Estimating future usage and system sizing

[sjsun]

Hi,

I live in the SF Bay Area (peninsula) and shopping for Solar. My current usage averages only about 500kW/month. So first, I am trying to estimate future usage. Right now, we are a young family (me, wife and a toddler) - that will likely grow and we are looking to add at least one EV in the near future and another depending on work/commute situation. For the first EV, I was eying the Mercedes B Class electric since it seems to be the only one with ample room for a small family. It is rated at 40kW/100 miles so with 1000-1200 miles a month of driving, the electricity usage would be ~500kW/month. Also, as kid(s) grow older, the current 500kW residential usage will climb up too. Maybe 700kW? In 3-4 years, with one EV, is 1000-1200kW/month a reasonable estimate of usage? (No pool, AirCon)

So far, I got quotes from Sungevity and Luminalt. Sungevity did not come out to the site yet and based on their remote survey, said maximum they can put up are 21 panels. For a 5.25kW system, they are quoting me ~21k before rebates/tax-incentives. Luminalt quoted me ~$25k for a 5.13kW system with Sunniva 270W panels (19 panels). They also quoted me $28k for a 6.2kW system with 23 panels (Luminalt did a physical site survey and said they can install 23 panels).

My questions are (1) Is my energy usage estimate reasonable? (2) If yes, then does it make sense to go for higher efficiency panels like the SunPower E20 or X21?

With a typical 250/270W panel, I can cover most of my residential usage (keeping me in Tier1 of the rate bands) with solar panels even if my usage goes up to 700/800kW/month.

But even though EV charging is only 10cents/kW right now (PG&E), it can go up depending on PG&E/CPUC policies where it becomes more expensive than Solar energy. At that point, I will pay a lot more to the utility than if I go for an over-sized system now.

Also, what is a rough method to calculate cost per kW generated? Assume 25 year life-span, estimate 1st year energy generation, adjust for panel degradation every year for 25 years and sum up the power generation over 25 years. And divide the total cost sunk into the system with the total power generated over the span of 25 years? (Not taking into account time value of money, inflation etc)

Would really appreciate either specific replies or in general, any replies explaining the larger context around which I should be making a decision.

Thanks

[solar pete]

Hi sjsun, and welcome to the neighborhood. You are on the right path by asking great questions, opinions will vary but there are some experienced people who will hopefully chime in at some stage. Probably wont hurt to get a another quote or two and check them all out on the reviews sites. Cheers and good luck with it

[Mike90250]

also factor in replacements of the power electronics (inverter) after about 10 years.

Consider the remaining life of the roof, BEFORE you install panels, maybe combine both jobs ? Let the roofer seal up the mounting brackets.

[sjsun]

also factor in replacements of the power electronics (inverter) after about 10 years.

Ah! Good point.

Consider the remaining life of the roof, BEFORE you install panels, maybe combine both jobs ? Let the roofer seal up the mounting brackets.

Fortunately, have a new roof, installed in 2012

The Sungevity guy dropped his quote for $21,5xx to $20,5xx today for the 5.25kW system (end of the month discount or something he said) And, I haven't even brought up my corporate discounts yet! I would've said yes but I think the Sungevity system will be inadequate for my needs very soon.

[thejq]

For the first EV, I was eying the Mercedes B Class electric since it seems to be the only one with ample room for a small family. It is rated at 40kW/100 miles so with 1000-1200 miles a month of driving, the electricity usage would be ~500kW/month. Also, as kid(s) grow older, the current 500kW residential usage will climb up too. Maybe 700kW? In 3-4 years, with one EV, is 1000-1200kW/month a reasonable estimate of usage? (No pool, AirCon)

The number sounds right. Since the EV will take most of your electricity usage, you need to make absolutely sure that you're getting one, otherwise you will end up with a hugely over sized system. 40KWh/100 miles sounds pretty high. I guess the SUV is much heavier than the Leaf or 500e which gets about 28KWh/100 miles. I guess you probably also want to consider leasing it instead of buying out right, since the development of EV is so dynamic.

So far, I got quotes from Sungevity and Luminalt. Sungevity did not come out to the site yet and based on their remote survey, said maximum they can put up are 21 panels. For a 5.25kW system, they are quoting me ~21k before rebates/tax-incentives. Luminalt quoted me ~$25k for a 5.13kW system with Sunniva 270W panels (19 panels). They also quoted me $28k for a 6.2kW system with 23 panels (Luminalt did a physical site survey and said they can install 23 panels).

I think you can do alot better than $4/W on those generic panels. Hopefully people from the Bay area will help you out.

My questions are (1) Is my energy usage estimate reasonable? (2) If yes, then does it make sense to go for higher efficiency panels like the SunPower E20 or X21?

Generally, unless you're limited by the roof area, SunPower offers few benefits for the extra 20% higher cost.

With a typical 250/270W panel, I can cover most of my residential usage (keeping me in Tier1 of the rate bands) with solar panels even if my usage goes up to 700/800kW/month.

But even though EV charging is only 10cents/kW right now (PG&E), it can go up depending on PG&E/CPUC policies where it becomes more expensive than Solar energy. At that point, I will pay a lot more to the utility than if I go for an over-sized system now.

I'd agree with that. But with the current PG&E EV-A structure, you might not need to budget 100% coverage in order to be net zero. Because of the rate difference between peak and off-peak, if you do it right you can use more than what you produce.

Also, what is a rough method to calculate cost per kW generated? Assume 25 year life-span, estimate 1st year energy generation, adjust for panel degradation every year for 25 years and sum up the power generation over 25 years. And divide the total cost sunk into the system with the total power generated over the span of 25 years? (Not taking into account time value of money, inflation etc)

PVwatts will calculate this for you.

[thejq]

As a reference, I have a 4.8KW system facing SW (230 degree az. 18 degree tilt). My consumption is around 1K-1.1K KWh/mon with an EV. My net metering statement says I draw roughly twice as much as generation. But based on the last 4 statements, I think I will only have to pay the fixed minimum charge at true-up time. FYI, you will most likely need to install a level II quick charger to make sure all your EV charging can finish in the off-peak hours. Also if you have a pool, you can run the pool pump in those hours too.

[silversaver]

I have 7.1kW solar with annual usage of 14,500 kWh including EV without pool. Not sure about your roof orientaion, but a 5.25kW might not be enough for an average of 1000-1200kWh monthly...

I would suggest a little larger system since panel addition were pricy if done later on and you need to follow original installer to keep the warranty without negotiation power. The B Class average about 38kWh for 100 miles on last 8000 miles. Here is an example of 12 months usage. EV starting in July and it adds up another 450kWh monthly (Luckily I can charge at work). Take your time before making decision. BTW, I'm comfort user on electricity AC and heater are on when needed.

[sensij]

A good understanding of your hourly consumption will help reveal of a time of use plan is appropriate for you. AS others have suggested, when the EV represents a substantial portion of your load and you can push the charging to off-peak hours, TOU can be a good deal.

25 years is a long time to use as justification for the system. Do you plan to be in the same house and benefit from the install for that long? If moving is a possibility, you might want to choose a timeframe that you can more confidently project, although in my experience, knowing what life will be like even 5 years from now is a crapshoot.

Also remember that this isn't an all or nothing thing. Even a small system that covers only a fraction of your consumption can still be of benefit. If you are using a tiered rate system, that smaller array may cover just the highest tier usage, providing a faster payback than perhaps a larger system would. If your usage is uncertain, I would err to smaller side. Any over production is paid at a very low rate, wiping out whatever return on "investment" in those excess panels you might have expected to get.

[J.P.M.]

Hi,

I live in the SF Bay Area (peninsula) and shopping for Solar. My current usage averages only about 500kW/month. So first, I am trying to estimate future usage. Right now, we are a young family (me, wife and a toddler) - that will likely grow and we are looking to add at least one EV in the near future and another depending on work/commute situation. For the first EV, I was eying the Mercedes B Class electric since it seems to be the only one with ample room for a small family. It is rated at 40kW/100 miles so with 1000-1200 miles a month of driving, the electricity usage would be ~500kW/month. Also, as kid(s) grow older, the current 500kW residential usage will climb up too. Maybe 700kW? In 3-4 years, with one EV, is 1000-1200kW/month a reasonable estimate of usage? (No pool, AirCon)

So far, I got quotes from Sungevity and Luminalt. Sungevity did not come out to the site yet and based on their remote survey, said maximum they can put up are 21 panels. For a 5.25kW system, they are quoting me ~21k before rebates/tax-incentives. Luminalt quoted me ~$25k for a 5.13kW system with Sunniva 270W panels (19 panels). They also quoted me $28k for a 6.2kW system with 23 panels (Luminalt did a physical site survey and said they can install 23 panels).

My questions are (1) Is my energy usage estimate reasonable? (2) If yes, then does it make sense to go for higher efficiency panels like the SunPower E20 or X21?

With a typical 250/270W panel, I can cover most of my residential usage (keeping me in Tier1 of the rate bands) with solar panels even if my usage goes up to 700/800kW/month.

But even though EV charging is only 10cents/kW right now (PG&E), it can go up depending on PG&E/CPUC policies where it becomes more expensive than Solar energy. At that point, I will pay a lot more to the utility than if I go for an over-sized system now.

Also, what is a rough method to calculate cost per kW generated? Assume 25 year life-span, estimate 1st year energy generation, adjust for panel degradation every year for 25 years and sum up the power generation over 25 years. And divide the total cost sunk into the system with the total power generated over the span of 25 years? (Not taking into account time value of money, inflation etc)

Would really appreciate either specific replies or in general, any replies explaining the larger context around which I should be making a decision.

Thanks

Since you ask:

1.) For starters, those prices look high. Perhaps other Bay Area folks could provide some insight. Also, Sunpower is pretty expensive, as mentioned about 20% up front, and some, me included have a hard time making the extra cost work for the extra oomph you get (not much, if any), particularly for low users such as you.

2.) +1 on the vehicle leasing comment. The future of EV's still seems a bit unsettled.

3.) Your use may go up, but you are still in control. Increased usage is not a mandated or foregone conclusion. Still Some increase is likely.

4.) Consider 2 of many possible scenarios: without and then with an EV.

Without it, and assuming your usage increases 20%: 500 X 1.2 = 600 kWh/month X 12 months/yr. = 7,200 kWh/yr. At that level, if you are on tiered usage, and things don't change too much (but they will, and soon, perhaps making all this more of a crap shoot than it already is), a good part of your usage, perhaps half or so, will be at the lower tier(s). Replacing that part of your load that costs $.18- $.20/kWh may not be cost effective until the price gets below some $/nameplate Watt figure, depending on how you figure your cost as f(time).

With an EV, assuming you DO get one and DO charge at super off peak, and that rate STAYS at ~ $.10/kWh, providing that much power (~ 1,100 X 12= 13,200 miles/yr.) X 40/100 = 5,200 kWh/yr. --->>> $520/yr. Now, part of the question becomes: How much will the additional solar equipment cost to provide $520/yr. worth of electricity ? And, is that extra cost worth it to you ? Or a bigger/different question: Is the cost of the fuel not used by an ICE worth the differential cost of a larger solar electric system?

Muddying the H2O are things like, rate reform, but mostly the fact that the future is unknown. The talk that often goes something like :"but electricity rates just keeps going up at a bazillion%/yr, and the POCO always screws me anyway, I don't care, I'm sick of all the bills ...etc ", are all valid opinions even if mounted on emotion and ignorance. Point is, consider what you think the future holds and take your best shot - just walk in knowing what the situation is. Learn how you are charged for electricity, what various rate plans may have to offer for your particular situation, and how rate reform is proceeding and how it may affect your situation and decision making. You'll probably wind up, like most of us taking your best shot. Unlike some folks, you started early enough to make that best shot a more informed one.

5.) To your last question, what you describe is one way to get a #. Some think that except in cases of VERY shot payback times, a couple - 3 yrs. or so, calcs of the sort you describe can be misleading and lead to less than optimum outcomes, and some consideration of the time value of money is appropriate.

6.) Respectful suggestion: Understand how you are charged for electricity - more than just a simple #. It's a real PITA, but worth it. Then run PVWatts, but take a few minutes and read ALL the help/info screen first. Note that PVWatts per kWh costs are somewhat oversimplified, especially for varying rate scenarios as may be applicable to your situation. Keep in mind that any # PVWatts spits out is an estimate (probably/perhaps somewhat conservative depending on input) of long term production, NOT a prediction, and NOT very useful for individual months.

7.) I'd do more homework and get more quotes. Don't be stampeded or scared, just be informed. with a little more knowledge, you'll probably know at least as much as some of the solar peddlers you'll talk to.

8.) Whatever you decide - Get your roof inspected/serviced as necessary. You will not be sorry.

Take what you want, need of the above. Scrap the rest.

[prhamilton]

I made the 'mistake' of trying to size my system to my consumption when I installed solar last year. I was targeting 80% of my usage on a per kWh basis. After getting about 8 months worth of billing I am finding that I make about 105% of my usage on a cost basis. I don't have an EV but I think that would skew the diff even higher.

I am in the North Bay and I used Sungevity. I was really happy with them but the thing to realize is they don't do the installs, they subcontract out the work to a network of installers. Some installers are better then others. My neighbor also used sungevity and got a different installer who had to make 3 or 4 followup visits to correct problems, they were not happy with the process. I would say that is the biggest problem with Sungevity. I can PM the name of the install company they are located in the East Bay. You might try to contact them directly, cut out the middle man and all that. I did my install for $3.60/w on a prepaid lease last May. I had an easy install panels on a composite low pitch roof with a single orientation, room in the panel, short conduit runs etc.

[Mike848]

When I was getting quotes for my system in S. California, I was told by all 5 installers I interviewed that SCE will not allow me to "over-size" my system. SCE will look at your last 12 months of electric usage and use that as a base-line to determine whether a proposed system is adequate or over-sized. If PG&E is the same way, you may encounter problems from them if you try to over-size your system for the possibility of a future EV or a growing family. I purchased an EV a few months before I started my solar project so I needed to install a larger system than what my historical 12 month usage would have shown I needed. My installer asked me to provide proof of the EV purchase that they can then show to SCE to justify my system size. BTW, my application was submitted in mid-December and I have yet to hear back from SCE.

[cbeam]

I just finished the install of a 6kW system on the upper Peninsula near San Francisco Airport.

We are low electricity users (family of four, average 425 kWh per month), and for years I did not pursue solar because my calculated return on investment was way beyond 10 years.
Then two things happened: I got an EV (Nissan Leaf) and I found an organization installing Solar for well below $3 per kW, which is far below what competitors quote.
Including cost of service upgrade (new main electric panel) and considering savings by replacing one ICE-car with the EV I calculate now an ROI within 6-7 years.

Solar is now up and running, the installer did an excellent job, and the average daily production is well above what PV Watts calculated, which means to me that the system performs very well (and the sunny January is definitely also a contributing factor). Of course I do not have enough data points to see how the system will perform over the years and if the solar system is slightly over- or undersized.

A few hints you might find useful:
Understand your current energy consumption. The PG&E website provides the usage patterns for your house (assuming you do have a smart meter). I also installed a Rainforest-Eagle Smart Meter Gateway and get real time consumption data on my computer if I want to. And I use a Kill-a-Watt to measure energy consumption of devices. I used this to identify where we wasted energy and made some changes to the house (LED lights instead of incandescent, switch to turn off the entertainment center instead of just stand-by, etc).
With Solar and EV you will probably want to switch to a TOU (Time of Use) tariff. If you do that and are technically interested (as I am) start to consider some kind of home automation so you are able to turn on the dishwasher, washer and dryer in the most beneficial time.
If you get an EV you definitely want to get away from the standard tiered rate system. You'll end up paying almost 40c per kWh to PG&E charging the car as your consumption will go into tier 4 really fast (Don't ask why I know that )

Let me know if you want more info about my system or my installer.

[J.P.M.]

I just finished the install of a 6kW system on the upper Peninsula near San Francisco Airport.

We are low electricity users (family of four, average 425 kWh per month), and for years I did not pursue solar because my calculated return on investment was way beyond 10 years.
Then two things happened: I got an EV (Nissan Leaf) and I found an organization installing Solar for well below $3 per kW, which is far below what competitors quote.
Including cost of service upgrade (new main electric panel) and considering savings by replacing one ICE-car with the EV I calculate now an ROI within 6-7 years.

Solar is now up and running, the installer did an excellent job, and the average daily production is well above what PV Watts calculated, which means to me that the system performs very well (and the sunny January is definitely also a contributing factor). Of course I do not have enough data points to see how the system will perform over the years and if the solar system is slightly over- or undersized.

A few hints you might find useful:
Understand your current energy consumption. The PG&E website provides the usage patterns for your house (assuming you do have a smart meter). I also installed a Rainforest-Eagle Smart Meter Gateway and get real time consumption data on my computer if I want to. And I use a Kill-a-Watt to measure energy consumption of devices. I used this to identify where we wasted energy and made some changes to the house (LED lights instead of incandescent, switch to turn off the entertainment center instead of just stand-by, etc).
With Solar and EV you will probably want to switch to a TOU (Time of Use) tariff. If you do that and are technically interested (as I am) start to consider some kind of home automation so you are able to turn on the dishwasher, washer and dryer in the most beneficial time.
If you get an EV you definitely want to get away from the standard tiered rate system. You'll end up paying almost 40c per kWh to PG&E charging the car as your consumption will go into tier 4 really fast (Don't ask why I know that )

Let me know if you want more info about my system or my installer.

Congrads. That you seem to have more production than PVWatts estimated is probably more of a function of 2 things: 1.) Using default inputs, PVWatts usually underestmates output. 2.) The weather is not the same as what PVWatts used.

Your thread is welcome confirmation of some of what's been discussed here for some time. Welcome to the party.

[ver2go]

I I can PM the name of the install company they are located in the East Bay. You might try to contact them directly, cut out the middle man and all that. I did my install for $3.60/w on a prepaid lease last May. I had an easy install panels on a composite low pitch roof with a single orientation, room in the panel, short conduit runs etc.

Would you mind PM'ing me? Or confirm/deny if they are YES?

[ver2go]

Let me know if you want more info about my system or my installer.

Hi,

I would be interested. Before the New Year I reached out to a few companies, as the folks here confirmed, the prices were a bit north. I'm ready to get new quotes again and negotiate for better pricing. I wasn't too pleased with my initial batch of installers so I would like to give yours a try. I