How to sizing of the Solar System.

[decaview]

Hello, I am a newbie to the site and to solar system..

I have a burning question about sizing the solar system that I couldn't figure out searching through online and reading forum threads.

Here is my electricity usage info. I am in NorCal in PG&E territory. I use about 700KWH and own two PHEV and under EVA electricity rate. Each month my off-peak usage is about 400kwh to 450kwh. Currently under EVA rate, peak = $0.43/kwh, part-peak = $0.23/kwh, off-peak = $0.10/kwh.

I asked one of my local solar installer to size my system, I would told that I would need about 5.2kw system to cover 80% to 90% of my usage. Also http://estimator.solar-estimate.org/ estimate about the same size system.

But my question is if I install the solar system and stay on EVA rate which will allow me to sell majority of my solar electricity production at peak and partial-peak rates and buy/use majority of my electricity at off-peak rate, wouldn't I be able to cover 90% to even 100% of my electricity cost ($$) with 1/2 the size of 5.2kw system? Given that peak and partial-peak rates are 2X to 4X more than off-peak. I understand that my yearly total generation will not be able to cover my total usage in kwh. In term of $$, let say 2.6kw system cover majority of the cost of usage?

Thanks in advance..

[solarix]

This is a complicated question and I congratulate you on even trying to figure it out - most solar salespeople don't even try that hard.
Without delving into the details of your utility's rate plan which is complex - having summer/winter variations, differing off-peak/on-peak times depending on what day.... I'll guess that about half your solar generation will fall on-peak and your most cost-effective solution will be to make that on-peak solar generation match your on-peak usage. The off-peak solar generation will mostly cover your off-peak usage but is just gravy on the deal as doesn't contribute to the payback near so much. Guessing your total insolation at 4.5 hrs/day you need 300kWh(on-peak usage per month)/30days/2.25hrs/day/70%efficient = 6.3kWstc array size. To find out exactly, spend a $100, get Andy Blacks solar estimator (OnGrid Solar) for a month, plug in all the numbers (it has a rate plan model for your utility) and let us know.

[J.P.M.]

This is a relatively easy thing to explain and nearly impossible to nail down: If you game the system by time shifting your use, you can do the same with less - a smaller system for the same residual bill - maybe.

With tiered rates, when you use is not a factor.

As you know with T.O.U., when as well as how much power you buy (use) and sell (generate) needs to be considered.

Assuming for a minute, for simplicity only, that you want a system that zeros out your bill. Also assume that T.O.U. does not have tiers within times as some do. With tiered rates that system will need to generate 100 or so % of your use. Now, on T.O.U., if you only use power at off peak, @ $0.10/kWh, and you only generate at peak time and so get "paid" @$0.43/kWh for what you generate, you'll zero your bill out with a system oly need generate approx. .10/.43 = ~23% of what you use. So the system can be much smaller.

As a practical matter, that won't happen and in reality, it much more complicated than that. My example was absurdly oversimplified to show a point.

With a decent and realistic handle on hourly use patterns - hardly an easy thing to come up with - it's probably possible to have a smaller system using a T.O.U. tariff than the same usage with tiered rate tariff - most of the time, but maybe not always.

Depending on your T.O.U. tariff, spreadsheets help to figure out how much "revenue" a specific system may generate in an average year over many years. That can then be used to "buy" power according to the same T.O.U. tariff.

Not an easy task, but a bit of thought and a spreadsheet helps make the task a bit more manageable. Don't let the 8,760 rows intimidate you.

The double edge of T.O.U. is that it gives consumers more control over their electric bill by allowing some choice as to when they use power and thus some say in how much they pay for it. The other edge is it gives consumers a choice to do things at odd hours (like laundry at 3 A.M.) in order to save money. That may create a situation some see as the POCO "forcing" them to do things they would not normally do - like laundry in the middle of the night.

[decaview]

This is a complicated question and I congratulate you on even trying to figure it out - most solar salespeople don't even try that hard.
Without delving into the details of your utility's rate plan which is complex - having summer/winter variations, differing off-peak/on-peak times depending on what day.... I'll guess that about half your solar generation will fall on-peak and your most cost-effective solution will be to make that on-peak solar generation match your on-peak usage. The off-peak solar generation will mostly cover your off-peak usage but is just gravy on the deal as doesn't contribute to the payback near so much. Guessing your total insolation at 4.5 hrs/day you need 300kWh(on-peak usage per month)/30days/2.25hrs/day/70%efficient = 6.3kWstc array size. To find out exactly, spend a $100, get Andy Blacks solar estimator (OnGrid Solar) for a month, plug in all the numbers (it has a rate plan model for your utility) and let us know.

Thanks for your advice. Much appreciated.

[decaview]

This is a relatively easy thing to explain and nearly impossible to nail down: If you game the system by time shifting your use, you can do the same with less - a smaller system for the same residual bill - maybe.

With tiered rates, when you use is not a factor.

As you know with T.O.U., when as well as how much power you buy (use) and sell (generate) needs to be considered.

Assuming for a minute, for simplicity only, that you want a system that zeros out your bill. Also assume that T.O.U. does not have tiers within times as some do. With tiered rates that system will need to generate 100 or so % of your use. Now, on T.O.U., if you only use power at off peak, @ $0.10/kWh, and you only generate at peak time and so get "paid" @$0.43/kWh for what you generate, you'll zero your bill out with a system oly need generate approx. .10/.43 = ~23% of what you use. So the system can be much smaller.

As a practical matter, that won't happen and in reality, it much more complicated than that. My example was absurdly oversimplified to show a point.

With a decent and realistic handle on hourly use patterns - hardly an easy thing to come up with - it's probably possible to have a smaller system using a T.O.U. tariff than the same usage with tiered rate tariff - most of the time, but maybe not always.

Depending on your T.O.U. tariff, spreadsheets help to figure out how much "revenue" a specific system may generate in an average year over many years. That can then be used to "buy" power according to the same T.O.U. tariff.

Not an easy task, but a bit of thought and a spreadsheet helps make the task a bit more manageable. Don't let the 8,760 rows intimidate you.

The double edge of T.O.U. is that it gives consumers more control over their electric bill by allowing some choice as to when they use power and thus some say in how much they pay for it. The other edge is it gives consumers a choice to do things at odd hours (like laundry at 3 A.M.) in order to save money. That may create a situation some see as the POCO "forcing" them to do things they would not normally do - like laundry in the middle of the night.

Thanks for the clarification. I have recently switched to EVA rate plan. I will let it run for a couple of months and revisit on this topic once I have better data of my usage/cost pattern. Ideally it would be great to have one whole year of data but I don't think I would want to wait that long. I used to be on PG&E E6 rate plan and my usage is pushing my rates to tier-3 and 4 levels. Now that I am on EVA rates, I will see how much electricity cost with this rate plan and reevaluate my option to invest in solar system.

[jimqpublic]

A few more things to consider-

Things that suggest not undersizing too much- future expansion can be difficult.

• Federal 30% solar tax credit is scheduled to expire at the end of 2016.
• PG&E requirement to offer a 20 year "net energy metering" (NEM) contract will end sometime in the next 2 years. The successor tariff probably won't be as sweet a deal for residential consumers. Southern California Edison's NEM contract limits future expansion of a "grandfathered" system to 10% or 1 kW; for more you will need to switch to a new contract.
• Even under a NEM contract, the TOU rates and rate periods can change. SCE recently changed the on-peak periods for some rate plans from noon-6 pm to 2-8 pm. That moves your two best production hours from peak to off-peak (DST solar noon is 1:00 pm or later in most of CA) , and moves two prime evening energy use hours into peak.

On the other hand, don't believe anyone on the idea that system prices have bottomed out.

• Component prices may have flattened, but they haven't bottomed and will probably drop more- especially after the fed tax credit expires. Racking designs are becoming more elegant and will either drop or save install labor.
• Permitting is becoming more streamlined.
• Utilities will probably be more helpful under the successor tariff.
• Installers are getting more efficient.
• Installers appear to be trying to squeeze more profit as they sell the 2016 tax deadline as a reason to rush.

[J.P.M.]

Thanks for the clarification. I have recently switched to EVA rate plan. I will let it run for a couple of months and revisit on this topic once I have better data of my usage/cost pattern. Ideally it would be great to have one whole year of data but I don't think I would want to wait that long. I used to be on PG&E E6 rate plan and my usage is pushing my rates to tier-3 and 4 levels. Now that I am on EVA rates, I will see how much electricity cost with this rate plan and reevaluate my option to invest in solar system.

If you don't already know, keep in mind that net metering as we know it will probably come to an end in the next year or so as the 5 % caps are reached. Also, unless things change via legislation, the fed. tax credit will drop from 30% to 10 % at the end of 2016.

[jimqpublic]

If you don't already know, keep in mind that net metering as we know it will probably come to an end in the next year or so as the 5 % caps are reached. Also, unless things change via legislation, the fed. tax credit will drop from 30% to 10 % at the end of 2016.

Clarification on J.P.M.'s post-
-New NEM signups will end- existing NEM contracts are good for 20 years and are transferrable to new owners.
-I believe the 30% federal tax credit for residential solar expires completely; the commercial credit drops to 10%.

[J.P.M.]

Clarification on J.P.M.'s post-
-New NEM signups will end- existing NEM contracts are good for 20 years and are transferrable to new owners.
-I believe the 30% federal tax credit for residential solar expires completely; the commercial credit drops to 10%.

You are correct.

My error on the residential tax credit. It does expire and drop to zero on 12/31/2016. The residential tax credit does not drop to 10%. The commercial tax credit drops to 10%.

I apologize for my error.

On the NEM - I could have been more clear. Those on NEM in CA will have 20 yrs. from startup date with transfer rights to new owners.

Unless changed by further legislation, those users installing after the 5 % cap is reached, or until 06/30/2017, whichever comes first, will fall under whatever NEM arrangements are in effect at that time.