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Robert Haylar
Interests: Electronics
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For those still following: Answering a question about a SolarCity spokesperson saying that the Powerwall doesn’t really make sense for most homeowners yet, Musk stated: “For the specific case of SolarCity, what they’re referring to is that, there’s two version of the Powerwall — there’s the daily cycling version, and there’s the Powerpack version. One’s energy optimized and one’s daily cycling optimized. For the daily-cycling-optimized one, the economics is true in the US, with rare exception, are more expensive than utility. So, if somebody wants to do daily cycling — basically, go off grid — it’s going to be more expensive than being on grid. “This doesn’t mean that people won’t buy it, cause there are people who want to go off grid on principle, or they just want to be independent. That’s what the SolarCity comment is about." I am not any happier about that, than those who believed.
Well, the above says nothing about how the Powerwall suddenly makes the difference. But... From the E.I.A: California Avearge monthy household consumption: 557/Kwhr Average tariff: 0.1619/kWhr Average month bill: $90.19 From Southern California Edison; Lowest tariff; $0.11/kWhr Highest tariff: $0.46/Kwhr ( summer only) It's worth noting that the average price paid is closer to the lowest tarrif, that the highest. Remember that the battery's sticker capacity is 7kWhr, but in the interest of cell life, depth of discharge will be 80% ( battery software will ensure that it is) Then, there are battery,charger and inverter losses. Say,10% Practically, the battery will output around 5kWhr - at least some of its claimed 10 year life. I will leave it to you to show how you can make a return on the $6000 investment made in the battery and inverter. I calculate that you can save no more than $500 per annum, even in this case where the tariff differential is large. Some advice: Don't go on vacation in the summer (or any other time), or you will miss the opportunity to 'save'. Also, ensure that you don't demand more than 2kW from the battery, and be at home during the day so that you can discharge the battery when the tariff is highest. Elron Musk will thank you for that.
By the way, I realise that the average tariff does not fully reflect the difference between high and low tariffs which the battery is said to exploit. As far as I can ascertain, other than exceptional cases such as Hawaii, the night tariff is typically 2/3 of the daily tariff. I doubt that difference would better my assumption that the stored 7kWhr were free.
As earlier stated by another poster, the matter does not concern home storage and generation in general, nor inverter costs, but just the Powerwall's contribution towards the aim. The Powerwall's storage is too low, and so is its power output. Figures are amusing. Taking the given dimensions, the energy density is 50Whr/litre. Power density is a laughable 10W/litre. Those results are, in part, due to the use of a large number of small cells. Tesla were by no means the first to see the cost advantages of using bulk commercial cells to make packs - but that approach has its limits. Cost of assembly and battery volume are large. In the car, that worked out because a very expensive and high performance vehicle was wrapped around the battery. The employed NCA cells have both high power and high energy density, but one can't be divorced from the other, and that can lead to unhappy combinations of storage capacity and power output. Cells must be arranged in series and parallel strings, dictated by the 18650 format. NCA is great for a performance car, but not storage. 18650 is great for portable applications, but otherwise, a millstone. Telsa need that specific NCA cell, or they have nothing, and new cells are becoming a threat. But, is the Powerwall useful? I doubt it. Volume sales are necessary to success, so one should use general numbers, and not those that flatter. The US is the current market. Average household consumption: 900kWhr/month. Average tariff: $0.12kWhr Average monthly bill: $108 Battery capacity: 7KWhr. Maximum storage over 1 month ( 100% daily cycling); 7kWhr*30days = 210kWhr Percentage of total household consumption; 210kWhr/900kWhr = 23.3% Monthly bill reduction assuming the 210KWhr is free; $108*.233 = $24 It would take a long time to get the $3000 battery cost back, even if the energy supplied to it were entirely free and without additional capital expenditure.
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May 6, 2015