The Cost to Time Shift Energy

The United States produces about 4.2 PWh (1 petawatt is 1x10^15) of electricity every year. Per day, that is about 12 TWh. Let’s say we wanted to time shift 25% of that energy from one point of the day to another, twice per day. For example, your wind farm output might be sagging in the early morning, and your solar output isn’t expected to ramp for another 4 hours or so. And then again in the afternoon.

That would require us to have 4 TWh of storage that went through two cycles each day.

Tesla’s Gigafactory is currently producing about 20 GWh of cells every year. Each cell can deliver 100% discharge for about 500 cycles. If you only discharge from 90% to 40%, though, you can get a thousand or more cycles. But let’s say the batteries will get discharged to the point that will yield exactly 730 cycles (365 * 2) and that will still give us 20 GWh of storage. It’s optimistic, but good enough for now.

To rely on cells to provide time-shifting for this 4 TWh of storage would require 200 Gigafactories, with 100% of their output making cells for the grid 24x7. And because the batteries would be used up after a year (700 cycles) you’d need to have those 200 factories making replacement batteries continuously.

A single 19 WH 21700 cell is about 70g. This means we’d need about 200B cells made per year, and about 15B kg of finished goods. Mining has roughly a 50:1 return by weight (gold is 150K:1, copper is 10:1, coal is very close to 1:1), which means you’d need to mine about 700B kg, or 700M tons of earth, to begin the process of making these cells. Mining costs for ore in general are perhaps $10/ton.

The US mining industry processed 1200M tons of coal last year .So, the mining effort for batteries would be on par with that of coal today. And just as we see train cars chugging across the midwest carrying and endless chain of coal, we’d see those same train cars carrying the other raw materials needed to make the cells. Non-stop. 24x7.

If the mining cost is about $7B annually, the selling price of these cells at $100 kWh would be $400B/year. Thus the mining cost would be about 2% of the delivered cell cost. Very small.

For comparison, hydro storage would be similarly challenging. Hoover Dam produces about 4 TWh of energy each year. We’d need 365 strategically located Hoover Dams to time-shift the energy target above. That has a much smaller chance of happening than a lot of Gigafactories.

The summary is this: Time-shfiting energy would require a massive undertaking. It’s doable, but it’s not something that could be started anytime soon. Just the mining aspect alone would prompt decades of fights in the court. That battle has already begun.

A final thought: The US drives about 3T miles per year. At 300 wh/mile, that’s another 900 TWh of electricity needed, or about 20% of our current consumption. In other words, the demand side will be going up a fair bit over the next decade or two.

Puget Sound Energy Fuel Mix

Tesla and Motor Metrics