Understanding Why Tesla Energy Settled on 7kWh Powerwall
There has been a lot of speculation on just how solar users will select the right size energy storage for their application, particularly in reference to the 7kWh size of the Tesla Energy Powerwall. For customers who have access to net metering, (the practice of selling excess solar energy at the same price as one purchases energy), there currently is little incentive for energy storage. So for the moment, the U.S. market for residential solar backup is limited.
For those that are exposed to Time of Use (TOU) rates, or wholesale pricing like that offered by energy co-operatives, or those who want to understand how residential solar could play a critical role in distribution of balancing peak load demands, then this attempt of explaining through an applied data approach may help in understanding how the Tesla Energy 7kWh Powerwall, and similar sized storage systems, could be integrated.
The above graph is a shaded representation of a 6kW DC rated manual tilt solar array located near Charlotte North Carolina receiving on average 5kWh/m2/day. The lighter data represents premium solar activity, while the darker indicates minimum solar production. Below are two sets of data. The first being the numeric kWh daily generation totalling 8860 kWh. On average, this array sells 6000 kWh per year from daytime excess, and purchases 6000 kWh for evening an inclement usage.
The second table is revealing. On average, 16 kWh are used daily directly from the array except during southern summer months where 25 kWh are directly used. The second table subtracts an average 16 kWh from the generated amount and 25 kWh for summer months.
Here is the takeaway. The displayed data shows how many days could be used for solar storage and how much. The orange shaded data reveals excess under 9kWh. The green indicates the potential for two Powerwalls or comparable energy storage. Non shaded data reveals days with no storage capacity.
This displays on one hand how difficult planning an off grid system would be, while on the other hand, how successful energy storage could be used in reducing the duck curve, or peak utility load by distributing power back to the grid for 4-5 hours. The latter, being the the real potential for residential solar storage.
Without energy storage, solar only offsets peak load until 3:00 – 4:00 PM. The ideal solution would be to distribute energy back to the grid from storage for an additional four – five hours. System size and usage varies greatly. As solar adoption grows, it will reach utility capacity faster than say if future systems are balanced with energy storage. It is difficult for U.S. individuals to visualize the need while they are being rewarded with the net metering model. The imbalance of peak loads, combined with the ability of an energy storage solution, will drive a future model that will likely be designed around this burgeoning industry.
Until utilities or government are willing to pay a premium for the stored energy, it will be difficult to achieve this elegant solution to peak load.
After 10:00 PM, the duck curve, or utility generation goes very flat. Night time EV charging will open a more balanced market for utilities.
In addition to managing peak load is adding grid stability. Enphase CEO, Paul Nahi, states that data collected by modern inverters and potentially by other parts of PV systems can be used to help utility companies or transmission and distribution (T&D) operators to manage networks.
This leaves us with what to do with long periods of intermittent weather. This final piece very well may be filled with utility scale power packs, and/or other methods of gathering excess renewable energy such as the use of hydrogen and similar methods which combined will bring the most balanced grid we have known to date.
The first phase of solar adoption has already begun with the rapid expansion of utility scale and residential solar. The next phase will include the addition of energy storage. Currently, utilities are making up the bulk of storage purchases and may very well have realized a future in energy storage. If the utilities choose to continue the offering of net metering, they very well will control energy storage indefinitely. If net metering goes away, the market will respond with an answer to both peak load and individual economics, and Tesla Energy may have sized it just right