Using TED-5002 “The Energy Detective”

In the ongoing effort to reduce his environmental impact on the planet, The Bear this week has installed a “TED-5002” on the home electrical panel. Other weblogs have described the T.E.D. installation process, so I won’t go into that here. Instead I’ll share some details about our particular grid-tied solar electric production system, and how the TED-5002 will help us monitor our solar production and total household electricity usage.

Here in Indiana, most of our electricity is produced from coal. By learning to live with less electricity, we can help reduce the production of global-warming greenhouse gases that result from burning coal. Our first steps in that direction focused on conservation — there  are many ways to reduce the amount of electricity we use. (For that story, listen to the 2005 TameBear Radio Podcast #11 – “Preparing for Solar.”) Then in 2006 we hired local solar contractor Home Energy, LLC to install a pole-mounted array of solar panels in our backyard, along with an inverter to convert DC electricity from the sun to AC electricity that our house could use. Our system is “gried-tied,” meaning that when the solar panels are not producing enough electricity to meet our needs, we get the extra juice from the local utility grid; and when our panels are producing more electricity than we need, the excess is pushed out onto the grid. (We also have a battery pack in case of a power outage; more on this in a moment.) We have a net metering contract with our local utility, Northern Indiana Public Service Company (NIPSCO), which specifies that we receive a credit for all excess electricity we generate, at the same rate that we are charged for electricity we draw from the utility. (For more details, check out TameBear Radio Podcast #26 – “Solar Powered.”) Here is a simple diagram of our solar electric system:

Our Solar Electric Generation System

After our system was installed, Treesh and I diligently read and recorded the numbers from two utility meters on the outside of our house. One meter measures how much electricity we’ve used from the grid, the other measures how much electricity we’ve delivered to the grid. We put those numbers into a spreadsheet and were pleased to see that the solar electric system was producing electricity to meet 90% of our needs. But after the first year, the nightly checking-of-the-numbers became too tedious, and we stopped recording. Furthermore, our total electric usage has been gradually creeping upwards over the past four years, and that’s been kind of a downer.

Because our power to and from the grid has this ebb and flow based on how much electricity our solar panels are producing on any given day, it’s been difficult to know exactly how much total electricity we’ve been using. We can see our net grid usage each month when we get our NIPSCO bill, but that doesn’t tell us anything about the total amount of electricity we’re producing, and how much of that we’re using.

So along comes T.E.D.to the rescue! The Energy Detective (TED) is a device you install in your electrical panel which measures and records electricity usage over time. We learned about this from a friend just last month. He helped us install two measuring devices (MTUs) and get our monitoring set up. One MTU measures that ebb and flow on the line connecting our house to the grid. Sometimes we are using power from the grid, sometimes we are pushing power to the grid (when our panels are pumping out more electricity than our house can use.) So this meter can read negative (drawing power from the grid) or positive (pushing power to the grid). The second MTU measures the total load our house is drawing. And the sum of these two numbers tells us the total amount of solar electricity our system is producing at any moment.

So for example, it’s about 10:30am on a sunny Wednesday morning, and MTU2 shows the house is using a total of 550 watts of electricity. MTU1 shows 60 watts going out to the grid. (It’s a positive number — excess solar electricity.) Adding these two numbers together tells us our solar production system is currently generating 610 watts of electricity (give or take a few watts). So at this moment, the solar panels are providing all the electricity our house needs, and putting a little excess out onto the grid. Nice!

MTU1+MTU2=solar power production

(These meter readings are from the “TED-O-Meter” app on the Bear’s iPhone.)

We learned early yesterday morning WHY it’s called “The Energy Detective.” We had expected that NET reading (our total solar production) should never go below zero — that would be negative solar production. But when I got up this morning at about 6:30am, NET was -0.800kW! Was something wrong? What could be sucking that much power, and why wasn’t it showing up on our house load? We’ve figured out that our Outback charge controller was doing it’s scheduled 3-stage battery charging. That’s something we had not been aware of prior to the TED. So keeping the backup batteries charged uses about half a kWh of electricity each day it recharges. (We don’t know yet if this recharging happens every day, or once a week or what. We will see!) Prior to the TED we had no way of knowing the “energy cost” of the battery backup power. Now we know.