Last August, we installed 24 SolarWorld solar panels on the south- and west-facing parts of the roof. They’re rated at 6.24 kilowatts under ideal conditions, and have on very sunny days can reach over 5 kW.
We were already almost one year through our lease of the all-electric Nissan LEAF, and had recently heard that a Washington State sales tax credit on solar panel equipment was due to expire, so we were thinking that this would be as good a time to invest in solar power as any, and signed up before the end of July. Because of this expiring tax credit, many other solar panel customers had placed their orders and so solar panel installers were all very busy. Installation actually took place late in August, so most of the sunny months of the year were behind us.
Just the other day, though, the web site that tracks how much energy was produced and consumed put a smile on my face:
What you’re seeing here is a sunny day interrupted by a few clouds (green bell curve), the Nissan LEAF being recharged (narrow and tall pink column), and my Current Motor Super Scooter being recharged (wide and short pink column). The spikes every 15 minutes are caused by the electric blower for the central heating. The heat itself comes from natural gas, so it can’t be measured here.
The numbers above the chart count how much energy was used, was generated, and the difference. The blue side shows what happened during the time period shown in the chart, in this case one 24-hour period in March. The green side shows what happened over a one-year period. We’ve only had the panels since last August, so this is currently the same as the total overall production.
What this means is that our house’s daily
energy electricity needs, plus the power necessary to recharge our electric vehicles for our daily commutes can be 100% covered by solar power, even if a couple of clouds pass overhead. (Edit: Strictly speaking, our energy needs are not met because solar power does not generate the natural gas consumed by the house’s furnace or hot water tank.)
When the sun is hidden by clouds or fog or are covered by snow, the panels generate less energy so production is less during winter. This is why the overall production shows a deficit. However, with spring coming, sunny days will be more frequent, so our production might look more like this 7-day snapshot from the end of September:
During the week featured in the chart, there were some days with less sun and some with more, but we still came out slightly ahead. As you can see, the peak solar wattage produced is roughly equal to what the LEAF draws to recharge, but commuting in the LEAF doesn’t empty its battery, so it doesn’t need all day to charge. This means there’s energy left over to power the house and my scooter…or sell back to the electric company.
Puget Sound Energy, our electricity and natural gas utility, reconciles the production vs. consumption numbers every April for those customers who generate electricity from solar, wind, or other sources. We have not yet had the solar panels long enough to know how this accounting is reported, so we’ll have to get back to you about that afterwards.
If we understand it correctly, there are several numbers that go into the calculation for what we owe the electric company.
- When our solar panels generate electricity, it gets measured by a new meter outside the house, and the utility keeps track of the total generated (even if we use some of it)
- When our solar panels generate more electricity than we need, then the surplus goes into the grid and the utility sells it to someone else
- When our solar panels generate less electricity than we need, then we pay the utility for the difference
So what do we actually pay for? We pay for what we actually use that came from the electric company. If the solar panels generate more than we need, we don’t pay the utility for that. If they generate less than we need, we pay only for the difference. Solar panels only work when the sun is up and not hidden behind clouds, fog, or snow, so we always pay for some electricity.
So what happens when the panels make too much electricity? That extra electricity goes back into the grid and is sold to some other customer. The utility keeps track of how much, though, so we will get a credit for that.
So how does the credit work? I’m not sure I understand their accounting method, but this is my working hypothesis:
- First, we owe for the net of what we pulled from the grid. That’s what we get billed for every month. Note that this is less than what our electrical devices and appliances actually consumed because some of that electricity actually came from the solar panels.
- Second, we are credited for the total amount that the panels generated. Note that this is not reduced by the amount of electricity consumed, unlike what we pay is reduced by how much we generate.
- Third, if the surplus exceeds the amount of electricity consumed, the utility does not owe us a credit. In other words, if the solar panels make more electricity than we can use (aggregated across the entire year), then the electric company gets the surplus for free and does not have to pay us for it. In other words, we have no financial incentive to make more electricity than we use (aggregated across the entire year).