Calculating Solar System Installation Monetary Outcome

[slinthicum]

I believe I'm now in a position to better calculate the actual financial benefit that has resulted in the solar system I installed a little over 4 years ago. My focus at this time is looking at the most recent billing cycle (4/22/22 to 5/21/22). Calculations are based upon an average kWh cost of $0.409, a number provided by the U.S. Bureau of Labor for April for San Diego. Solar energy production for this timeframe according to the SolarEdge web application was 819.9 kWh. Solar production in excess of consumption was 148.8 kWh according to the Emporia application. Subtracting that amount from the total solar energy production leaves a balance of 671.1 kWh, an amount that is likely reasonably close to the actual household energy consumed during the month. That figure is not unreasonable, given the fact that I have an EV and a spa.

Multiplying the 671.1 kWh consumption by $0.409 suggests an energy cost of $274.48 for the billing period. As time goes on, it will be interesting to see the numbers for subsequent months as solar production increases and decreases. Given the mild weather near the coast, I do not expect a large variation in monthly consumption. There will of course be a variation in solar energy productiion as we move into the winter months.

What this analysis tells me is the decision I made in moving forward with the solar system installation was wise. Four years ago my average bill for electricity was approximately $200/month. The system cost after deducting the 30% Federal tax credit was around $10,500. My annual true-up bill has averaged around $200. I suspect I'm reaching the "break-even" point where I will have fully recovered the money I spent installing the solar system.

[foggysail]

Wise YES as electrical generation costs soar so will utility bills. I just renewed with my suppler with a 35% increase over the prior contract. If I delayed until 1 June the increase would have exceeded 50%. Smart move to install when you did. OH, now congress is pondering another substantial tariff on imported solar which already costs us about twice that of those in Europe.

[J.P.M.]

Q: What would your actual bill for electricity have been if you did not have a PV system ? Seems easy to calculate.

[RichardCullip]

Q: What would your actual bill for electricity have been if you did not have a PV system ? Seems easy to calculate.

It is an interesting and somewhat challenging exercise to accurately calculate what a bill from SDG&E would have been without a solar installation. I have had my solar install since April of 2019 and have been tracking solar production from SolarEdge and net electrical use from SDG&E downloads since day one of turning the system on. In year one I used 4,894 kWh which would have cost me $1,456.53 without solar. In year two I used 5,978 kWh which would have cost me $1,813.08 with solar. In year three I used 5,350 kWh which would have cost me $1,971.79. Without solar, assuming my usage patterns didn't change, I would have spent $5,241.39. With solar my bill from SDG&E for the three years nets out to a credit of $31.22 which has carried over into the start of year four.

In order to get fairly accurate estimate I need to track changes in SDG&E's TOU rates and keep track of the TOU periods I use electricity. Not a trivial task but doable with a bit of Excel expertise.

[slinthicum]

I wish it was, but as near as I can tell the data provided by SDG&E focuses on electricity consumed when there is no sunshine and the difference between the electricity consumed and the solar system production during daylight hours. That difference in good times measures excess solar production and in not so good times the amount consumed less the system production.

[Ampster]

It is an interesting and somewhat challenging exercise to accurately calculate what a bill from SDG&E would have been without a solar installation.........

In order to get fairly accurate estimate I need to track changes in SDG&E's TOU rates and keep track of the TOU periods I use electricity. Not a trivial task but doable with a bit of Excel expertise.

Yes, TOU time periods and two to three rate changes a year add complexity to the exercise..

[J.P.M.]

It is an interesting and somewhat challenging exercise to accurately calculate what a bill from SDG&E would have been without a solar installation. I have had my solar install since April of 2019 and have been tracking solar production from SolarEdge and net electrical use from SDG&E downloads since day one of turning the system on. In year one I used 4,894 kWh which would have cost me $1,456.53 without solar. In year two I used 5,978 kWh which would have cost me $1,813.08 with solar. In year three I used 5,350 kWh which would have cost me $1,971.79. Without solar, assuming my usage patterns didn't change, I would have spent $5,241.39. With solar my bill from SDG&E for the three years nets out to a credit of $31.22 which has carried over into the start of year four.

In order to get fairly accurate estimate I need to track changes in SDG&E's TOU rates and keep track of the TOU periods I use electricity. Not a trivial task but doable with a bit of Excel expertise.

Thank you for the info. Kind of nice to see what a home constructed to Title 24 standards is capable of.

As for the spreadsheet work, it might not be as bad as you think. Once the spreadsheet template is done and checked, it's actually quite simple, and that's writing as one who only knows enough about spreadsheets to be dangerous and not much more. HelI, I don't even know what a macro is, much less know to write one and I get along just fine. I'm pretty sure a spreadsheet jockey with any facility at all would shake their head and laugh at my simplistic spreadsheet skills, but my stuff seems to agree with what SDG & E sends me and my neighbors. It was much more difficult back in Buffalo and way back in the days before PC's and excel (or LOTUS). Prior to that all my solar and conservation work was done with a TI- 30 until the early '80's when PC's came along. All my programming before then was done for work and done in Fortran IV for technical stuff, or COBAL for engineering economic work. PC's changed everything.

Through it all, I've been tracking and storing POCO rates, and reading/recording all my gas, electricity and water meter readings 1X/day with the good eyeball method and a notebook almost first thing in the A.M. The spreadsheet's the easy part. Making sense of the numbers is the rigorous and fun part.

For you or anyone in SDG & E's service area all that's needed for what you may consider a daunting task is the available green button data from SDG & E and an appropriate T.O.U. rate schedule, both or all of which are readily available - both current and past. Once the spreadsheet templates are done it's a rather easy task to get within a penny or two of an SDG & E bill - I've done it many times for neighbors - to see the impact on billing for various changes in quantity and manner of use (as in time shifting of the type Slinthicum mentioned he had something published about such things), or PV contributions to use reduction and its impact on bills.

All the necessary info is in plain sight for the price of the looking and some spreadsheet work of a couple of hours. It then becomes a plug and chug task and the guesswork is over. It was also a rather satisfying educational experience, but that's just me.

FWIW, for the little work involved to get it right, thorough, rigorous and flexible, I've been a bit surprised and somewhat amazed for years now that no one has published/marketed something just like it or better.

[RichardCullip]

I use a combination of three spreadsheets. One spreadsheet downloads the hourly solar generation values from the SolarEdge monitoring website using the SolarEdge Monitoring API. A macro takes care of the nitty gritty details to download the data each evening and convert it to the appropriate TOU periods. Each morning I use a second spreadsheet to convert the SDG&E Green Button 15min usage data and convert it to the appropriate TOU periods including the NBC amounts. Each of these spreadsheets automatically transfers the data to a third spreadsheet which does the monthly billing calculations. At the end of the monthly billing cycle I match SDG&E's bill totals with a penny or two.

Once I have the daily solar production and the SDG&E net usage converted to the appropriate TOU periods it's trivial to add the two together to see the total household energy use.which is used to calculate what my SDG&E bill would have been without solar.

Before retirement part of my job duties included Excel programming. I have kept up that skill with projects like this. Once properly programed, the macros do almost all of the work. It was an interesting intellectual challenge.

[RichardCullip]

Thank you for the info. Kind of nice to see what a home constructed to Title 24 standards is capable of....

.

FYI - our 2,000sq ft house was built in 1974. We bought it in 2009 and have upgraded the windows, added more attic insulation and installed a whole house fan to take advantage of the cool evening we often get in Poway. 4kW of Solar was added in 2019 which is generating more electricity than we use each year.

[J.P.M.]

FYI - our 2,000sq ft house was built in 1974. We bought it in 2009 and have upgraded the windows, added more attic insulation and installed a whole house fan to take advantage of the cool evening we often get in Poway. 4kW of Solar was added in 2019 which is generating more electricity than we use each year.

Apologies for the confusion. My bad.

Congrads on sensible energy conservation.

[slinthicum]

To further complicate rate calculations here in SDG&E country, many residential ratepayers have to deal a "baseline allowance" factor. A key component in calculating this number is the "microclimate" where your home is located. The utility provides a baseline calculator. With Oceanside in the coastal zone and Poway in the inland zone, two residents having matching power utilization (both in terms of quantity and TOU), can have different bill totals as the result of being in different zones.

[J.P.M.]

To further complicate rate calculations here in SDG&E country, many residential ratepayers have to deal a "baseline allowance" factor. A key component in calculating this number is the "microclimate" where your home is located. The utility provides a baseline calculator. With Oceanside in the coastal zone and Poway in the inland zone, two residents having matching power utilization (both in terms of quantity and TOU), can have different bill totals as the result of being in different zones.

Well, there's a little more to it than that.

Richard and I seem to be doing pretty much the same thing which doesn't surprise me since we have the same starting point and probably most of the same goal. His stuff is probably much cleaner and more elegant than my stuff.

My stuff accounts for climate zone by adding flags (a bunch of "if "statements) for climate zone that's part of the baseline calc that gives you the daily baseline allowance in kWh. That is then multiplied by the number of days in the billing cycle. There are 22 billing cycles each with 12 billing periods/yr. The number of days/cycle varies between 28 and 32 and the billing cycle year changes annually (see SDG & E for a copy).

The 4 climate zones with different daily baseline kWh allowances for both summer and winter are further broken down within each climate zone with a somewhat anachronistic modifier of the daily allowance that makes that daily allowance larger for homes that do not have natural gas service from SDG & E. Those users are what's called "all electric".

I've not bothered to investigate the billing cycle logic beyond some statistical analysis that seems to point to a slightly lower number of days/billing period for all billing cycles in the summer which would mean lower monthly allowance and so more billing at the 2d and the +400 % of baseline tiers and so more backdoor revenue for SDG & E.

Then, there are the rates themselves (there are currently 19 residential rate schedules in the current SDG & E tariff book) with all the rates changing several times /yr.
Some residential rate schedules have tier levels that use the baseline daily allowance as part of the rate calculation and some do not.

Having the different schedule info available can be helpful for users who want to examine the scenario of changing billing schedules to lower their bills in some way or season, but I suspect most users would stay on schedule for at least a year or more and probably have some but not much need for the other schedules.

As I wrote, getting an accurate algorithm that includes all the stuff SDG & E does to generate a residential bill is not conceptually difficult. Seems to me the hardest part was finding and deciphering all the nuance that SDG & E doesn't seem eager to divulge. It is there however for the diligent to uncover.

After that and the spreadsheet is done and checked, it's a plug and chug process with only updates for rates and CPUC forced (or allowed) changes necessary.

Take what you want of the above. Scrap the rest. If you do use it, I'd request you credit and reference the forum as a source.

[Mike 134]

Glad I'm in the Midwest surrounded by nuclear and coal fired plants. Flat 13 cents an hour, although they are starting to offer time of day billing.

[slinthicum]

I think a specific "takeaway" from all of this is utilities, with their complex rate structures make it challenging for consumers to make decisions about what plan is best for them.

So let me add a little fuel to the fire. Living in the Coastal zone results in having a lower baseline rate than those living in the other climate zones. As a consequence, I guess it could be argued that coasters are effectively subsidizing the lower cost per kWh those in other zones are paying.

[J.P.M.]

Glad I'm in the Midwest surrounded by nuclear and coal fired plants. Flat 13 cents an hour, although they are starting to offer time of day billing.

Enjoy your flatness, be it geographical or utility rate paying.

The source of the energy for generation makes little difference except probably as a bunch of red herring methods that use crap logic for others, be they POCOs or tree huggers or anything in between to complicate things, usually for not much in the way of good purposes.

The methods to do a bill with a spreadsheet are not as complicated as they sound (read). It just takes persistence.

Once the spreadsheet or other method is done - which doesn't take long - then checked and debugged, it's only a matter of kWh input either for design or estimating billing and keeping an occasional eyeball on the rate changes to stay current.

As I wrote, I'm a bit surprised (but maybe not) that someone hasn't done what I and folks like Richard have done already and published it, either for profit (IMO, the most likely motive) or as a public service.

It just isn't that intellectually difficult or challenging or time consuming to do.