Considering solar panels, looking for sizing/financing advice

[willow_hiller]

Hi Tesla Energy Forum!

I've been a regular poster on the auto/investment side of Tesla, and now I'm considering a Tesla solar system for my home. I would be very appreciative to hear any advice you can give me on sizing a solar system and financing it.

To start, I live in Maryland (near DC), and since we've got net metering I am not considering Powerwalls at this point. In the 2.5 years we've lived in this home we've only ever had brown-outs, no full power outages (mostly buried power lines, so I'm lucky in that regard). We had a windstorm blow off a quarter of our roof in early 2018, so we've got new architectural shingles. For that reason we're probably considering panels instead of solar glass. My wife and I have done a series of small improvements to our home (insulation, new water heater, etc.) to try and reduce our energy consumption as far as possible, but we do have all electric heat/AC/appliances. Attached is a CSV of our electricity bills for the last 2.5 years. We're averaging 14.74 cents per kWh and 1364 kWh consumed per month (15,474 kWh consumed in the last 12 months). A new heat pump could probably reduce our consumption further (we're working with an old SEER 12 unit), but I figured that now would be a good time to invest in a solar system given the incentives.

When I enter my details into Tesla's solar calculator, they recommend an extra large (15 kW) system, but I think that for our needs, a large system (11 kW) will likely cover 100% of our annual usage, is that correct? We have 2-3 large sections of roof that face Southeast and Southwest respectively, and we have no shade whatsoever. Picture of roof attached below:

After sizing, I would also appreciate any advice you can give on financing. We're currently in the process of paying off a low-interest auto-loan for our Model 3, and almost have enough savings to pay that off completely. But I was thinking that it might make the most sense to take the money we would use to pay off the car, and use that to buy the Large solar system in cash instead and avoid a long-term solar loan. And just as one additional question, when would you recommend getting the ball rolling with Tesla to ensure that our Solar system is fully installed before December 31, 2020?

Thank you for your time in reading my post.

 

[bob_p]

We planned for a system that would provide 50% of our daily needs, which ended up with 15.4 KW of solar and 4 PowerWalls.

When have several days of full sun, we'll run 100% of our house from solar/PowerWalls (except for EV charging), with a little excess power going to the grid.

However, most of the time we don't get full sun all day - and will have stretches when there are a lot of clouds. During those days, the solar power will only provide part of the house power, with the rest coming from the grid.

We aren't using a net metering - so if we'd scaled our system larger, we would have been at risk of having more excess power on sunny days - and the larger system probably still wouldn't have enough power to keep us off grid on cloudy days.

When estimating how much solar power you will get, be careful about relying on average measurements - because the amount of solar power generated will vary based on factors like cloud cover, time of year (length of day , and north/south angle) and time of day (east/west angle).

If you want to be off grid, you'll need a system sized providing more than the average power consumed each day.

As for when you should get started, the later you wait, the greater the risk that you'll encounter shortages on the type of solar panels you'll want or in getting the PowerWalls.

Plus, due to manufacturing and supply chain disruptions, there's likely to be a several month impact on getting components for new systems.

 

[willow_hiller]

If you want to be off grid, you'll need a system sized providing more than the average power consumed each day.

Thanks for your fast reply! Our primary motivation isn't really being off-grid, I just want to offset as much of our electricity consumption with solar as possible. I wouldn't mind positive electricity bills in the winter as long as they're roughly offset by negative electricity bills in the summer.

 

[getakey]

I'm in Calif near Sacramento. Have a 11.8kW system. Last month, we generated 1590kWh. Panels are facing East and West, with slightly more in the East side
In June and July we are welll above 2100kWh per month (no rain in the summer here)
Lowest production is Dec at ~500kWh

 

[willow_hiller]

I'm in Calif near Sacramento. Have a 11.8kW system. Last month, we generated 1590kWh. Panels are facing East and West, with slightly more in the East side
In June and July we are welll above 2100kWh per month (no rain in the summer here)
Lowest production is Dec at ~500kWh

Thanks for that data. Does anyone know how accurate the default settings are on PVWatts Calculator? PVWatts Calculator

Here are my results using their default settings for an 11.34 kW system and my home address:

This is part of what has me thinking that on average, an ~11 kW system will cover 100% of my consumption.

 

[Zaxxon]

What does Tesla estimate would be the annual production for the Large and Extra Large systems? Is your Model 3 your only car? If not, do you anticipate potentially moving to 2 EVs in the future?

I'm generally of the opinion that you should go as big as you can within reason--if there's any chance that the Large won't quite cover your usage, or that your usage will rise in the future with another EV or other changes in your consumption, go big. It's generally cheaper to get 'too many' panels now than it is to get too few and then add on later.

To answer your question on production, per PVWatts, A generic 11 kW system in Maryland would be right on the edge of producing what you need (of course I just picked Maryland rather than entering your home's specific location and roof facings). With this generic Maryland run, PVWatts has you ~150 kWh short/year with an 11 kW system. However, remember that the system will lose a bit of efficiency each year, so were I you, I'd also consider going larger. Also keep in mind that you can generally work with Tesla after placing the initial order to customize. You could end up with a Large-but-not-quite-extra-large system if that's your goal. They won't generally let you customize to the extent that a smaller installer would, but if you explain that Large might not quite cover you but Extra Large is more than you're looking to spend, they may magically find a way to build you a 12 kW system or something.

As for timing, I'd start the ball now. I would think you've got plenty of time, and my own experience with Tesla in Colorado last year took less time (signed in Aug, installed in Sept, approved by power company in October). But with the tax credit dropping on 12/31, why risk it by waiting?

Good luck, and congrats on joining the nuclear-fusion-powered EV club.

 

[Zaxxon]

Ha, nice timing--I see you played with PVWatts yourself. The defaults are pretty accurate in my experience--I don't have enough data on my Tesla system yet since it was just installed late last year, but the system I installed in 2014 has been right in line with the PVWatts estimates at the time, using default values, my address, and the system size I installed.

 

[willow_hiller]

What does Tesla estimate would be the annual production for the Large and Extra Large systems? Is your Model 3 your only car? If not, do you anticipate potentially moving to 2 EVs in the future?

I'm generally of the opinion that you should go as big as you can within reason--if there's any chance that the Large won't quite cover your usage, or that your usage will rise in the future with another EV or other changes in your consumption, go big. It's generally cheaper to get 'too many' panels now than it is to get too few and then add on later.

Thanks for your reply. Tesla is estimating 34-43 kWh/day on the 11.34 kW system, and 45-57 kWh/day on the 15.12 kW system, but both of those seem a little conservative considering the PVWatts annual estimate for 11.34 kW / 365 is almost exactly 43 kWh/day.

Near 0 chance of getting a second EV, at least within the life of the system! It took 5 years of marriage before I saw my wife behind the wheel of a car, and even that took significant encouragement.

Do you know of any good tools for estimating whether a roof can accommodate all the panels? If I'm reading it right, the large system is 36 panels (maybe I'll try and ask for 37 or 38 just for some extra breathing room). Our home actually isn't all that big ~1,600 sq feet interior space.

 

[Zaxxon]

Do you know of any good tools for estimating whether a roof can accommodate all the panels? If I'm reading it right, the large system is 36 panels (maybe I'll try and ask for 37 or 38 just for some extra breathing room). Our home actually isn't all that big ~1,600 sq feet interior space.

I don't, but the panels they installed for me last year are 66.3"x40", if that helps.

 

[TheTalkingMule]

If you're not concerned about backup in a blackout, then I think it's a pretty straightforward decision. If your net metering program compensates you beyond 100% of usage(to any degree), it's probably a good move to go with the largest array that fits and you find aesthetically pleasing. In PA I get net metering to 100% then something like $.08/kWh from PECO(Exelon) for anything over that.

IMO we'll see rule changes coming down the line over the next few years that might even compensate you more than today for production beyond 100% as utility lobbyists continue to lose power. We'll probably look back at these purchases and wonder why we would buy anything lower than max production since it's so efficient and cheap.

Hopefully we'll look at demand from a more holistic perspective by 2025 or so. There's 5 or 6 people on your transformer and not all of them can install solar, might as well plan to sell them cheap sustainable energy for the next 35 years.

 

[willow_hiller]

If you're not concerned about backup in a blackout, then I think it's a pretty straightforward decision. If your net metering program compensates you beyond 100% of usage(to any degree), it's probably a good move to go with the largest array that fits and you find aesthetically pleasing. In PA I get net metering to 100% then something like $.08/kWh from PECO(Exelon) for anything over that.

IMO we'll see rule changes coming down the line over the next few years that might even compensate you more than today for production beyond 100% as utility lobbyists continue to lose power. We'll probably look back at these purchases and wonder why we would buy anything lower than max production since it's so efficient and cheap.

Hopefully we'll look at demand from a more holistic perspective by 2025 or so. There's 5 or 6 people on your transformer and not all of them can install solar, might as well plan to sell them cheap sustainable energy for the next 35 years.

Thanks for your reply. Looks like the net metering policy from Pepco and the state of Maryland is a bit more complicated than I thought, but I'm pretty sure it will still work out how I imagined it will: 2020 Pepco (Potomac Electric Power Company) Net Metering Rates | EnergySage

I double checked their Restricted Circuits map and my neighborhood is not covered by any (so it's presumably open). That just means that I'm restricted to 2 MW or 200% of my annual energy usage. That is of course, assuming that the state cap of 1,500 MW isn't met on a regular basis. It looks like we met about half of that last year: Maryland Net Metering Installations Reach 50 Percent of 1.5-Gigawatt Limit: PSC Report » EnerKnol

If anyone on the forum has experience with Net Metering and Pepco I would love to hear it.

 

[Yanquetino]

I'm guessing that you might be right that a 11 kWh system would suffice, but perhaps it depends on your home's square footage and how many kWh per day you use to charge your Tesla.

Our 8.28 kWh system (36 panels x 230W each) powers both our 2,700 sq. ft. home and our Model 3, with just a tiny excess "donation" to our local utility averaged over a typical year, BUT… We live in a southwestern desert that is much sunnier than Maryland. The good news is that panels are now much cheaper —half the cost!— than when we installed our system over 9 years ago. Still, it has already paid for itself.

 

[jrweiss98020]

I live just north of Seattle, and the installers guarantee my 9 KW system will generate around 9,000 KWh/year. I haven't had it a full year yet, so I don't know how much more it will actually generate. Since you're a bit further south, the 11KW system may well generate the 15,000 KWh/year you need.

Check the rules for production credit in your area. One of the decision factors is whether you can bank all your solar credit indefinitely. In WA, the credit resets to 0 every Apr 1, so an oversized system does not make financial sense. Also, I have to pay a minimum of 53¢/day (~$17/mo) regardless of production credit, so there are no "negative bills".

As for financing, I am a fan of paying cash for everything, ESPECIALLY non-critical items. You may "need" a house and a car, but you don't "need" the solar system...

 

[charlesj]

I would not cut it too close to your current needs. Hard to know what the future outlook will be with weather or your consumption.

CA has true-up with net metering or at least on mine. Pay at end of year if needed.
If you over generate you will get a credit.

 

[charlesj]

... Also, I have to pay a minimum of 53¢/day (~$17/mo) regardless of production credit, so there are no "negative bills".
...

Need to move to CA with PG&E, only $10/mo.
And if you are in a community power for generation like in Monterey Co, they pay $0.06/kw excess generation.
So, this year I am waiting for an $80 check back, not to mention the CA Rebate for something of $60.

 

[getakey]

at $.06/kWh for excess, that would take a long time to justify over-sizing the system.
Mine is sized for ~zero dollar true up bill even though I use slightly more than I produce. PG&E only pays about $.04/kWh excess

 

[willow_hiller]

at $.06/kWh for excess, that would take a long time to justify over-sizing the system.
Mine is sized for ~zero dollar true up bill even though I use slightly more than I produce. PG&E only pays about $.04/kWh excess

Do you have a fixed rate for excess? As far as I can tell in Maryland, excess is bought via solar credits that have a floating price, and are priced per MWh: Flett Exchange | Maryland SREC Pricing

 

[MorrisonHiker]

You might also check Project Sunroof for a quick estimate.

Also, depending on where you live, you might be limited on how much solar you can actually install. We were only allowed to install a 16.5 kW system originally, even though we knew it wouldn't cover all of our usage. A year later, we were allowed to install an additional 4 kW system. Both of our systems and all of our Powerwalls were installed by Tesla.

Our power company does allow us to roll over any credits month after month with no annual true-up and they are earned 1:1 depending on which period we are in (peak, part-peak or off-peak).

 

[getakey]

I'm on a PG&E EV plan. I have 3 rates:
Peak 2pm-9pm Weekdays
Partial Peak 9pm-11pm and 7am to 2pm
Off Peak 11pm to 7am (lowest rate for charging the EV)

We are on a Net Metering annual true-up bill. During the summer, I am over-producing and getting "credit" at the time of production, i.e., Solar back to Grid is credited at the Partial or Peak rates which are $.27 and $.54 respectively. Off Peak is $.12/kWh. Thus, better to charge car at this rate and bank the solar at the Partial or Peak rate.

Going into Winter, I have built up a negative true-up amount. Through the Winter, I am using more than I produce, so I am charging against the credit at what ever rates are in effect when I am using. The true-up is the 12 month cumulative amount. However, true-up can not be negative. If you have over-produced and the true-up amount would have been negative "on paper", this amount gets zeroed out. Instead, they take your excess production and pay you $.04/kWh. That's why here (in PG&E land), you should not over-size the system. It does not cost out economically for the excess capacity.

I've left out the PW complication. Basicaly, with PW, I am completely off of Peak Rates. I charge PW on Partial PEak and run entire house off of PW during Peak Rates while getting credit for all of my solar production during Peak (at the Peak rate)

 

[SMAlset]

Don’t know what Tesla is spec’ing for solar panels on the east Coast but here in my area of California they set is up with Hanwha’s Q Cell panels. Tesla’s layout is done by 12-panel arrangements. Our house will accommodate a Medium system of 24 panels that will go on our south-east and south-west facing roof structures. Placement is guided and restricted by your roof hips and ridge as there is some minimum distance that is required from them. Any venting, solar attic vents, chimneys, etc on the roof may cause placement issues for them. I wouldn’t spend too much time on the coverage they can do as they will determine that.

I’d recommend placing your order deposit, which will fix your place in line, and then submit the photos of your main panel box etc that they ask for. They will turn that over to their planning team and submit to you a diagram with the proposed roof layout (they used a Google map aerial image of our yard and roof to plan from). Also submitted to you will be the equipment such as the inverter that will used for your project. We’re getting PWs as well so not sure what else goes along with just the solar. If you think you might want to add PWs later I’d let them know that upfront. For the work they did including our initial phone conversations with them on equipment and project questions, it was a $100 well spent. The $100 will be applied to your purchase if you proceed. I think otherwise it’s money you spend for the work they’ve done up to that point. And I believe there will be a quote given to you at that time for the project.

We requested a change to the layout and they were able to revise it. Based on the info you submit on your electrical panel (they will need make and model and see layout of the breakers) and roof condition, things could need modifying. In our case our main panel needed to be upgraded. Stayed 200As but it wasn’t a solar ready panel and it used a center feed as opposed to end feed and it needed the bus bar to be upped to 225A for us to add the solar portion. But that was our project. This was a modification we didn’t find out until they were pulling the permit because we didn’t have info on our panel make and model at the time we sent Tesla our photos. Had they know it would have been caught sooner. The City pulled our developer’s plans from a decade or so before and that’s when the upgrade was determined.