~~Per this 7 pm to 1 pm is off-peak, which is 18 hours if I can do math. 10 cents/kWh in either season.~~

1-3 pm is mid peak. 19 cents/kWh summer, 13 winter.

3-7 pm is peak. 27 cents/kWh summer, 17 winter.

The question therefore is with that 11.5 kW and 13.5 kWh, how much could I wind that meter back at peak hours (and mid peak if there's enough capacity, which there won't be trivially): the answer would be 13.5 kWh worth of energy, over 1.17 hours, which the astute reader will note is << 4. The best arbitrage one could do is to buy 13.5 kWh at 10 cents and sell that back, if you will, at 27 cents in the summer, every day.

13.5 kWh * $0.10 / kWh / 89% == $1.52 to fill it up, using 89% charging efficiency

13.5 kWh * $0.27 / kWh == $3.65 credited to empty it, not applying that factor since that 89% is round trip. might be doing this wrong

$2.13 per day.

On the cost side, I'm seeing $11.5k estimated installed cost for one Powerwall 2 (which has the same kWh, slightly lower kW power but irrelevant since still well less than 4 hours worth).

There's a 30% Federal tax credit from 2023-2032 for standalone with existing solar. Xcel has local incentives if one allows them to use the battery as a buffer: $500/kW up to 50% of equipment cost, + $100 annually. That'd be $5,750 + $100/year trivially but I think that caps due to the 50% bit, so let's call it $4,750. Denver furthermore has a credit for either 80% of project cost up to $500 or up to $2,750 if one also jumps to a heat pump or heat pump water heater. Colorado has a new 10% state tax credit as well, and exempts systems from sales tax.

(I don't think the heat pump or heat pump water heater options would make sense for me as the cold climate air source heat pump rebate is 40% of cost up to $3,500, and the smart heat pump water heater credit is 60% of project cost up to $1,750. That remaining 40-60% of cost would increase my net costs more than the $2,250 difference, I'm guessing... but maybe not for the smart heat pump water heater potentially? will mull this and tease it out separately. Going with $500 here.)

Up front cost +/- timing of rebates would therefore be $11.5k - 30% * $11.5k Fed - 10% * $11.5k CO - $4,750 Xcel - $500 Denver == $1,650.

Ignoring time value of the money spent on it here. Smoothing in that $100/year makes the arbitrage price $2.40/day. Trivially it's therefore $1,650 / ($2.40 / day) == 687.5 days.

With all these various and sundry incentives stacked that's actually an order of magnitude shorter than what I was expecting. The new as of 2023 30% Fed and 10% CO incentives really help. Hey @stoney after your test please come sanity check this, as I'm thinking these tea leaves indicate I should get a Powerwall or 3. All the credits scale with total cost other than the Denver one already hitting its cap at $500.
edit: math wrong due to my parsing Xcel's formula incorrectly due to them using wrong/mixed terminology. see below.

So I messed up here, but it's not entirely my fault as Xcel used the wrong/mixed terminology. As I quoted, they'll pay $500… but per kW (power), not kWh (capacity). My math used capacity.

Why it's not my fault is that

their literature says

Yeah, they used kW, but also "capacity". Thus my parsing it wrong. What led me to notice the error here is their little sidebar example:

View attachment 197401
Here their terminology is in order: kW matches max continuous power, and a Powerwall 2 is indeed a 5.0 kW continuous battery. Anyway, that kills the economics of the Powerwall completely because 5.0 vs 11.5, not to mention their install cost is $6k through Powerwall Direct for 1, $7,500 for 3.

1 Powerwall as installed by Tesla: $8.7k + $6k install - $500 Tesla summer/fall rebate per unit - 30% * $14.7k Fed - 10% * $14.7k CO -

*$2,500 Xcel *- $500 Denver == $5,320.

3 Powerwalls as installed by Tesla: $33,288 installed - $1,500 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $7,500 Xcel - $500 Denver == $10,472.

Via electrek, official specs

View attachment 200515
will update math with this assuming that Xcel’s payout is the same for Battery*Connect, $500/kW continuous grid tied power. Big difference from 5 kW!

Circling back re Powerwall 3, at 13.5 kWh, 11.5 kW continuous on grid power.

Same arbitrage through discharging/charging it along with the $100/year ongoing credit from Xcel factored in. As of today I'm still seeing $33,288 pre-incentive price for 3 installed current-gen Powerwalls, $24,527 for 2 (with Tesla installation as that proved to be cheaper in my explorations). Let's assume these prices stay constant for simplicity.

Let's also assume that Xcel's Battery*Connect credit at $500/kW holds for the higher power Powerwall 3. That'd therefore be $11,500 for 2, $17,250 for 3. Tesla fall incentive appears to have been extended to March 2024, now a winter incentive as well: $500/installed Powerwall. Fed 30% and 10% CO still apply as does the $500 Denver that doesn't scale with size.

Ah, a fly in the ointment:

Xcel's incentive is capped at 50% of pre-tax, pre-labor cost of the battery, so $500/kW won't actually be paid out, but instead 50%. I have to go back and note the pre-installation costs for each:

2 Powerwalls: $24,527 installed, $6,900 nominal installation, $16,200 cost alone

3 Powerwalls: $33,288 installed, $7,500 nominal installation, $23,700 cost alone

4 Powerwalls: $42,149 installed, $8,200 nominal installation, $31,200 cost alone

5 Powerwalls: $51,309 installed, $9,200 nominal installation, $38,700 cost alone

6 Powerwalls: $60,470 installed, $10,200 of that installation nominally, somewhat inexplicably $46,200 listed as cost alone as that doesn't add up but go figure

And in turn the amount of the price arbitrage depends on the number of Powerwalls. $2.13/day * # Powerwalls + $100/365, so $4.53 for 2, $6.66 for 3, $8.79 for 4, $10.92 for 5, $13.05 for 6.

So regardless of the funny internal Tesla site math, we'd be capped at 50% of battery cost alone. Resultant fixed math:

2 Powerwalls: $24,527 - $1,000 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $8,100 capped Xcel - $500 Denver == $5,116 net cost. 1129 days simple payback.

3 Powerwalls: $33,288 - $1,500 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $11,850 capped Xcel - $500 Denver == $6,123 net cost. 919 days simple payback.

4 Powerwalls: $42,149 - $2,000 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $15,600 capped Xcel - $500 Denver == $7,189 net cost. 818 days simple payback.

5 Powerwalls: $51,309 - $2,500 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $19,350 capped Xcel - $500 Denver == $8,435 net cost. 772 days simple payback.

6 Powerwalls: $60,470 - $3,000 Tesla rebate - 30% * installed cost Fed - 10% * installed cost CO - $23,100 capped Xcel - $500 Denver == $9,682 net cost. 742 days simple payback.