11kW system in SoCal - need advice please?

[J.P.M.]

The size of the system is based on our electricity usage for the past 12 months (15765 kW - which includes daily charging of my wife's electric car for about 2/3 of that period), with additional projected usage based on the recent addition of a second electric car (to replace my ICE car), for roughly around 16500 total projected annual electricity usage.

I'm still a little fuzzy on how the estimated first year production is calculated, as I see different calculators/maps that produce materially different results. I'm trying to get further info on how the estimates for my bids were calculated. However, based on a couple of the PVWATTS online calculator (recommended in the Dummies guide), the system size in the above bids seem correct for our projected electricity usage.

Performance models used to estimate sizing can have inputs manipulated. Run PVWatts on your own after reading the help/info screens a couple of times. that may clear some of the fuzzies.

Keep in mind that a system that offsets 100 % of an annual bill, while a choice, may not be the most cost effective choice.

As for how vendors estimate output, usually some model such as PVWatts, or SAM (sort of PVWatts on steriods), or sometimes proprietary or purchased canned stuff. Vendors tend to underestimate performance to sell more product. Also, the solar ignorant do not see oversizing as such, as much as they think they got a deal when they see more production than the vendor told them to expect. Run your own est. w/ PVWatts and use an 8 - 10% derate instead of the default 14%. You'll get closer to reality. Just remember that PVWatts output is a long term AVERAGE ESTIMATE and not a predictor of any one year's/month's performance.

[SolarFamilyGuy]

The size of the system is based on our electricity usage for the past 12 months (15765 kW - which includes daily charging of my wife's electric car for about 2/3 of that period), with additional projected usage based on the recent addition of a second electric car (to replace my ICE car), for roughly around 16500 total projected annual electricity usage.

I'm still a little fuzzy on how the estimated first year production is calculated, as I see different calculators/maps that produce materially different results. I'm trying to get further info on how the estimates for my bids were calculated. However, based on the PVWATTS online calculator (recommended in the Dummies guide), the system size in the above bids seem about right for our projected electricity usage.

Got for it. In two weeks I am having a 9.8kwh solar array installed onto my roof. Why? No utility bill. No sales tax. Anything over 10 kWh I have to pay sales tax. I am wanting the maximum payout of $5,000 per year from my PUD. With the 30% tax credit and $5,000 per year until June 2020, I will almost recoup my up front cost of $45,000. If after 2020 my PUD stops the incentive of $.54 per kWh, I can still bank up the extra electricity as credits and use these credits to carry me through the winter months. Up North the weather can get nasty and rain a lot. For you down in California, you guys get sun almost year round. You would do good with a 11 kWh system.

[dusterbuster]

Got for it. In two weeks I am having a 9.8kwh solar array installed onto my roof. Why? No utility bill. No sales tax. Anything over 10 kWh I have to pay sales tax. I am wanting the maximum payout of $5,000 per year from my PUD. With the 30% tax credit and $5,000 per year until June 2020, I will almost recoup my up front cost of $45,000. If after 2020 my PUD stops the incentive of $.54 per kWh, I can still bank up the extra electricity as credits and use these credits to carry me through the winter months. Up North the weather can get nasty and rain a lot. For you down in California, you guys get sun almost year round. You would do good with a 11 kWh system.

Wow, that's great that you are able to get cash back from your utility. No such luck here with SoCalEdison.

[dusterbuster]

Performance models used to estimate sizing can have inputs manipulated. Run PVWatts on your own after reading the help/info screens a couple of times. that may clear some of the fuzzies.

Keep in mind that a system that offsets 100 % of an annual bill, while a choice, may not be the most cost effective choice.

As for how vendors estimate output, usually some model such as PVWatts, or SAM (sort of PVWatts on steriods), or sometimes proprietary or purchased canned stuff. Vendors tend to underestimate performance to sell more product. Also, the solar ignorant do not see oversizing as such, as much as they think they got a deal when they see more production than the vendor told them to expect. Run your own est. w/ PVWatts and use an 8 - 10% derate instead of the default 14%. You'll get closer to reality. Just remember that PVWatts output is a long term AVERAGE ESTIMATE and not a predictor of any one year's/month's performance.

Interesting. Using a 10% derate versus the default 14% makes somewhat of a difference, and would indicate I could downsize the system by about 0.7kW. Not a huge difference I suppose in the grand scheme of things, since I think that equates to about 2-3 panels, but still good to know.

[J.P.M.]

Interesting. Using a 10% derate versus the default 14% makes somewhat of a difference, and would indicate I could downsize the system by about 0.7kW. Not a huge difference I suppose in the grand scheme of things, since I think that equates to about 2-3 panels, but still good to know.

You can make any choice you want, including oversizing, but having what is possibly better information does not hurt. The choice is still yours.

[dusterbuster]

Update: I've added a forth option listed as Option D below. I'm narrowed my decision between Options A and D, and would love to hear any thoughts/opinions people have regarding: (1) comparison of the two SMA SB5000TL-US inverter setup versus the SolarEdge 9000 + SolarEdge P320 Optimizers setup, and (2) the LG 305 panel versus Solar World 285 panels. Thank you!

Option D: ($3.37/watt)
Solar World 285 panels (39 panels) = 11.115 kw
(x2) SMA SB5000TL-US inverters (10 year warranty)
20 year workmanship warranty
1st year production estimate: 16,739 kWh
$37,500 base price (before federal tax credit)

[jflorey2]

Update: I've added a forth option listed as Option D below. I'm narrowed my decision between Options A and D, and would love to hear any thoughts/opinions people have regarding: (1) comparison of the two SMA SB5000TL-US inverter setup versus the SolarEdge 9000 + SolarEdge P320 Optimizers setup, and (2) the LG 305 panel versus Solar World 285 panels. Thank you!

If backup power for power outages is at all important to you I'd lean towards the SMA inverter.

[ulrich]

If backup power for power outages is at all important to you I'd lean towards the SMA inverter.

I believe SolarEdge would be compatible with Tesla's Powerwall, which would give an even more flexible backup than SMA's inverters.

I believe SMA features a separate 1500 watt circuit that's available when the grid is down. This option is definitely better than a complete outage but has two drawbacks in my mind:
1. You need to run a power cord from this outlet to the appliances you want to run
2. The power is (obviously, I suppose) available only when the panels are generating power

The Powerwall/SolarEdge combination has the advantage that it provides a "real" backup in that it feeds your home through the same path the PV panels do and is always available, even at night. It will also potentially allow you to do some load shifting to take extra advantage of TOU plans during the winter when the higher rates are in effect during non-productive hours for solar.

Of course Powerwall and the SolarEdge upgrades to support the Powerwall are a significant expense - you may want to decide whether that's worth it.

I'm currently researching systems and leaning towards SolarEdge even though I don't intend to install Powerwall immediately. I figure it "future proofs" me a little to be ready for Powerwall. I also like the panel-level diagnostics that the optimizers will be able to provide, but that's more of an entertainment benefit than a real benefit.

[dusterbuster]

Contrary to what I had previously read/heard, I was told most recently by the solar consultant I've been working with that the particular SolarEdge inverter model specified in the bid would actually not be compatible with the Tesla Powerwall. This was a reversal from what the solar consultant previously thought/explained to me, and was the result of having specifically following up with a SolarEdge rep to check on future compatibility with the Powerwall. I'm not sure how accurate that is, but on further reflection, I think it's unlikely I'll actually be adding a Powerwall, so I've decided to not let that be a material factor in my decision.

At this point, I'm leaning towards Option D with the two SMA inverters, primarily because that particular installer offers a 20 year workmanship warranty, and according to the consultant, this installer is their top rated installation company in terms of quality of work, etc. Interestingly, this particular installer is also the only installer to provide a first year performance guarantee.

[inetdog]

From what I have seen, interfacing with the PowerWall requires the StorageEdge inverter unit. That may have some hardware in common with the SolarEdge grid tie inverters but the firmware is vastly different.
I have not seen any indication that any SolarEdge model can be field or factory reconfigured into a StorageEdge.
In any case the functionality appears to still be evolving.
And possibly not compatible with the backup version rather than the cycle version of the PowerWall.