PVWatts - I'm confused about it - any advice appreciated

[sqc]

Hi all,

I have been looking at and playing with PVWatts and - within reason - it's an easy to use/understand tool.

HOWEVER, I keep seeing references here about using it to help size a system for my needs.

I don't really understand how you would do this, when all it gives you is an estimate of your yearly energy production.

If I don't have any component pricing or a list of system components (panels, inverter, etc.) upfront, how am I supposed to make use of this data?

Apologies if this seems like a totally noob question, but I just don't get it.

If I am starting from scratch (no pricing, no component details, just an address & knowledge of my utility's rate, etc.) exactly HOW do I use PV Watts to help me get to whatever the next stage is?

Thanks in advance for your help.

SQC.

[inetdog]

Hi all,

I have been looking at and playing with PVWatts and - within reason - it's an easy to use/understand tool.

HOWEVER, I keep seeing references here about using it to help size a system for my needs.

I don't really understand how you would do this, when all it gives you is an estimate of your yearly energy production.

If I don't have any component pricing or a list of system components (panels, inverter, etc.) upfront, how am I supposed to make use of this data?

Apologies if this seems like a totally noob question, but I just don't get it.

If I am starting from scratch (no pricing, no component details, just an address & knowledge of my utility's rate, etc.) exactly HOW do I use PV Watts to help me get to whatever the next stage is?

Thanks in advance for your help.

SQC.

If you are designing an off-grid system, then the first step is to size the components using the information in the sticky threads on the off-grid area.
Part of that will involve using PVWatts to determine how much panel power you need to cover the lower amount of sun in your particular area in the winter.
If you are designing a grid tie system, then PVWatts will let you figure out how many watts of panel you need to cover any particular percentage of your total year's grid power use. After that determination you can look at inverters that will handle that much power.
If you have issue with shading, then you will want to explore using optimizers or microinverters.
First step is to size the system, then ask about component parts.
If you want a grid tie system, get several quotes from installers in your area. Depending on the local permit and inspection process and the requirements of your Power Company (POCO) as well as your skills a DIY installation may not be economical for you.
If you will not DIY, the installer will suggest components rather than asking you for a list, and we can comment on the merits of different quoted equipment.

[sqc]

PVWatts - I'm confused about it - any advice appreciated

If you are designing an off-grid system, then the first step is to size the components using the information in the sticky threads on the off-grid area.
Part of that will involve using PVWatts to determine how much panel power you need to cover the lower amount of sun in your particular area in the winter.
If you are designing a grid tie system, then PVWatts will let you figure out how many watts of panel you need to cover any particular percentage of your total year's grid power use. After that determination you can look at inverters that will handle that much power.
If you have issue with shading, then you will want to explore using optimizers or microinverters.
First step is to size the system, then ask about component parts.
If you want a grid tie system, get several quotes from installers in your area. Depending on the local permit and inspection process and the requirements of your Power Company (POCO) as well as your skills a DIY installation may not be economical for you.
If you will not DIY, the installer will suggest components rather than asking you for a list, and we can comment on the merits of different quoted equipment.

Hi & thanks for your fast response inetdog.

So, I'm looking at a grid tie system, and I'm still confused about how PVWatts will let me work out how many watts of panels I need.

So, in running the numbers just using all default settings, PVWatts tells me I will generate 6,325 kWh/yr & my avg solar radiation is 5.76.

How does that tell me how many watts I need in terms of my system?

Sorry if that's obvious, but it still has me stumped.

Thanks.

SQC

pvwatts.jpg

[solar_newbie]

Hi & thanks for your fast response inetdog.

So, I'm looking at a grid tie system, and I'm still confused about how PVWatts will let me work out how many watts of panels I need.

So, in running the numbers just using all default settings, PVWatts tells me I will generate 6,325 kWh/yr & my avg solar radiation is 5.76.

How does that tell me how many watts I need in terms of my system?

Sorry if that's obvious, but it still has me stumped.

Thanks.

SQC

[ATTACH]7975[/ATTACH]

Default is 4KW. You can change it along with other parameter as need. From beginning, just change the kw value to get the production match with what you want to cover.

[sqc]

Default is 4KW. You can change it along with other parameter as need. From beginning, just change the kw value to get the production match with what you want to cover.

Thanks solar_newbie. I do understand the default is 4kW and that I can change that number, but how does that tell me what production match I want or need to cover?

At the moment, I just know that a default 4kW system will produce approx 6,325 kw/yr. How do I figure out what I need?

Is the idea to keep changing that number until I get close to 6,325/yr or something else?

That's the bit that's confusing me.

Do I need to get data from my utility first to see how much power I used per year, or how much power used per year during daylight hours, or something else again?

This should seem obvious to me, but it isn't.

Thanks again.

[solar_newbie]

Thanks solar_newbie. I do understand the default is 4kW and that I can change that number, but how does that tell me what production match I want or need to cover?

At the moment, I just know that a default 4kW system will produce approx 6,325 kw/yr. How do I figure out what I need?

Is the idea to keep changing that number until I get close to 6,325/yr or something else?

That's the bit that's confusing me.

Do I need to get data from my utility first to see how much power I used per year, or how much power used per year during daylight hours, or something else again?

This should seem obvious to me, but it isn't.

Thanks again.

Yes. You need to get the total power you use for 12 months. That are good enough as starter.
Later, you can play with time of use, but it is quite advance.

Next, you can play with the direction of your roof Use google map. measure degree from North. True South = 180, True West = 270 ...

[sqc]

Yes. You need to get the total power you use for 12 months. That are good enough as starter.
Later, you can play with time of use, but it is quite advance.

Next, you can play with the direction of your roof Use google map. measure degree from North. True South = 180, True West = 270 ...

Ok, so just to clarify, I start by getting my yearly power usage from my utility. Obviously this is going to assume day & night time usage? If I am going grid-tied withOUT batteries, I guess I can approx halve the amount of power used last year as a guide to how much I might need to generate now with solar?

With that number (x kWh/yr), I can modify the system rating in PV Watts from 4kW to whatever other test number and keep tweaking the calculation until the yearly energy production roughly equals half my yearly consumption of power?

Is that correct, or am I making some wild assumptions about the power I would have used during daylight peak hours?

Thanks again.

[solar_newbie]

Ok, so just to clarify, I start by getting my yearly power usage from my utility. Obviously this is going to assume day & night time usage? If I am going grid-tied withOUT batteries, I guess I can approx halve the amount of power used last year as a guide to how much I might need to generate now with solar?

With that number (x kWh/yr), I can modify the system rating in PV Watts from 4kW to whatever other test number and keep tweaking the calculation until the yearly energy production roughly equals half my yearly consumption of power?

Is that correct, or am I making some wild assumptions about the power I would have used during daylight peak hours?

Thanks again.

Since it is grid tied system, total is good enough. You can start with either 85% or 100% production to offset your usage.

Once you know well, you can do model to make some TOU stuffs, but it does not matter much.

I would suggest to size with 100% now, so solar system will degrade over time it will come back to match your usage ... But some may not agree with me.

That is my mistake. I measure 85% of my usage With TOU, it is match to my usage, but I can not do anything else. Sometime learn more info is not good I do calculate using spreadsheet and all data. I find out to have system to cover 85% of my usage and TOU plan. I will have zero in my bill (beside min payment). However, I do not think that I should make my family a bit more comfortable with solar, like turn on AC, get pool pump run more frequence ..... Those can not be done ... I wish I size the system with 100% my usage from beginning.

[foo1bar]

Ok, so just to clarify, I start by getting my yearly power usage from my utility. Obviously this is going to assume day & night time usage? If I am going grid-tied withOUT batteries, I guess I can approx halve the amount of power used last year as a guide to how much I might need to generate now with solar?

1st: determine if you can use net metering.
(many/most utilities do, but not all. For example all the big utilities in CA do, and I think most of the small ones)

2nd: If net metering is available learn how it works for your state/town/utility. ( google "net metering my_homes_power_company"; ex. 'net metering xcel energy' - you may need to add additional terms if it's a multi-state utility or something.)

3rd: if net metering is available, typically you can put 1kwh during the day and get 1 (or maybe more than 1) back out at nighttime.
Some places you can put 1kwh into the grid during the summer and effectively get that 1kwh back out in winter.

4th: determine your total power usage for the year. (On mine it's on the bill, and also easily available from the POCO website)

5th: If you have net metering and it's a 1:1 ratio, plan on having your system be sized to generate during the year about the same kwh as you purchased last year. (More than that and you're paying more for a system whose energy will go into the grid and you'll get nearly no benefit from it.)

6th use pvwatts with the right slope and direction/angle for your roof (if roof mount) to determine how many kwh a year you get for a 1kw array. Then divide that result by the total kwh you need and that's the # of kw you need for your array. (NOTE that I used 'kwh' and 'kw' correctly in that sentence - try to use them correctly yourself when writing - it makes your life easier. If you don't know the difference already, go learn. )
Many people recommend tweaking the pvwatts inputs a bit to use a higher efficiency number than the default - you can search this forum and probably find threads on that. IMO you're going to be off by a few percent no matter what. It's never going to be perfect representation - just do the best you can.


7th: talk to a solar salesman or two - at this point you're probably better informed than 90% of the people they deal with. You'll be better positioned to ask intelligent questions.

[J.P.M.]

sqc:

Suit yourself, but as a respectful suggestion, before you go any further, do yourself a big favor: Download and read " Solar for Dummies". The way you're going about this is like trying to get information on running when you haven't learned to walk yet. Start at the beginning. A small investment in time can pay big dividends.

Also, don't take everything you read here as gospel. While everyone's opinion is to be respected, and a lot of what you'll read here is from folks with a lot of experience and education - like foo1bar and Inetdog,, some others are not.

[sqc]

sqc:

Suit yourself, but as a respectful suggestion, before you go any further, do yourself a big favor: Download and read " Solar for Dummies". The way you're going about this is like trying to get information on running when you haven't learned to walk yet. Start at the beginning. A small investment in time can pay big dividends.

Also, don't take everything you read here as gospel. While everyone's opinion is to be respected, and a lot of what you'll read here is from folks with a lot of experience and education - like foo1bar and Inetdog,, some others are not.

Hi JPM. Thanks for the suggestion to download Solar For Dummies. Is it available on this forum or elsewhere? I could only find "Solar Power Your Home For Dummies" by Rik DeGunther on Amazon. Is this the book you mean?

It is for sale, not free, which makes me think you might be referring to a different book. A quick google search found a reference but was suspiciously stored on a russian site and I didn't want to risk any issues with viruses, etc.

For what it's worth, I do understand solar fairly well. I'm just confused about how best to use PVwatts to help me decide what sized system I should get. It's very pretty but appears to have been designed for solar engineers, not the general public.

[Naptown]

Ok
Since this is grid tie and we assume you have net metering available (what state do you live in)
find out how many KWH you used last year by either adding up usage or going to the POCO website and getting the information.
Next go to PV watts change the system size to 1 KW
Plug in premium panels
roof mount
angle of roof and azimuth of roof ( pitch and direction) direction can be found on Google earth.
This will predict how many KWH that system will produce in a year.
Lets for example it will produce 1200 Kwh in a year.
Divide that 1200 by 1000 and you come up with 1.2 KWH per year for each watt DC you put on the roof. This is the production factor as I like to call it.
Lastly divide your annual usage or whatever percentage you want to offset by the production factor.
Say you use 6000 KWH a year
6000/1.2=5000W
So to offset 6000KWH per year would take a 5000 watt system.

[sqc]

Ok
Since this is grid tie and we assume you have net metering available (what state do you live in)
find out how many KWH you used last year by either adding up usage or going to the POCO website and getting the information.
Next go to PV watts change the system size to 1 KW
Plug in premium panels
roof mount
angle of roof and azimuth of roof ( pitch and direction) direction can be found on Google earth.
This will predict how many KWH that system will produce in a year.
Lets for example it will produce 1200 Kwh in a year.
Divide that 1200 by 1000 and you come up with 1.2 KWH per year for each watt DC you put on the roof. This is the production factor as I like to call it.
Lastly divide your annual usage or whatever percentage you want to offset by the production factor.
Say you use 6000 KWH a year
6000/1.2=5000W
So to offset 6000KWH per year would take a 5000 watt system.

Thanks Rich

[Naptown]

you are welcome

[J.P.M.]

Hi JPM. Thanks for the suggestion to download Solar For Dummies. Is it available on this forum or elsewhere? I could only find "Solar Power Your Home For Dummies" by Rik DeGunther on Amazon. Is this the book you mean?

It is for sale, not free, which makes me think you might be referring to a different book. A quick google search found a reference but was suspiciously stored on a russian site and I didn't want to risk any issues with viruses, etc.

For what it's worth, I do understand solar fairly well. I'm just confused about how best to use PVwatts to help me decide what sized system I should get. It's very pretty but appears to have been designed for solar engineers, not the general public.

Yes. That's the one. And iot is free for download. Try this: Google: :"Solar Power Your Home for Dummies". Then, choose the option that says : "Solar Power Your Home for Dummies - Internet Archive". hit enter. Choose the PDF download option. I just did it again.