Solar panel sizing?

[gabeincal]

Hi

I just had a Solarcity person visit me for initial consultation. We moved into a brand new home a month ago and I'm keen to get solar on the roof.

This is what I'm working with:

Baseline consumption about 6-8kWh / day
Extras that are not calculated in baseline:

New hot tub - max 2-3 kWh / day
Tesla charging - don't drive a lot locally, so estimate 20kWh / every 5 days
Fiat 500e charging - 20kWh / every 5 days

This gives me a total consumption of about 520 kWh / month * $0.20 = About $120 electric bill.

I asked for a quote on a 6kW solar system and the loan would be about $200 a month for 10 years.

And here is the question. Is the 6kW system a right size for us? I'm not going to invest in battery storage for now, so any extra electricity is going back to the grid, and as I understand, PG&E does not pay much for production over usage (something like 5-6 cents per kWh).

So all in all, I want to future proof my investment should I need to drive more, or use a little more electricity in the house, but at the same time, I want to minimize my monthly cost.

Can you please suggest some data points, and / or advise on what size should I aim for? I'm more and more thinking a 4kWh system will be sufficient.

Oh, SW facing roof, all day sun in Northern California.

Thanks!!

 

[gabeincal]

And of course the other thing, if somebody is familiar with this, do the ridiculously cheap 'exported' kWh's offset the gas bill, especially in the winter over a 1 yr period. So perhaps, sizing up a little on the panels would be the gain there. I don't want to make money on electricity as such, but I don't want to pay for any electricity and gas charges either. (4 bedroom 2 story brand new home)

 

[miimura]

I think you're under-estimating your usage. Your EV charging estimates will only give 400 miles per month. Is that really all you drive on each car? My wife and I each have a commute that's less than 20 miles per workday yet we put 800-900 miles per month on each car. Our off-peak usage alone is 500-750 kWh/month. Most of that is EV charging.

I would go for the 6kW solar system. Mine is only 4.32kW DC and it's too small. I still pay $1,000/year at true-up including charging both cars. The solar that goes back to the grid in the middle of the day earns full retail credit. The low reimbursement of ~4c/kWh is only if you are a net generator - ie. you have pushed out more kWh into the grid than you pulled in. You won't be a net generator with only 6kW, so don't worry about that. You can still almost zero out your electric bill, which is really the goal.

You cannot offset the gas portion of your PG&E bill with electric. Well, I shouldn't say that. You will probably be put on Silicon Valley Clean Energy (CCA) unless you opt-out so you could potentially get a check back from them for generation credits which you could consider offsetting your gas bill. If you really want to offset your gas bill, get a hybrid-electric (heat pump) water heater. However, nobody wants to discard a brand new water heater...

 

[AudubonB]

Did they include in their discussions any talk of one or a gang of Powerwalls to assist with load balancing? I'm wondering if the SolarCity aka TeslaEnergy team is up to speed on this rather significant change in what they can or presumably shortly will be able to provide.

 

[nwdiver]

I asked for a quote on a 6kW solar system and the loan would be about $200 a month for 10 years.

Is that before or after the 30% tax credit? I would try to negotiate a little... a 6kW system shouldn't cost >$18k.

 

[gabeincal]

Is that before or after the 30% tax credit? I would try to negotiate a little... a 6kW system shouldn't cost >$18k.

After. When I get home I'll try to find the numbers and post them! Is there negotiating room with Solarcity?

 

[gabeincal]

Did they include in their discussions any talk of one or a gang of Powerwalls to assist with load balancing? I'm wondering if the SolarCity aka TeslaEnergy team is up to speed on this rather significant change in what they can or presumably shortly will be able to provide.

They did but for my use case, I don't think it makes sense. At least not right now...

 

[timk225]

Instead of 6 Kw, the right size solar array is.... more. Much more. That 6 Kw is at peak performance under perfect conditions, so go for around 15 Kw to greatly extend the time of day and weather conditions under which you can get at least 6 Kw out of it.

 

[gabeincal]

Instead of 6 Kw, the right size solar array is.... more. Much more. That 6 Kw is at peak performance under perfect conditions, so go for around 15 Kw to greatly extend the time of day and weather conditions under which you can get at least 6 Kw out of it.

But.. Whilst logical, that would mean I'd be paying at least twice as much for a loan every month than my current maximum electricity bill scenario, which to me doesn't make any sense. I'd be selling most of my energy to PG&E for chips... Unless I bought like 4 powerwalls with the install, but then we're talking $70k or so for the install...

 

[swaltner]

PVWATTS PVWatts Calculator is the standard for estimating production of a PV solar system in the US.

Taking a guess on the unprovided parameters, I entered the following details into PVWATTS:

Inputs:
Location: Sunnyvale
DC Size: 6 kW
Module Type: Standard
Array Type: Fixed (roof mount)
System Losses (%): 10 (this gives accurate estimates for me)
Tilt (deg): 20 (pitch of roof in degrees; 0 = flat, 90 = vertical)
Azimuth (deg): 180 (orientation of roof line; 90 = east, 180 = south, 270 = west

Results:
9,290 kWh per year
Peak production: 1,086 kWh (in June)
Lowest production: 466 kWh (in January)
May to August are all estimated to produce over 1,000 kWh of electricity

Entering accurate details on the proposed system will give you a good idea on the estimated energy production. PVWATTS uses historical weather data to predict how much sunlight the system will receive and then converts that into kWh of energy that will be produces.

 

[gabeincal]

PVWATTS PVWatts Calculator is the standard for estimating production of a PV solar system in the US.

Taking a guess on the unprovided parameters, I entered the following details into PVWATTS:

Inputs:
Location: Sunnyvale
DC Size: 6 kW
Module Type: Standard
Array Type: Fixed (roof mount)
System Losses (%): 10 (this gives accurate estimates for me)
Tilt (deg): 20 (pitch of roof in degrees; 0 = flat, 90 = vertical)
Azimuth (deg): 180 (orientation of roof line; 90 = east, 180 = south, 270 = west

Results:
9,290 kWh per year
Peak production: 1,086 kWh (in June)
Lowest production: 466 kWh (in January)
May to August are all estimated to produce over 1,000 kWh of electricity

Entering accurate details on the proposed system will give you a good idea on the estimated energy production. PVWATTS uses historical weather data to predict how much sunlight the system will receive and then converts that into kWh of energy that will be produces.

Thanks for this, I'll try today and post my results! I'm also seeing a couple other providers this week, Solarcity is rushing me and I haven't even seen the price breakdown before they got me to sign a preliminary contract. Not sure I'm liking their style...

 

[Ampster]

Not sure I'm liking their style..

That may be to keep you from comparing bids using the metric of cost per Watt of capacity. Today you should be able to get bids at around $3 per Watt. My 4kW system cost $4 per Watt three years ago and my little beach town is installing a 100kWh system for $2.40 per Watt.
Once you get a competitive price you can run some scenerios on PVWATTS or better yet get the vendor to recommend the best rate. As others have mentioned it appears you may be able to take advantage of the incentive rates offered by the new Silicon Valley CCA.

 

[gabeincal]

That may be to keep you from comparing bids using the metric of cost per Watt of capacity. Today you should be able to get bids at around $3 per Watt. My 4kW system cost $4 per Watt three years ago and my little beach town is installing a 100kWh system for $2.40 per Watt.
Once you get a competitive price you can run some scenerios on PVWATTS or better yet get the vendor to recommend the best rate. As others have mentioned it appears you may be able to take advantage of the incentive rates offered by the new Silicon Valley CCA.

Thanks Ampster! For now, I think I'm going to cancel the Solarcity contract as I've only got 10 business days to do so. Without explanation or the breakdown of pricing, I'm not going to commit. And also, I'm expecting a fair amount of discount based on the fact that my new house is prepared for solar, there are conduits running from the roof to the garage and breaker panel, already fitted inside the walls. Surely, that should get me something...

 

[Ampster]

Smart move

For now, I think I'm going to cancel the Solarcity contract

Smart move. Time and research will only give you better choices. SVCCA is just starting up and others may have more inputs about their NEM rates that may help you find the best size configuration.

 

[idleuser]

I got my 7.2kw Solar System installed through Solar Universe and I paid 25000 OTD before the 30% ITC rebate. The cost per KW installed cost me around 2.43kW.

 

[miimura]

Thanks Ampster! For now, I think I'm going to cancel the Solarcity contract as I've only got 10 business days to do so. Without explanation or the breakdown of pricing, I'm not going to commit. And also, I'm expecting a fair amount of discount based on the fact that my new house is prepared for solar, there are conduits running from the roof to the garage and breaker panel, already fitted inside the walls. Surely, that should get me something...

I would certainly be more inclined to work with someone who gave a detailed quote after doing a site survey. This would be heightened if they specifically called out a discount for existing conduits or unusually favorable installation conditions.

 

[Ampster]

I got my 7.2kw Solar System installed through Solar Universe and I paid 25000 OTD before the 30% ITC rebate. The cost per KW installed cost me around 2.43kW.

In the example which I used above, I calculated the quoted installed cost per Watt since the tax credit belongs to me. In the above example the math comes out to $3.47 per Watt. If you want to use kWatts as your metric then it calculates to $3,470 per kWatt.

 

[bkp_duke]

Instead of 6 Kw, the right size solar array is.... more. Much more. That 6 Kw is at peak performance under perfect conditions, so go for around 15 Kw to greatly extend the time of day and weather conditions under which you can get at least 6 Kw out of it.

15kW is major overkill for what he described.

I have 12kW in a new house right now, with two Model S' that are driven about 700 miles per month each, and I currently have a 50% power overage. I know this will go down in the winter, and 12kW is probably "right size" for winter months, but 15 is a huge expense that pushes the pay-back for the OP way way into the future.

 

[Skotty]

In my state, according to info I found online, anything over 10kW requires an additional $100K in liability insurance. Have no idea what the case is in other states, but it's another possible thing to consider. I have no idea, yet, how much added monthly cost that results in.

 

[bakerboy]

Im right in the middle of getting my 7.35kwh system installed, its 21 panel 350watt system including upgrading my service panel from a 100amp to 200 amps and a 14-50 plug for the MS!!! Everything was 27k.