Self Consumption Sizing

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bendesa

Member
Location
USA
Occupation
Retired
Hi All,

Just curious about your opinions.

How do you design / size the maximum capacity of a PV System that you want to be self-consuming. (No export to the grid)

So it's a grid tied PV System. No storage equipment. Just verry simple Grid Tied.

Let's assume there is an average evenly distributed consumption (load) during the daylight hours of 150.000 Watt

How would you calculate / size the system ?

Many thx in advance for your contribution

Kind regards

B
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
At some point you will have to put some finer criteria on what the goal is. For example, are you trying to desing the most cost effective system, or are you trying to supply the load as much of the time as possible?

It should also go almost without saying that at 150W average load there's pretty much nowhere in the real world that this idea would have a point. But change that to 1500W or 15000W and that may change significantly.

Conceptually, you probably want an inverter(s) whose nameplate is close to the average load being served. If you want more cost effectiveness, you'd connect modules slightly above that nameplate, and if you want more assurance that the load will be served, you'd connect more modules that are higher above inverter nameplate. You don't want to pay for more inverter power that would likely only be used for export.

This approach doesn't necessarily work if the load is much more variable than you've postulated. There is probably a software analysis approach that could handle such situations, but I haven't seen any such product on the market (and I've been keeping my eye out). And such an approach would be way beyond anything that could be described in a forum post.
 

bendesa

Member
Location
USA
Occupation
Retired
At some point you will have to put some finer criteria on what the goal is. For example, are you trying to desing the most cost effective system, or are you trying to supply the load as much of the time as possible?

It should also go almost without saying that at 150W average load there's pretty much nowhere in the real world that this idea would have a point. But change that to 1500W or 15000W and that may change significantly.

Conceptually, you probably want an inverter(s) whose nameplate is close to the average load being served. If you want more cost effectiveness, you'd connect modules slightly above that nameplate, and if you want more assurance that the load will be served, you'd connect more modules that are higher above inverter nameplate. You don't want to pay for more inverter power that would likely only be used for export.

This approach doesn't necessarily work if the load is much more variable than you've postulated. There is probably a software analysis approach that could handle such situations, but I haven't seen any such product on the market (and I've been keeping my eye out). And such an approach would be way beyond anything that could be described in a forum post.
Jaggedben,

Thank you so much for your extended reaction. I do appreciate that. I also do understand your remarks.

The goal is to design a system that is as much self consuming as possible. So try to limit the export to the grid.

I've seen calculations for this made quite simple like:

The (Max) Load is 150,000 Watt
The performance ration for the PV System is 75%
So the max capacity for the PV System is

150,000 x 1.4 (PR) makes a System with a max capacity of 210,000 kWp

Personally I think that's to simple. First of all all data like load and performance is not static.
I can imagine that oversizing can be more profitable at the end.

The like they sometimes do with clipping, see the attachment. But what is the right formula to calculate the optimal capacity?

Curious about your thoughts on this

Kind regards

B
 

Attachments

  • solar-inverter-output-AC-power-change-curve-throughout-the-day.jpg
    solar-inverter-output-AC-power-change-curve-throughout-the-day.jpg
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Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Why wouldn’t you want excess to go to the grid?
Don’t most inverters have an export override?
 

Carultch

Senior Member
Location
Massachusetts
Why wouldn’t you want excess to go to the grid?
Don’t most inverters have an export override?
A: your ISA doesn't permit it.
B: your service is on a particular section of the grid, that cannot accept backfeeding
C: you get penalized for exporting, being billed for it as if it were consumption, which penalizes you in two ways.
D: you get no credit for it, and do not want to set a precedent of giving away energy for free.

I don't know what you mean by "export override", but there are ways to stop your inverter from exporting, by feeding it back a measurement of the service's net power. This is a feature most manufacturers I've seen call zero export, rather than export override. This ultimately will "leave sun on the roof", so there's potential production for which you won't get any credit.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
A: your ISA doesn't permit it.
B: your service is on a particular section of the grid, that cannot accept backfeeding
C: you get penalized for exporting, being billed for it as if it were consumption, which penalizes you in two ways.
D: you get no credit for it, and do not want to set a precedent of giving away energy for free.

I don't know what you mean by "export override", but there are ways to stop your inverter from exporting, by feeding it back a measurement of the service's net power. This is a feature most manufacturers I've seen call zero export, rather than export override. This ultimately will "leave sun on the roof", so there's potential production for which you won't get any credit.
I know the reasons you stated.. I deal with some of them daily.
I was asking the OP specifically why he didn’t want to export.
Export override, Zero export, probably some other names different manufacturers use to stop excess from exporting.
Depending on his setup and inverter, it could be a simple as a setting.
 

bendesa

Member
Location
USA
Occupation
Retired
Ok let's open up and give you more information.

The project is realized under BOT Built Operate Transfer. With a zero investment for the client.
The PV System will be installed on the factory roof of the client.
An Investor will finance the whole system
The client buys the produced kWh by the PV System against a tariff of minus 10% of the utility company
The investor operates and maintains the system for a period of 20 years
After 20 years the system is transferred to the client.
The country is Indonesia

All kWh that are exported to the grid will not be payable for the investor so no revenue
The Utility company does not give any credit for the exported kwh

So I'm looking for the balance between maximum capacity PV System and maximum savings for the client and maximum
revenue for the investor.

How would you determine the maximum PV System Capacity?

And yes I know many variables are involved and that the numbers are not static

I'm just curious what are your thoughts on this

Many thanks in advance

Kind regards

B
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
Can the utility provide any data on load profile throughout the day? It probably wouldn't cost much to throw a logger on the service equipment for a few days of typical operation. Without that level of data, it is going to be really tough to guess. Just assuming a static load of 150kW sounds like a shot in the dark. I am assuming the . was supposed to be a ,
 

pv_n00b

Senior Member
Location
CA, USA
If you want to do it without curtailment controls then you identify the lowest site energy usage during the time of PV production and size the PV system to some percentage of this to keep production always less than consumption, like 50%. If the utility requires export protection then you need a reverse power relay. This can put a low cap on the size of your PV system though.

If you want a more complicated system you can have controls that allow the PV system output to track the site energy usage and curtail the inverters if they would overproduce and export. In this case, you can go up to the maximum site energy usage. But depending on the variation in energy usage you might end up with a PV system that rarely runs at full power, making the energy produced more expensive.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
A: your ISA doesn't permit it.
B: your service is on a particular section of the grid, that cannot accept backfeeding
C: you get penalized for exporting, being billed for it as if it were consumption, which penalizes you in two ways.
D: you get no credit for it, and do not want to set a precedent of giving away energy for free.
Reason D is kinda silly, IMO. Why would you want to spend extra money to keep from giving energy away when giving it away doesn't cost you anything?
 

pv_n00b

Senior Member
Location
CA, USA
Reason D is kinda silly, IMO. Why would you want to spend extra money to keep from giving energy away when giving it away doesn't cost you anything?
Oh yeah, some people just have a thing about not giving anything to the utility for free. It just sticks in their craw. And they will pay not to do it, but there is usually a dollar limit on how much they dislike giving it away. :D
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Oh yeah, some people just have a thing about not giving anything to the utility for free. It just sticks in their craw. And they will pay not to do it, but there is usually a dollar limit on how much they dislike giving it away. :D
Sticking it to the man by sticking it to themselves.
 

paulengr

Senior Member
You size a ZPV system based on a lot of factors such as the latitude and average weather conditions. So these factors define a system with an average desired output but the actual production varies continuously. Since we are effectively “over sizing” the system we will get a maximum that can be high enough to become a producer at times.

The reality is that there is one way to guarantee power flow never goes “backwards”. Size the maximum system output (not average) so that it is never greater than the minimum customer usage. This will not maximize revenue though.

This is how I would do it. It’s a simple boot strap method. If we can model the output as a distribution curve (average and standard distribution) and we have actual consumption data from a meter, or a second model, we can simply run a simulation. At each iteration we randomly and uniformly pick both a solar system output and a customer demand. Then calculate the value (currency) for that condition. Run it for say 1000 iterations and then you can average that (as well as standard deviation) to determine revenue. The idea here is rather than attempting a highly complex if not impossible analytical model, just side step it. This is exactly what bootstrap statistical methods are all about and easily constructed in spreadsheets.
 
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