Putting the battery together with the solar system also makes it eligible for 30% tax credit.
If there are incentives, then best planning is to support PowerWall 7kWh system with PV, to shave the peak off home use, so the fossil hit to the utility is lower. If the electronics are streamlined, I could see PG&E being asked to require peak shaving batteries for all subsidised PV installs of the future. This will reduce peak capacity issues, reducing utility costs and fossil fuel use, so both the people at large and the utilities should be requiring battery installs, not discouraging them.
This design would require software upgradable controllers that are capable of negotiating smart grid cooperation with the utilities. The problem if we let government or utilities design it is they will want to have full control. Instead, prenegotiated SLAs between homes and utilities could be done on a daily basis, coded by the programs and protocols of smart grid, and enforced by best practices coding of software upgraded on every controller. Typical SLAs would just be agreements to do what's probably smartest, like local peak shaving with a % pool reserve for utility demand (utility asks for power, PowerWall provides it as per utility demand). The standardized SLAs could fit in 30 bytes or less; negotiation would be about % for this or that with associated $, upper and lower limits, ratios, curves. Pretty much the utility states its typical desire, then the house counteroffers what it thinks it can do. After some back and forth refining with a 7 ping pong limit or so, some side accepts, usually the utility (especially if it reaches the refinement limit). If the average requirements of the utility aren't being met to certain thresholds that day, evening, night, month, whatever, certain incentives can be adjusted by the utility for each home to encourage more of what the utility wants.
This is safe for the utility, its banks, generators, wires, the grid operator, the environment, homes, the home pocket book, the engineers and designers of backup and grid tie systems, and everyone. Each day the smart grid decides what it wants for the next day and gets commitments in place, and can plan for and order the next day's energy with plenty of time to spare. The next morning when the weather forecasts get more precise, some renegotiation can be attempted by both sides -- no more than two renegotiation requests every ten minutes (max any type 10/10 minutes), let's say, just to keep the computers happy. The utilities won't have to have insane reserve capacity because they can just tap home batteries as needed. The banks won't be mad that reserve capacities reach unpredictability levels that make them nervous. The smart grid can move electrons from above-normal sunny areas with more battery charge to below-normal cloudy areas, softening the blows from weather. Days with all cloudy weather everywhere and no wind would define the worst case, but the utilities would plan for those. Perhaps they have, in addition to generation, some battery farms, and some large multi level reservoirs they can use gravity to pull energy hydroelectrically from until the bad weather abates or they get bigger generation online.
A company such as Tesla needs to be the leader in this.
If it's left up to the utilities, it will be a mess because the utility will want to own the home batteries and everybody is going to get mondo confused and do everything ass crap. Government would be even worse.
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Looking to the future, I see it like this: at present, Tesla or some better (smaller team size) engineers design the first standardized SALe and computer programs and equipment. Then, through time, after decades, the utilities, grid operators, regulators and markets can wrestle over the SLAs, how they work, software protocol adjustments if absolutely necessary, etc.. But they wouldn't be able to go too far astray, because the initial example shows the minimum quality achievable, and it will just work so well, paired with the fact the homes would make their impacts known and felt directly to the various push and pull of the other parties by making PV array size, battery array size, use curves and monetary commitment decisions of their own.
