Thought of the Day
Confused...
OK, so that is how most of my thinking happens… I’m confused!
What’s next - dig in and see if we can find a way to get UN-confused.
I have this really nifty app that Sonnen (the Shell company) sent me that explains how my solar and battery system are working. I’ve been trying to see how I can increase our self-sufficiency. (As one REAL critic explained, self-sufficiency is not the only goal one might try to achieve. But that discussion is for another post.)
But as I review the results today I was really confused.
How can I have generated 15.7kWh so far today, and I have only consumed 16.9kWh, be just 53% self-sufficient? And why is there only 8.9kWh applied to my needs (energy world calls needs, demand)?
Looking at this diagram above, we can see the battery is discharging (the green line) throughout the evening. In my attempts to increase my self sufficiency, I have configured the battery to discharge from 4 PM (when the premium utility rates kick in) until 10:30 AM (when the sun usually starts reaching my solar panels.) With the goal through that period of minimizing grid imports, the battery is configured to allow discharge all the way to 30% while the grid is available.
As you follow the green line, today it only discharged to 40% until solar energy began to be collected (the yellow line). At that point the battery began to refill. The battery is a 12.5kWh pack, so with 60% discharged it needs 7.5kWh of energy to be fill back up. The solar array is just under 6.7kW, but this is late October, so that peak we see at 11:45 is just at 4.2kW. Any surplus solar energy not necessary to power the house begins to fill the battery.
The dog and I decided to find a new place to hike this morning and went for a drive. I kinda got lost, so the battery in the EV drained for 48 miles. The way I drive, that equates to about 12kWh of energy. When we returned home we plugged in the car to take advantage of the solar power available.
We see the demand (the blue line) changing dramatically as soon as I plug the car in. But the car doesn’t know about the limits of the solar array and so it begins to pull 7.7kW of energy to refill its battery. Over the time it takes to refill the car battery, the solar array averaged about 2.8kW, so the grid needs to supplement the supply to meet the demand.
Now I am starting to get a hint at why the self-sufficiency is so low. It has to do with the definition of consumption.
SolarEnergy - BatteryRecharge + GridEnergy = Consumption
And yes, the system does measure consumption directly. The ah-ha for me is that recharging the battery is not consumption, nor is the discharge of it supply. The battery’s machinations are nothing except a shift of the energy supply. The battery energy doesn’t care whether it sources from the solar or from grid supply. So it simply does NOT enter into the calculation of Self Sufficiency. But it does drain the supply that is used to calculate self sufficiency since that energy was NOT available to satisfy demand!
Let’s check and see if the numbers confirm this epiphany.
OK, up till now today (about 2PM) The house has been using around 300W/hr (5.2kWh/14hr) and the car took 12kWh. Adding those together (17.2kWh), we are pretty darned close to the 16.9kWh the app has reported. And we know that the car was going to drain at least 7.9kWh from the grid to supplement the solar. That’s where we see most of that grid contribution of 8kWh.
WOW! this all matches well with the self consumption reported.
The curious part to watch for the remainder of the day, up till midnight, is how the Self Sufficiency increases. We should have a little more demand that the battery will continue to offset after we reach 4PM and the battery is allowed to discharge.
But in any case, the 7.9 kWh supplement required to charge the car will keep self sufficiency from reaching to 100%, That is because regardless of the past experience, the battery is not allowed to discharge in excess of the realtime demand.
(It can never catch up. Kinda like the 5 year old can never become older than the 7 year old regardless how long they each live.)
OK, so now I stand Un-confused.