Jhm, why do you think stores need more than 400kWh of storage? Do you know how the system works? Most likely it is for CA based stores. walmart took advantage of rich tax credits to lower their power costs with solar in CA and now want to hedge against peak load pricing by the utilities during times when their solar array does not fully support the store power draw during peak times or there is intermittancy. This can be done with a 400 kWh storage device offering peak output of 200KW. The way peak load shaving works is the battery puts out power enough to lower the grid draw for a period of time when the peak load occurs. Usually mid afternoon with the highest air condition usage on a sunny day. But on a cloudy but hot day, you can get periods of high AC draw and low solar output leading to higher peak charges from the utility. Even though stores have solar, on cloudy days, they still may pay peak load fees. The battery can also help with selective power for blackout periods or when signaled from the utility to go into higher output mode due to grid constraints. That is the future of smartgrids which tell downstream customers to cut back due to limits in the systems' capability.
one thing to think about is utilities in CA are looking to limit net metering in the next few years. Big arrays on walmart stores equate to a big savings in store electric bills while lowering revenues to the grid. Utilities want to cut net metering down to at least wholesale feedback pricing. That may be less than half the benefit of net metering. On top of that, peak load fees make having solar a lower return on investment long term. With battery systems, businesses like walmart can get ahead of this and cut their costs by shaving peak load fees now and storing solar power themselves rather than sharing it on the grid. It actually makes the community "good" of solar lessen because now your array does not feed your own needs plus your neighbors, it tries to keep the power local so that it limits feedback to the grid. As net metering is cut, businesses will look for tax grants and credits to help pay for battery systems to be installed to help cut their power bills once again. It is all about the money, really. Cutting expenses through peak load shaving has been around a long while. Various other companies already offer products to do this. It is not much different than computer room UPS systems which have been around for nearly three decades. Just google peak load shaving.
in CA, the grid peak usage is between 5pm and 8pm. This is the time when businesses may have to use the batteries to slow down their grid draw if they are getting hit with peak load fees. Solar is still producing during those hours and batteries can also put out power lowering the grid draw. However if the solar array was charging batteries from say 10am through 3pm then it was perhaps not sending excess power back to the grid as much, lessening the community sharing nature of peak sun. That in turn raises grid demand earlier in the day.
What some companies have done to cut peak load is a full lighting system replacement to LED or newer forms of FL lighting. More light with less power and heat. Reducing demand need is always cheaper than adding renewables and batteries. Unless there is government money paying for it. Cutting lighting-heat cuts AC usage. Ever walk into a Home Depot lighting department with all the light bulbs glowing? They have replaced most of the demo lights with LED bulbs over incandescents of a few years ago. That is one example.
final thing about CA. Much of this is due to state legislation. They seem to be unwilling to install new base load plants, such as nuclear. By not growing base load and messing around with various forms of renewables and pricing structures, they are becoming an island of their own in terms of power pricing and local regulation. It may not end well for the common solar homeowner customer who finds that net metering goes away and their smart meters report and factor in peak load pricing too. Solar city is hoping to sell you a battery to solve this problem and I am sure the CA government and federal incentives will be there to help pay for it.
it wasn't long ago that Nat Gas fuel cells on site were going to be a clean way to shave power. Heard of a company called Bloom Energy? The Bloom Box? Many hydrogen fuel cell companies are also trying to get into the peak load shaving and green energy business too. Getting crowded and diluted.
some good info:
Making the Case for Smart Grid to Shave Peak Power : Greentech Media
Bloom Energy Server - Wikipedia, the free encyclopedia
