out a staggering 240 watts per panel. This was an almost unthinkable level of efficiency just ten years ago, and is still about 30-50% more efficient than many, if not most, of the other leading panels on the market today.
An array of these E19 panels would fit very nicely on our roof, and would provide all the power we need over the course of a year. Now it is worth keeping in mind that it is all a balancing act – we’d run a surplus some days, but not others, and a surplus some months but not others, but we now knew we had a system design and equipment list which, over the course of a year, should allow our meter to run backwards at least as much, if not more, than it runs forwards.
This post is part of a series about our net-zero residential solar project – see a list of links to the full series here, a list of frequently asked questions here or click here to bring up all Green-related posts. Next Post in Series / Previous Post in Series.
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