Net Zeroers, Peak Shavers, and Beyond

Now that we have a pretty good understanding of how the grid works and how a home with a battery can aid the grid, it's time to get more into the specifics of what one can do with a home battery system. I'll be using data from our own house to provide examples. First we need to outline the options. I'm going to assume that all of these options involve a home with Solar Panels (as opposed to a wind turbine, etc...) In later posts I'll examine each possibility in more detail.

  • Net Zero homes have Solar Panels only and are connected to the grid. The Solar Panels make excess electricity during the day and return it to the grid. During the night the home draws from the grid. These two equal out, thus the "net" zero.
  • Peak Shaver homes have Solar Panels and a Small Battery and are connected the grid. Basically I'm arguing that this is what most people who buy a home battery should do with it. The battery gets charged from the sun, then the homeowner uses the battery to go "off grid" for a few hours a day during peak electrical demand times.
  • Off Grid homes have Solar Panels and a Large Battery. In this option the home is not connected to the grid at all. The home must generate excess electricity during the day, store it in a home battery, and then use the battery to power the home overnight. Usually these are remote homes that use a diesel generator for backup if the battery runs out.
  • Islanded homes are similar to Off Grid but the home is connected to the grid. Instead of using a generator for backup power, the home uses the grid. Note that this home can return power to the grid just like a Net Zero home if the Solar Panels are making more electricity than the home needs.
  • "Behind the Meter Storage Aggregated Home" -- this home doesn't exist... yet. I'm hoping that our home can do this someday. This home has Solar Panels, a Large Battery, and is grid connected. What makes it different from an Islanded home is that the utility remotely controls the battery so that the utility can give or take power from the battery depending on what's needed at the time. For example, during a really hot day with very high electrical demand the utility would take power from the battery. On the flip side, if there was excess wind energy one night (this has happened several times in Texas) the utility could store the excess power in the battery. Obviously having just a single home with this capability would be meaningless to a utility -- but if the utility could aggregate many homes like this... it would be the same as if the utility had a huge battery of its own!

So those are the basic "types" of homes with home batteries. Now on to what our home has been doing the last few months. Our home has a decent sized Solar PV array (7.7kW) and a large home battery (60kWh). There is more detail on the system in the "Our Home, Home Batteries, and Solar Panels" post. Currently it is late April 2017 -- the system went live in late September 2016 -- so I have 6 months of data. In our specific situation we're going to make plenty of power during the summer, but not very much in winter. This is because we have a huge (and very beautiful) Elm tree that causes shading problems when the sun angle is low in winter. But from April to September the sun is high enough that there isn't any shading of the panels. Because of this our home alternates between a Peak Shaver home in the winter and an Islanded home in the summer. I'll show you this below.

First let's look at a Winter day where we Peak Shaved:

The electricity our home used is represented by the grey bars, the power we take from the grid is red, the power made by the Solar Panels is green, and the line on top represents the state of charge of the battery. On this day our home used 37kWh total (we don't have natural gas, so that includes heating). Our Solar Panels only made 11.5kWh. The home was powered from the grid except from 5 to 9pm when the home disconnected from the grid and powered itself using the battery. From the utility's perspective, our home "disappeared" during peak electrical demand hours. 

Next let's look at a Spring day where we were purely Islanded:

Here we can see that we didn't use any electricity from the grid at all. Our home used 20.1kWh total, and the Solar Panels make 35.6kWh, so we had plenty of power. You can see how the battery is getting discharged overnight and into morning, then gets charged up during midday, then is getting discharged once again during the evening.

Finally let's look at another Spring day where we were Islanded but also returned excess power to the grid:

On this day the battery started out almost all the way charged. The battery discharged some during the morning, but pretty quickly gets charged back up to 100%. The system then returned 10.9kWh of electricity back to the grid from noon to 3pm (the light green bars). Once again we didn't need any power from the grid at all.

I hope this post makes it a little more clear what a home with Solar Panels and a large Home Battery can actually do "in the wild," and that a home can and will behave differently depending on the season. In the next posts I'll look at each type of home in more detail.