I was recently contacted by a woman who had purchased a home with a solar electric (photovoltaic or PV) system on the roof. Let’s call her Britt. She had found my name in the paperwork left by the previous owner since I did the initial consultation and site survey. Britt sounded very nice on the phone and wanted to know why her energy bills were so high. “Is my PV system even working?” she asked.
I reviewed the original customer record and took a look at her roof on Google Earth. Britt had her most recent energy bill on hand. Her home and PV system are very similar in size and capacity to mine, yet her summer electric bills were $200 higher. It made for an interesting comparison. As we worked through the problem, I was able to help Britt understand more about her system as she provided additional information that ultimately explained the discrepancy.
Due to the orientation of her roof, the PV system faces almost due east which results in about 12-15% less annual energy production than an optimal south-facing array. Also, there are some power lines and a chimney to the east that may impede some of the available energy. My site survey also recorded some pine trees that, if still there and now larger, may also impact production. This information was provided to the original owner to help them make their solar power purchase decision.
For the monthly service period of July-August, my south-facing system produced 994 kilowatt-hours of energy while Britt’s produced 653, a difference of over 34%. We expect her east-facing system to produce up to 15% less and it is about 4% smaller. While it is possible that there is a problem with the system, it is more likely that the remaining 15% loss is due to shading and perhaps the accumulation of pine needles on the panels. Cleaning panels is usually not required but there are exceptions.
Net Metering Statement
Once we had an idea of the site impacts, we reviewed Britt’s net metering electric bill. Net metering refers to the Nevada program that allows NV Energy customers to produce their own electricity and send excess back to the grid for credits against future energy use. Each of the five lines on the statement express the unit of energy called a kilowatt-hour (kWh):
- KWHD = energy DELIVERED to your home from the NV Energy grid.
- KWHR = energy RECEIVED from your home by the grid (excess solar energy that you do not use on site).
- KWHA = energy ACCUMULATED on your account, if any (can be negative for the month if subtracting from previously accumulated credits).
- KWHN = the NET energy you pay for (KWHD – KWHR plus effect of KWHA if applicable).
- KWHG = total energy GENERATED by the solar array (AC).
In Britt’s case, she had no accumulated credits and her net was over 2,000 kWh, thus the higher bill. We discussed the impact that home efficiency and lifestyle choices have on energy use. She said she was using efficient lighting and appliances, but hers was an older home and was probably less efficient. A home energy audit could be helpful but overall, Britt was still perplexed by the higher bill.
I again mentioned lifestyle and thought of pets. I asked if she had a doggy-door. She said, “No, we have cats so we just leave the door open.” I laughed at her joke. No one leaves the doors open in the middle of summer in Las Vegas. As it turns out, she was not joking. We had solved the “mystery” of her high energy bills.
I had just been reminded how disconnected from reality some can be when it comes to energy use. Common sense is the most important ingredient, but apparently it is not as common as one might expect.