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Comment by dougb | Hacker News Reader

It sounds like they are trying to game the system by lowering their Peak Load Contribution (PLC.) Good article explaining Capacity charges in PJM http://energysmart.enernoc.com/bid/188545/Deconstructing-You... They only have to use these batteries for a few hours a year to save money all year round.

But, the capacity charge for PJM for 6/1/2013 - 5/31/2014 is $27.73 MW / day. So if you can drop your PLC by 54kW, you would save $27.73 * 0.054 * 30 = $44.91 a month. Hardly enough to make the $100k 54 kW system worth it.

Disclaimer: I know enough about the energy markets to be dangerous. I mostly know about PJM, so YMMV in CAISO.

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Interesting!

Looking at what I think is the right PG&E tariff for them [1], I can't quite figure out what it means. However, I found a helpful guide [2] that includes an actual hotel electrical bill.

Reading that, their summer difference is $0.07/kWh, so the box only could save them $3.50 a day on cheaper energy. There they are paying $13/kW for peak capacity, $8.58 for off-peak, and $2.99 for "partial peak", whatever that is.

So it sounds like being able to level peaks in your power consumption could be a big win. Even if you level it within peak, therefore saving nothing on a kWh basis, a little curve smoothing could pay big dividends.

The interesting question for me is how much this is incentive gaming versus solving a real problem for generators.

[1] http://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_A-10.pdf [2] https://sites.google.com/site/greenwrenco/san-francisco-bay-...

I've met one of the Stem management execs, and heard the pitch.

What they're doing is to smooth out the spikes in the day to day energy usage. Their software will learn the usage pattern over time, and try to predict when spikes will occur. The battery will then discharge during spikes to prevent triggering additional peak demand charges.

> Hardly enough to make the $100k 54 kW system worth it

Perhaps the operative bit is "California state incentives cover about 60 percent of that price."

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