Ask a Renewable Energy Expert

Hi Everyone,

I had a 5kw system installed last year that is not fully offsetting our usage. I'm wondering if it's common to add to an existing PV system and how difficult/expensive it typically is.

By my calculations (and I'm no expert) we need an additional 190kWh per month to eliminate our power bill. The system currently has produced an average of 677kWh per month over the past 7 months. There is room on the roof and we have a 5kw inverter so I'd assume that would need to be upgraded as well.

I'm asking this because the company that I went through guaranteed that the system would match our average usage and fully offset the power bill. I'm trying to gather as much info as I can before contacting them.

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Samuel Gray
Web Developer
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Hi Samuel,

Thanks for posting your question here!

It's quite doable to add to an existing system. Based on my understanding of the issue, the primary hurdles you may face are:

• Inverter capacity -- If your system uses a single string inverter rather than microinverters on each panel, you may need a new inverter (depending on the existing inverter's capacity).
• Permits -- You'll want to check if you need another permit to add additional panels to your roof.

If you go back to the original company and ask them to perform the upgrade, they will obviously be the most familiar with your system and know its ins and outs. They should also have access to more of the same panels and/or microinverters. If you were to approach a different installer, they may be hesitant (or outright refuse) to work on another installer's system due to warranty and liability concerns.

In terms of pricing, I'll leave that to those on here who actually work for solar installation companies. It will be a smaller job, and some companies may not even wish to take up such a small job since it won't be profitable for them.

I'm curious what those who work more directly in the installation space would recommend in terms of seeking out that guarantee for a full usage offset from the installer. Does your contract with the installer provide any information on your potential means of recourse should they fail to meet their guarantee?

I hope this helps!

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Kyle Combes
Software Engineer & Technical Product Manager
Oceanworks, Inc
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Original Message:
Sent: 02-15-2023 11:03 AM
From: Samuel Gray
Subject: Expanding existing PV system

Hi Everyone,

I had a 5kw system installed last year that is not fully offsetting our usage. I'm wondering if it's common to add to an existing PV system and how difficult/expensive it typically is.

By my calculations (and I'm no expert) we need an additional 190kWh per month to eliminate our power bill. The system currently has produced an average of 677kWh per month over the past 7 months. There is room on the roof and we have a 5kw inverter so I'd assume that would need to be upgraded as well.

I'm asking this because the company that I went through guaranteed that the system would match our average usage and fully offset the power bill. I'm trying to gather as much info as I can before contacting them.

------------------------------
Samuel Gray
Web Developer
------------------------------

Did your installer provide you with a simulation report of the predicted system production?  Typically this would provide the monthly production expected from the system.  If not, you can go to PVWatts and run your own simulation, by putting in the size, location, orientation and tilt of your array. I suggest you choose premium panel type and leave all the other paramaters at default.

You say your installer guaranteed the system output would offset your bill, was this a written guarantee or just a verbal claim?  It is not common for installers to guarantee output, there can be year over year variation. It is more honest to provide the simulation using local irradiance data and tell you that things like shading and snow cover can impact production, as well as grid outages if your inverter is grid-tie with no battery. If there is significant shading from trees or buildings, they should have acccounted for that in the modeling.

You describe the last 7 months of production, but what specific months are included in those 7?  If July thru Jan, you have not yet seen production from May and June which often ar ethe highest production months.  Still, sounds like that would not be enough to make up 190kWh per month shortage.

Expanding the system is possible, but let this be a lesson for everyone reading this that you should ask questions and get a production simulation and try to install as much solar as you need up front because installing it twice is a lot more expensive than sizing it right the first time.

------------------------------
Ken Nadsady
AviSun Renewable Energy
Hudson OH
------------------------------

Original Message:
Sent: 02-15-2023 11:03 AM
From: Samuel Gray
Subject: Expanding existing PV system

Hi Everyone,

I had a 5kw system installed last year that is not fully offsetting our usage. I'm wondering if it's common to add to an existing PV system and how difficult/expensive it typically is.

By my calculations (and I'm no expert) we need an additional 190kWh per month to eliminate our power bill. The system currently has produced an average of 677kWh per month over the past 7 months. There is room on the roof and we have a 5kw inverter so I'd assume that would need to be upgraded as well.

I'm asking this because the company that I went through guaranteed that the system would match our average usage and fully offset the power bill. I'm trying to gather as much info as I can before contacting them.

------------------------------
Samuel Gray
Web Developer
------------------------------

I'll second Ken's comments and add a few:

It definitely makes sense to wait an entire year to get a complete picture of your system's annual production.  If you run a PVWatts report for your system, you'll see projected average production by month of the year.  You can see that, as Ken suggests, certain months are estimated to produce a lot more than other due to the sun being higher in the sky and the days being longer (more avg sun hours per day). 

If your installer gave you an estimated year 1 production from your system but then your actual first 12 months of production are a lot lower, it can be due to a cloudier than usual year, snowier than usual, etc (it does happen). I would guess your installer didn't guarantee production but you can go back to them and express your concerns.  They should be able to show you the PVWatts report for your system that they used to create their design (or some similar report) and you can see how your system's production compares for each specific month of the year (Jan actual to Jan est, Dec actual to Dec est, etc.)

If they made a error in sizing and it's consistently underproducing month after month, you can ask if the installer would maybe add a panel or two on their own dime (even though they may not have guaranteed production).   It doesn't hurt to ask, but you should have a bunch more data points than it sounds like you currently have (at least 12 months).

I hope that's helpful.

Dara

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Dara Bortman
Sustainable Solar Systems
Philadelphia PA
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Original Message:
Sent: 02-20-2023 11:13 AM
From: Ken Nadsady
Subject: Expanding existing PV system

Did your installer provide you with a simulation report of the predicted system production?  Typically this would provide the monthly production expected from the system.  If not, you can go to PVWatts and run your own simulation, by putting in the size, location, orientation and tilt of your array. I suggest you choose premium panel type and leave all the other paramaters at default.

You say your installer guaranteed the system output would offset your bill, was this a written guarantee or just a verbal claim?  It is not common for installers to guarantee output, there can be year over year variation. It is more honest to provide the simulation using local irradiance data and tell you that things like shading and snow cover can impact production, as well as grid outages if your inverter is grid-tie with no battery. If there is significant shading from trees or buildings, they should have acccounted for that in the modeling.

You describe the last 7 months of production, but what specific months are included in those 7?  If July thru Jan, you have not yet seen production from May and June which often ar ethe highest production months.  Still, sounds like that would not be enough to make up 190kWh per month shortage.

Expanding the system is possible, but let this be a lesson for everyone reading this that you should ask questions and get a production simulation and try to install as much solar as you need up front because installing it twice is a lot more expensive than sizing it right the first time.

------------------------------
Ken Nadsady
AviSun Renewable Energy
Hudson OH

Original Message:
Sent: 02-15-2023 11:03 AM
From: Samuel Gray
Subject: Expanding existing PV system

Hi Everyone,

I had a 5kw system installed last year that is not fully offsetting our usage. I'm wondering if it's common to add to an existing PV system and how difficult/expensive it typically is.

By my calculations (and I'm no expert) we need an additional 190kWh per month to eliminate our power bill. The system currently has produced an average of 677kWh per month over the past 7 months. There is room on the roof and we have a 5kw inverter so I'd assume that would need to be upgraded as well.

I'm asking this because the company that I went through guaranteed that the system would match our average usage and fully offset the power bill. I'm trying to gather as much info as I can before contacting them.

------------------------------
Samuel Gray
Web Developer
------------------------------