Electricity Bills

People often ask how my solar panels affect my electric bills. My total cost of electricity for 2017 was $521.37. Here’s a chart from my utility that illustrates my monthly electric utility bills.  Note that my average monthly bill is $45.

If one assumes that the average monthly bill for a south Florida home is around $200, then I have saved $18,600 in electricity bills over the ten years my solar panels have been in operation. If one assumes that the average home’s bill is $250 a month, then I have saved $23,600 in ten years. Projecting my system’s operational life out to 50 years shows a saving of nearly $100,000.

New Mini-Split Greatly Increases Efficiency

I’ve been running my house using my new air conditioning procedure.  I turn off the central air for most of the day.  I run the Mitsubishi, high efficiency,  mini-split in my bedroom at night.  In the morning I run the central air for about half an hour to take the humidity out of the house.  It’s proving to be very efficient to operate this way.  Yesterday, the solar panels on my roof generated 94.26% percent of the electricity needed to run my home.  The days are the shortest of the year and many of them have been cloudy.

First Days of Mini-Split Operations

Percent from solar, last ten days.

My new SEER30+ mini-split air conditioner has been operational for a few days now.  I’ve been running it at night with my central air conditioner off.  It’s difficult to say how much impact that has had in increased efficiency, but there is no doubt it has to have contributed greatly.  Other factors to be considered are cooler temperatures and cloudless days.   Looking at the percentage of electricity from my solar panels over the last ten days, and considering that these are the shortest days of the year, I can only conclude that it seems to be a big step in efficiency for my house.

Daily usage chart for November 19, 2017. Green bars show buying from the grid, gray bars selling to the grid from my solar panels.

Mitsubishi Mini-Split A/C

Ten years ago, when we conceptualized our photovoltaic system as an emergency power source when the grid was not available. The plan included a mini-split air conditioner for the master bedroom. The idea was that during an extended grid outage, such as occurs after a hurricane, we would be able to power our house and have an air conditioned refuge powered by solar panels and battery backup.

Recently, Hurricane Irma caused a six day power outage and the system functioned wonderfully, powering the house day and night. However, the refuge room concept had not been implemented, even though planned for. Those hot sweaty days and nights motivated me to implement this final component in the system.

On Wednesday, November 14, 2017, JWair and Urban Solar installed a Mitsubishi, inverter based, SEER 30+, 9,000 btu mini-split air conditioner in my master bedroom running on a 240 volt dedicated circuit pulled from my two inverters.

In the weeks and months to come, I will be testing the system by running it on solar panels and battery backup. I will collect and publish data on its performance here. The unit will provide increases in efficiency by allowing me to turn off the central air at night and sleep with only the master bedroom cooled by the Mitsubishi. In extended outages it will provide air conditioning powered by solar and battery backup.

Six Days On Solar Power Emergency Backup

My emergency circuits powered by solar panels and battery backup.

We lost power at 3:30 pm on September 10, 2017, as the outer bands of hurricane Irma, impacted West Palm Beach. At that time, due to the extreme weather conditions, little if any power was being generated from the rooftop panels, but the system switched instantly to battery power (120 volts from the inverters to all emergency circuits).  We continued on battery power until  3:30 am. when battery voltages reached the low-level cutoff of 46 volts and the charge controllers switched the system off.

My 8, 240 ah, Concorde battery, emergency backup bank

The next morning after the sun came up and the panels started supplying power, energy was first directed to the batteries to bring them up to charge.  When the batteries reached a level of 50 volts, the system restored power to the house, while continuing to bring the batteries up to 52 volts.  At that point, the system switched to float to maintain the batteries at a fully charged state through the day while supplying power to the house, including two refrigerators, lights, ceiling fans, and TV.  As the sun went down, the system switched to the batteries as the source. The second night the system shut down at 6:30 am when the batteries again dropped to 46 volts.

For six more days the process continued but with no shutdowns due to reaching the lower battery voltage limit.  With a little more care in usage, I was able to have continuous, uninterrupted power for four more days.  Power from the grid returned in the afternoon of September 16, 2017.


Outback Power Systems charge controllers.

Early in the morning on September 16, I became over confident about how much I could run while on battery backup. While still running on the last of the battery power, I started a load of laundry, made toast and coffee, and squeezed out the last of the battery reserve. Had I waited until the sun was fully up, I would have been okay. The charge controllers shut off the house at the low voltage threshold and directed all of the energy from the rising sun into topping off the battery bank. As soon as the batteries get up to around 50 volts, the house was powered up again.

All around me during the six days the grid was down generators were running noisily with their owners refueling them every few hours.  In conversation with my neighbors, I found their noisy generators were running no more circuits in the house than I was.  There was one exception, one neighbor was able to coax his air conditioner online for a couple of hours by shutting down all other circuits in his house.

Outback Power Systems, solar emergency backup system.

I would say that my grid-tie photovoltaic system with battery backup performed exactly as it was designed to do.  I was able to run two refrigerators, all lights, ceiling fans, and 60 inch Samsung TV.  All of my lights are LED lights except for one halogen light which I avoided using.  I am most appreciative of the design work by Roger Messenger and the great installation by Mike Vergona and his crew at Urban Solar.

Spreadsheet for first four years of operation.

I kept daily readings of my electric meter and my charge controllers for the first four years of operation.  These thousands of data points are a valuable resource for examining the performance of my system and learning its specifics.  For example, I searched all of the total generated column for a maximum value and learned that the most my system ever generated in a day was 30 kwh.  That seems like a lot but on that particular day it was only 46 percent of our energy.  It was a hot day and the house had yet to be tweaked for energy conservation.  I also had a large RV plugged in with its air conditioner running.