For a residential customer placing a new order with Tesla for Powerwalls today, I thought it would be interesting to comparing pricing on 1, 2, or 3 Powerwalls, based on the assumption that Tesla's capacity for the SGIP Small Residential Storage is full booked in all steps, but that there is availability for SGIP Large Storage Step 2. [For SCE customers, Step 2 is already sold out, so it would be Step 3.]
My quote for 1 Powerwall was $7,700 and change, which includes the hardware ($5,500 for the Powerwall, $700 for the Gateway), $800 for installation, $150 for permitting, and sales tax (9.25%) on the hardware. So I'll assume 2 Powerwalls would be $13,700 and 3 Powerwalls would be $19,700. This is pretty much the best case scenario, no other upgrades required.
For 1 or 2 Powerwalls with no SGIP rebate, the only incentive is the ITC. [Edit: I'm assuming an adequately large solar system and charging the Powerwalls only from solar.] After that 30% incentive the net cost without SGIP is $5,400 for 1 Powerwall or $9,600 for 2 Powerwalls.
For 3 Powerwalls, the SGIP Large Storage category requires that the system size (in kW) be no greater than the customer peak demand over the last 12 months. Residential rate plans don't include any demand charges, so residential meters just record energy used over an interval of time (either 15 minutes or 60 minutes). The imputed demand for the interval is the just the average power used over an interval. So to qualify for 3 Powerwalls under SGIP Large Storage, you need to have had an interval during the last 12 months in which you used 15 kW average over that interval. 15 kW = 62.5A @ 240 V.
Assuming you make it over that hurdle, the Step 2 incentive rate is $0.40/Wh without the ITC, or $0.29/Wh with the ITC. Using a discharge duration of 2.7 hours (some use 2.64 hours), the incentive per Powerwall is $4,700 without the ITC, or $3,400 plus change with the ITC.
So for 3 Powerwalls under the Step 2 SGIP Large Storage, the out of pocket cost if not taking the ITC would be $5,600. If taking the ITC, the cost after SGIP before ITC would be $9,500, and after the ITC it would be $6,650. Odd.
The upshot is that for a new Tesla Powerwall residential customer who could qualify for SGIP Large Storage for 3 Powerwalls (12 month peak demand is at least 15 kW), with the above minimum installation costs, it's bascially a buy 1 get 2 free situation: $5,400 net for 1 Powerwall, or $5,600 net for 3 Powerwalls. Of course, if you can find a non-Tesla developer who won't gouge you on the installation costs, then under SGIP Small Residential Storage Step 2, the cost of 1 Powerwall would be less then $5,400. [If my attempt to self-develop pans out, then it would be $3,000 for me with SGIP only, or $2,100 if I also took the ITC.]
Cheers, Wayne