Batteries Make Everything Great About Solar Even Better

Solar is great. Solar plus batteries is even better. 

When you generate your own electricity with solar, you have a level of energy independence. You have some insulation from electricity price fluctuations from your utility. When you add batteries to the mix, things get even better. 

Batteries allow you to time-shift your solar use and they allow your solar system to continue to operate when the grid is out. That's right, most solar PV systems shut down when the grid is down. But if you have batteries, you can continue to produce your own power and keep the lights on. 

In addition to these benefits. Batteries buffer grid demand and fill in the solar production gaps that passing clouds can cause... To illustrate this, I'll show you the energy use of our home on a hot August day. You can see the energy perspective from our home and how solar plus batteries radically changes how this appears to the grid.

Home Energy Use on a Hot August Day

The above image shows our home electricity use. You can see that our electric cars start charging at midnight and both were fully charged by 3AM. The spikes that happen throughout the day are for the air conditioner. As you can see, as the AC cycles on and off, our electricity demand cycles up and down accordingly. 

Now let's look at the solar production for this day. 

Solar production on a hot, cloudy summer day

This particular day was bright and cloudy. The passing clouds caused spikes and dips in solar production.

Looking at the two images above, you can see that we have spikey production caused by clouds and spikey demand caused by an AC unit turning off and on. How did these two interact and what was the resulting grid demand? See below: 

Day's Grid Usage

You can see that at 6AM we stopped using the grid. In the afternoon, we exported some solar production. We didn't start using the grid again until late in the evening at 10PM. This is a big deal. During the day, electricity prices are at their highest and we aren't using the grid at all during this time. 

How can the grid use be negative during the day? How were all of these spikes in both production and supply resolved? Batteries. 

Battery Usage (~charging AM, discharging PM)

Batteries supplied energy before the sun came up; they were the buffer that supplied energy when the clouds passed. Batteries filled in the gaps when the AC kicked on and we needed more than the solar was currently providing. After the sun went down at 8PM, batteries exclusively powered our home until 10PM. 

Looking at the image above, you can see at 6AM the batteries started discharging (above the line). By 7:30AM the sun was producing more than our home needed and the batteries started charging (below the line). By 1PM the batteries were full. From here the batteries alternated between charging and discharging always making sure that the grid didn't need to be touched with anything other than surplus solar production. 

By 5PM the solar production was not enough to power our home. The batteries started discharging, working cooperatively with solar, and taking on more and more of the home load as the sun was setting and solar production was fading for the day.

At 10PM the price to use the grid is back to its cheapest rate, so the battery shuts off and our home went back on the grid.

For us, our 12 kW solar array is not enough for us to be 100% self-powered, but (with batteries) it is enough to ensure that we're off-grid when electricity prices are the highest. We're on a time-of-use fee schedule. From 10PM till 6AM is off-peak time. This is when rates are the cheapest. This is when the grid has surplus capacity. This is when wind energy production is generally at its highest. This is when the dams have to keep some minimal amount of water flowing downstream for water quality, even if the power is not needed. 

Shifting our grid demand to 100% off-peak means that the existing infrastructure is better utilized. And it means that we don't have to pay the higher prices for electricity at mid-peak or peak times.

So if you have solar or you are considering solar, I suggest adding batteries to the plan. 

fin 

Disclosure: I am long Tesla