Optimizing Power Usage

[Ampster]

..... I would be kind of pissed for such an obvious dumb ass move.
....

Yes, that was my initial reaction when I purchased a 5.7kW system with a 3.8kW Solar edge inverter. There is another thread discussing this so I wont go into details here. Here is a link:
https://www.solarpaneltalk.com/forum...f-solar-panels

[J.P.M.]

Wow, you guys offered a lot of information, we have 26 295w Canadian Panels on the south facing roof and a 6kw SolarEdge Inverter, not sure why the inverter wasn't 8kw but we don't heat our pool, we run our pool pump 6-8 hours a day and we haven't seen our first bill yet so I'm curious if NEM 2.0 actually pays according to the TOU Tier system, see TOU Tier attached screenshot. I'm still not sure if we should run our pool pump when we're producing the most power or at night during lowest rate tier?

On the other issue of pump run times and costs: For a minute, think of your array's input to the grid as completely separate from what you draw from the grid. So, every kWh you use (draw) from the grid during peak pricing time costs more than if drawn at non peak time. Similarly, every kWh you put into the grid during peak pricing hours is worth more than if put in during non peak pricing hours.

Now, put the two together. Every kWh you generate during peak pricing can offset an equal amount of usage for power you draw during that time. The same is true for other times. That means - and this is a blinding flash of the obvious - minimizing your draw during peak pricing times and/or shifting that use to lower pricing times will save you $$ and will result in a lower bill.

However, and to elaborate a bit, you really have no control over when your system generation takes place - that's under the control of old sol.

What you do have is some control over with T.O.U, in addition to not using the power through conservation measures, etc., is how much you pay for that power by controlling when you use that power. If you do not have a solar thermal water heater or other reasons why you'd need to run a pump during peak pricing times, then, if you are on some type of T.O.U. pricing, you'll most likely be better off financially running the pump when electricity is cheapest - during off peak, or better yet if available, super off peak times - that's often between midnite and 5 A.M. or so.

Provided you have a system that does not overgenerate over a 12 month period, under strictly T.O.U. billing tariffs, the amount of net bill savings a PV system will generate (the annual bill before PV minus the annual bill after PV) is mostly (but not entirely) independent of how much electricity you use and if you think about it, because generation offsets usage when the two take place simultaneously.

A very oversimplified example: Suppose you have a PV system and you go away for one billing period and use NO power (I know, won't happen, but say for a minute that it does and follow along with me here). So, all the system generation for that billing period will be excess and will generate a certain amount of "revenue" at retail rates - revenue that can only be used to offset future use. Say that generation is 1,000 kWh for that billing period, and it generates $ credit against future use at an average rate of $0.275/kWh or $275 for the 1,000 kWh.

Now, same (theoretical) month, but you stay home. The system generation is the same as when you weren't home == 1,000 kWh., and because it's the same hypothetical month, the solar energy distribution is the same and so the value of the generation to offset a bill is also the same == $275. On the usage side, since you're home, and as an admittedly unrealistic example, say you use 1,000 kWh for our same hypothetical billing period but use it (again and admittedly unrealistically) for 2 different usage patterns. The first pattern has you using all 1,000 hours at super off peak time at a rate of, say, $0.23/kWh between midnite and 5 A.M., and no usage at other times. That means your charges for that billing period will be $230 + some NBC and some other (small ?) charges. The second pattern has all your use at peak pricing times at a rate of, say, $0.52/kWh with no usage at other times, which means your charges for that billing period for the same 1,000 kWh of use will be will about $520 + some NBC and some other (small) charges like in the all super off peak usage scenario.

See where this is going ? The net generation will be the same for all 3 scenarios ($275) and because it's for the same (theoretical) billing period and so with the same weather and solar irradiance pattern, the value of the system generation to offset a bill is the same for all three scenarios = $275. What's changed is how much electricity is used (zero or 1,000 kWh) and (useful for your pump timing question) when the power is used., either at super off peak or peak pricing times.

For the use scenario that has all use at super off peak, you'll have about $275-$230 = ~ $45 surplus to carry forward. For the scenario that has all usage at peak pricing times you'll carry forward a charge of ~ $520 - $275 = $245. Note that the value of the power generated stayed the same. What you get to control is WHEN you use the power. In that sence, you get to control the size of your electric bills to some degree.

Reality is, for most residential users, simultaneous generation and use cancel one another out (Sidebar: Except for NBC which is charged on all POCO power input regardless of net + or - use at the end of a billing period and year) to the extent of the absolute value of the smaller of the two quantities, with what you usually see for kWh usage or $ credit on a billing statement is the net amount for all 15 minute periods in the billing period integrated over that billing period. That is, in most single meter cases, at any instant, if your home is using more power than your PV system is generating, the POCO "thinks" you are only using (actual use - actual system generation). That's why, for true T.O.U billing (that is, for billing tariffs without a tier system layered over the top of them), as long as the system does not overgenerate on an annual basis, the net $ value of a system model's generation (such as from PVWatts for example) is a constant for any given tariff schedule and set of rates over a year. The actual will of course be different from year to year and from the model's output, mostly due to weather variation but also due at least partly as f(model input vs. actual system parameters).

[John_Dumke]

J.P.M. - Thanks for the clarification. After looking at my first months bill. Your explanation was pretty much the conclusion that I came to. That SCE and my TOU rate plan was not zeroing me out at the end of the year based upon total KWh's produced and consumed, but zeroing me out more on the value of that energy produced and consumed.

This is actually pretty surprising, that SCE would make a TOU plan mandatory for NEM 2.0 as it actually seems like the TOU plan favors the solar owner.

While I did design my system to cover a little more than 100% of my usage, based upon the favorable treatment of solar generation, I may really have to crank up my AC during the summer to use excess generated $$'s

[Ampster]

J.P.M. - Thanks for the clarification. After looking at my first months bill. Your explanation was pretty much the conclusion that I came to. That SCE and my TOU rate plan was not zeroing me out at the end of the year based upon total KWh's produced and consumed, but zeroing me out more on the value of that energy produced and consumed.

What is even more interesting is one year I had over a megaWhrs of net consumption and a $100 accumulated credit that all zeroed out..

This is actually pretty surprising, that SCE would make a TOU plan mandatory for NEM 2.0 as it actually seems like the TOU plan favors the solar owner.

There was a policy decision at the state level that encouraged the Investor Owned Utilities to implement TOU rates for consumers. I think they let each utility had some latitude in how the do it. Interestingly I just got SCEs notice of new TOU rates for commercial. The rate was not a shock but the peak shifted late in the afternoon and mid peak now runs most of the night and the super off peak runs almost until Noon. I have two buildings with solar and I will be interested in the effects. Most of my loads are lighting and EV charging late at night. I will have to change the timers on the EV chargers. The trend has been to shift the rate times to times less favorable for solar.

[Ampster]

Ampster, sounds like you understand the dilemma, I've just ordered the Sense Energy Monitor with Solar Electricity Usage Monitor to Track Solar Production and Energy Usage in Real Time. Hopefully this accessory proves to be a helpful tool.

We must think alike. I also have the Sense. My inverter gives me generation but with the Sense I can see generation and consumption in real time on my phone.
I am a TOU fanatic and now that it has recognized the dryer I am going to get some Hue lights and maybe have them flash red if the electric dryer is turned on before super off peak rate.LOL

[J.P.M.]

Never a good move? From a compensation from the POCO you are correct. However there are three reasons to disagree.

1) Usage can be flexible OR the "Not having to care and bug your wife to turn off the lights or not use the AC" reason. - Might my usage pick up a little knowing that I am not in Tier 3 of my old rate plan at 40 cents a KWh? Heck yeah! With a large home, five computers running all the time and a pool. I would tend to not run the AC during a couple of hot months. Plus I had to constantly either bug my wife to turn lights out after leaving a room or be very annoyed if I chose not to bug her. Either option is not fun. But now I just bought a couple of small electric space heaters for heating bedrooms in the winter, which may help me get out of tier 3 gas usage.

2) Predicting your actual production can be difficult. - While I have done all of the calculations on PV watts, my annual usage has ranged between 13,000 - 15,000 KWh. Combine that with the PV watts calculator possibly being 10% too conservative what is the "Right" number?

3) Incremental install of solar is much cheaper than after the fact install of solar - My self installed system consists of 24 x 385 watt panels. These panels costs about $230 plus add a $70 optimizer. So overbuilding my system by 2 panels (around 10%) only costs $600. The inverter stays the same and the additional wiring, racking, shipping cost and my labor is negligible. But if done after the fact, I would have the following additional expenses. Re engineering and re-permitting the system $600, additional shipping costs $200 for shipping 2 panels separately. My additional time to install the panels at a later date, which would include, ordering, install and walking permits through the city, probably at least 10 hours. If I billed my additional time at $100 an hour which is reasonable and that is what an electrician would charge then that is an additional $1,000.

So I can spend $600 now and have piece of mind, have added expand-ability and not have to bug my wife and possibly waste a portion of this money, But even if the entire $600 is a waste, the alternative would be to spend $1,800 ($600 + $200 + $1,000) of my time and money at a later date. The risk / reward favors getting the extra capacity at the time of install rather than at a later date.

Opinions and situations vary, and there are a lot more than 3 reasons to disagree. I do not disagree with what you post, but only offer some opinions about what I think I've learned about some things and offer them for what they may be worth. I often suggest folks take my mental and written spoor for what it's worth to them and scrap the rest.

To your points, which I'd take as 3 of many to disagree with me about, I would comment on a FWIW basis:

1.) I've found it's more cost effective not to generate electricity than to use more of it.

2.) Predicting actual system production is about as accurate as predicting next year's weather - and mostly for the same reasons. That's why I know it's folly in the short term and marginally better as the period of measurement gets longer. Using PVWatts or any PV design model's hourly output to predict any single day's output in the future is absurd. Using a period of, say, 30 days starts to get into the realm of the difference between weather and climate so the variance gets smaller, but with the variation between model and actual probably about what the PVWatts help screens suggest.

On such variance and for those interested, I track and record my system's output in 5 minute increments and have done so since 10/17/2013 on a continuous basis. One way of several I analyze the data is to use running 31 day totals of actual output to what SAM (That's PVWatts on steroids) says for my system and location for the same running 31 day periods. Then, I compare that data to some stuff I wrote that is similar to SAM but pukes out additional data. I also compare it to another model called TRNSYS that I've had a license for since 1980. I then take PVWatts and modify the system loss parameter, inverter ratios and inverter efficiency that I've measured several hundred times so as to match the PVWatts model's annual output of the average of the other 3 models and compare each PVWattts so modified running 31 day output to the actual system's running 31 day output for the same period and plot/analyze the difference between the actual system output and the PVWatts output.

The system loss parameter for PVWatts needs to be 10.3 %. The DC to AC size ratio needs to be 1.046 (= 5.232/5), and the inverter efficiency needs to be 97% to match what I've measured several hundred times. Doing all that, the PVWatts model shows an annual output of 9,446 kWh/yr. The actual system 365 running day average output before shade correction 9,449 kWh/yr.

Bottom line: Over 2,236 consecutive running 31 day periods:
The average actual output for all 31 day periods has been has been 99.48 % of the adjusted PVWatts model output for the same periods.
The minimum was 60.19%.
The max. was 123.39%.
The population standard deviation was 9.9 %.

I'm not sure why the data is skewed toward the min., but otherwise, I'd suggest the distribution is about or roughly in line with what the PVWatts manual suggest as a variation for monthly data, that is, as much as ~ +/- 30 %. So, quantitatively, I think I agree that +/- ~ 30 % is about as good as I can hang my hat on for saying what my, or maybe any nearby system's output might be for any one month period in the future - at least for planning purposes.

Qualitatively, as for predicting output, it looks like the best anyone can do is about as good as long term weather forecasting which brings the old saying to mind : Climate is what you expect. Weather is what you get.

3.) Incremental install after original install is indeed much more expensive than adding to a system. However, looking at the bigger picture, goals and priorities come to mind - as in setting them up front while knowing and accepting the fact that things will change. A little oversizing is probably wise. How much ? Individual choice and hail the freedom to choose. Still, oversizing based on ignorance, which is IMO the most common single reason for oversizing in a world where most homeowners don't as much as the difference between a kilowatt and a kilowatt-hour. Oversizing coats money and affects system cost effectiveness in ways most homeowners are completely clueless about.

To my somewhat limited experience and opinion, that ignorance usually or at least often results in systems being installed that wind up increasing the NPV of the cost of proving electricity to a dwelling after adding a PV system to beyond the NPV of the cost of providing electricity to the same dwelling had the system never been installed. That puts a lot of treehuggers and solar lemmings behind the 8 ball and they're clueless about it.

I've been around alternate energy and in particular solar energy since the mid '70's. I'm one of its biggest fans. It's the initial and a large reason why I changed careers to engineering. But, I've got to think, if more folks knew what I think I might know, there would be a lot fewer PV systems on residential property, and those systems that were installed would be a lot smaller and a whole lot better designed.

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

[J.P.M.]

The interesting effect is that a panel which is open circuited and a panel which is short circuited will both get hotter than a panel delivering output at its MPP, and by roughly the same amount.

One other way to look at it is that at MPP the cell/panel/system will be operating at a greater efficiency (output/input). So, from an energy balance on the device, assuming (quasi-)steady state operation, and regardless of the mechanisms in the cell that achieve it, less of the incoming energy will rejected as waste heat. Assuming the overall heat loss coeff. is a weak f(temp.), particularly over the small delta T's we're dealing with here, less heat rejection will be manifested as a lower device-to-ambient temp. diff.

[John_Dumke]

J.P.M. - I can certainly see how in context the never oversize a system statement makes sense. It took me a fair amount of time wrap my head around the solar process and I am a relentless researcher and also have a BSME degree. And I only have just learned that my TOU billing may even make my system even more productive.

So I have no doubt that what is sold to most consumers is often times inappropriate on many levels. It was this initial distrust of solar sales people and their lack of understanding of what they were selling that led me to do my own install.

While my in-depth and educated decision to oversize my system by 10% may be relative, it required a more in-depth analysis than what any homeowners or solar sales people would possess.

So in general the statement to not oversize your system applies as a general rule. Because 99 times out of 100 somebody is wasting your money. I definitely get it.

[J.P.M.]

J.P.M. - I can certainly see how in context the never oversize a system statement makes sense. It took me a fair amount of time wrap my head around the solar process and I am a relentless researcher and also have a BSME degree. And I only have just learned that my TOU billing may even make my system even more productive.

So I have no doubt that what is sold to most consumers is often times inappropriate on many levels. It was this initial distrust of solar sales people and their lack of understanding of what they were selling that led me to do my own install.

While my in-depth and educated decision to oversize my system by 10% may be relative, it required a more in-depth analysis than what any homeowners or solar sales people would possess.

So in general the statement to not oversize your system applies as a general rule. Because 99 times out of 100 somebody is wasting your money. I definitely get it.

Understood. I'd also say that for most residential installs, at least in most of the U.S, 10% oversizing is probably justifiable economically and practically, but that's a matter of individual choice.

When I was designing process equipment/boilers/system for power plants/refineries, etc., some overdesign was often prudent and, in spite of what some may think of my opinions about oversizing from my rants around here, I was usually a fan of sane approaches to the surplus sizing question, but know that it always and also prudently required very careful analysis of the costs, benefits and possible consequences, both expected and educated guesses about possible unknown consequences. Most of the time, low initial buck was the driving force by those with the power of the purse with conservative design considerations the poor stepchild.

Problem with most residential decision makers (if one exists at all ?) is that the best choices are those made by informed folks and most are not - for the most part they seem to be simply running in near panic from high electric bills they neither understand or care to learn about.

As for what passes for formal education these days, and while not trying to paint everyone with the same brush, I offer a recent anecdote as to putting too much trust in the value of alleged education and capabilities: I've got a neighbor who claims to be an EE who recently bought a system based on low buck from Larry with a Ladder Solar. His choice of panels was made (so he told me) by the one with the highest OC spec sheet voltage (which, BTW, it was not), with the justification being that gave the lowest line loss and so highest system efficiency (!). The guy's about 30 yrs. old and so probably just beyond the 20 something range of the mostly clueless.

[Guardian7]

Ampster, we do think alike and have Hue lights! Today our Sense kit is due to arrive from Amazon, looks like an easy DIY install, is your antenna outside your service panel? I realize that it could actually pay for itself if Solar owners leverage the information like you have and it identifies your appliances properly. How long have you had your Sense and what is your opinion of the product, do you find it to be a worthy investment for Home Solar?

[Ampster]

My service panel is outside on a wall of my garage. Behind my service panel, inside the garage I have my two sub panels. I mounted the Sense on the internal wall and found a short and easy path to run the CT wires from the inside of the garage to the service panel. My WIFI router signal only has to travel through one other wall. I am not sure it is recommended to have the Sense outside the panel per code..I will find out when I get the two sub panels inspected next month. If I do change it i will still have the antenna on the inside..

i have had my Sense for 6 months and i like it enough that i gave one to my daughter and her husband for Christmas. They have a PPA with Solar City which they acquired when the bought the house two years ago. Their consumption has been way over their generation and they were getting into expensive tiers. I did the analysis and convinced them to move to a TOU rate after their last true up. My son in law is a tech geek and I think it will help them manage their consumption.

[Ampster]

hii
<link removed>

Wow, amazing. Will it make my inverter larger too. I am getting some clipping on sunny days. LOL