People talked earlier about round-trip efficiency of lithium energy storage. I think it is appropriate to share my own experience here.
I am adding a new block of battery storage of 12 kWh to my solar system. It is divided into 28 modules, and I carefully measure each module capacity so that I can later check their degradation with age and usage.
I have a voltage and current logger which records the I and V every 10 seconds during charging and discharging. It gives me a data file to import into a spreadsheet for me to compute the power (P = V*I). The modules are 120 Ah nominal, and I use a relatively low current of 10 A, so that the charging and discharging take 12 hours each.
My spreadsheet showed the round trip power loss being 5.7%. That's decent I thought, but what could be expected with this type of cells? Then, just out of curiosity, I measured the voltage loss due to the wiring of the setup. I had 7 milliohms of resistance that was not accounted for. That alone caused a loss of 3.5%. The cells themselves are better than 2.2% at the C/12 rate.
But there's more. I adopted Tesla's method of attaching each cell to the bus using small wires that serve as fuses at the cell level. That caused roughly a 1% loss. The intrinsic cell loss could be down to 1%. Lithium cells are very, very good!
With a much better professional cell assembly, along with top-notch electronics for the charging circuit and the inverter, I think total system efficiency can easily get to 10% loss or better for a round trip.
But even if the loss is 20%, it is still quite good. Pumped hydro power storage has 20-30% loss by comparison.
California is in dire need of more batteries to store the excess solar energy it generates in early spring early in the day, and has to pay Arizona and other neighbor states to take some off its grid. Yes, California has to pay to give away power some time. Yet, it still has to import around 30% net over a year period. When you have solar power out the wazoo some time, you need a place to store it. You do not care if it is 50% efficient if it is cheap. Lithium batteries are very good, but they are still too costly.
If you cannot afford enough batteries to store for use at night, when will you be able to store for a rainy day or two?
I shared this article in LA Times in a past thread, and it is worth repeating here.
On 14 days during March, Arizona utilities got a gift from California: free solar power.
Well, actually better than free. California produced so much solar power on those days that it paid Arizona to take excess electricity its residents weren’t using to avoid overloading its own power lines.
It happened on eight days in January and nine in February as well. All told, those transactions helped save Arizona electricity customers millions of dollars this year, though grid operators declined to say exactly how much. And California also has paid other states to take power.