Sunday, March 20, 2011

More about where we are with solar power

Up to now, the peak time of electrical generation for our system has been roughly between midday and 3pm, when the sun is shining directly onto the panels. Increasingly over the last few weeks, as the sun has got higher in the sky, the solar panels have been reaching their "peak" earlier in the day. The peak on a sunny day corresponds to an output of between 1500 and 1800 W, though we have measured outputs close to the maximum output possible of the inverter (2000W). The power tends to drop off after around 3pm, as the sun gets close to the treeline behind the house. Again, this should get better as the sun gets higher in the sky.

The most effective way of using the generated power would be for us to use as much of it as possible. We are paid (3p per unit!) for half the units we generate anyway, regardless of how much (or little) goes back into the grid, so we might as well use what we can.

Our house seems to have several "modes" of using power, and none of them really fit with the output of the solar system. During the day, the house tends to tick over at somewhere between 300W and 500W, assuming nothing much is happening. At night, once lights go on, it tends to sit between about 600W and 800W - though there's not much the solar system can do about that. As soon as appliances are on, this increases - but appliances rarely draw power continually, even if they need quite a lot. The kettle, for example, uses around 3kW - but only runs for a couple of minutes. The washing machine, ovens, the iron and so on can draw around 2000W, but not for the whole time they are on. The electric shower is by far the most demanding electrical thing in the house, but it's not used a large fraction of the time (fortunately!!).

So assuming the solar system is producing a healthy output, even having appliances on is not likely to consistently use all of its output. Unless a load of things are on at the same time, a significant amount of the power is likely to end up going into the grid.

However, on even a reasonably bright day, within three months of the summer solstice, the solar system seems to be able to cover the bulk of the "background" power use from say around 10am to 3pm - 1.5-2 units, and perhaps some of the use of appliances during the day. So we are trying to shift our behaviour - to put the dishwasher on after breakfast, to delay the washing machine and dryer to later in the day, or run it at weekends. The difficulty of managing our electricity usage to get the most out of the solar system during the daytime highlights how significant the feed-in tariff is in making the cost of the system bearable. Micro-generation of electrical power is an asset, however small, for the country as a whole. Whether it is worth 41p a unit is debatable, but without the FIT encouraging the take-up of such schemes, it would be hard for a person to justify its take up in economic terms.

The new option of a domestic fuel cell is potentially very interesting. This adjusts its supply of electricity to match demand - day or night - and when it can't supply enough, the grid supplies the shortfall. The grid will get a lot less from this form of micro-generation - but the proportion of a household's electricity that it is likely to cover would be substantially higher.

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