Oversizing inverters

[jflorey2]

Also if using PV Watts as a tool to get an estimate of production it should be noted that PV Watts assumes a 1.2 to 1 DC to AC ratio. Therefore if one wants to get a reasonable estimate of production they should calculate the DC to AC ratio and enter it into PV Watts along with other system parameters such as location, azimuth and tilt.

Definitely. I'm not saying it's not important. I'm just saying that starting out by saying "I want a 1.2 to 1 DC/AC ratio so I will . . . " doesn't generally give good results.

[Ampster]

Definitely. I'm not saying it's not important. I'm just saying that starting out by saying "I want a 1.2 to 1 DC/AC ratio so I will . . . " doesn't generally give good results.

I agree with the concept that starting out with an idea of a randomly picked ratio can give bad results. . The ratio of 1.2 is the assumption in PVWatts and it is roughly what you get from a table from Enphase which matches micros to panels. I have noticed that when doing some scenerios that a ratio of 1.2 often gives the optimal resuts compared to lower or higher ratios depending on other factors. That ratio might be a good starting point unless there are good reasons to start with another number. It does have a probability of providing an optimal result, but other factors as you mentioned earlier may have more significance or influence on the initial design. One can enter other configurations to find an optimal mix if the data suggests.

[fraser]

I agree with the concept that starting out with an idea of a randomly picked ratio can give bad results. . The ratio of 1.2 is the assumption in PVWatts and it is roughly what you get from a table from Enphase which matches micros to panels. I have noticed that when doing some scenerios that a ratio of 1.2 often gives the optimal resuts compared to lower or higher ratios depending on other factors. That ratio might be a good starting point unless there are good reasons to start with another number. It does have a probability of providing an optimal result, but other factors as you mentioned earlier may have more significance or influence on the initial design. One can enter other configurations to find an optimal mix if the data suggests.

I think the problem is the definition of "optimal results". Optimal results should be based on ROI, not on the output per panel or inverter. A 1.2 ratio allows clipping where 1.0 does not and the marginal cost is usually not much (eg. it was $30 more for me to go from a 15kw to 16kw inverter combination). You start to see price jumps when you have to have multiple inverters. Plus oversizing the inverters allows you to add panels easily in the future. The other main problem that is not solved is sourcing. This by far has the largest impact on ROI as opposed to tilt, azimuth, etc. Better sourcing will cut your overall cost by 20-30%. I'm all in on materials for a 16kw system at $17k, not including an elec panel upgrade (another $4k), which I probably could have gone without, but would limit me switching over to elec car, heat pump, dryer, etc. On flat roofs, optimizing for tilt gains you maybe 8-16%, but you lose much more in in roof space efficiency because of additional inter-row spacing required. If you can tilt in a single plane, so there is zero interrow spacing, that would be optimal for the panels, but you have to have more secure racking and it would be ugly with one side being that high off the roof. So, you need to optimize for roof space vs the cost of panels/racking/optimizers/inverters (with sourcing being an additional component to optimize for each item). That gets you the true max ROI. No one is doing this. So, rules of thumb?
- Use the largest panels possible that fit on your roof (eg. you may be able to get 5 slightly smaller panels in a row on the roof vs 4 larger ones, resulting in higher power), with larger panels, you may have lower racking cost/panel , lower wiring cost/panel, lower optimizer cost/panel.
- Minimize tilt. With my latitude (37.45°), 10 degree east/west tilt produced the most power, by far. I lose 16% efficiency vs optimum tilt, but with almost no inter row spacing, I gain about 30% in power with more panels.
- If you tilt, do it in a way that minimizes interrow spacing so you can get an extra row of panels in. Eg. Tilt 2 rows of panels in a single plane, instead of 2 planes, or optimally tilt only the most northerly row (in N. hemi), or tilt rows that have mandatory pathways or setbacks behind them
- don't use microinverters, use optimizers and hybrid or string inverters

[davidcheok]

That gets you the true max ROI. No one is doing this. So, rules of thumb?

- Use the largest panels possible that fit on your roof ....

- Minimize tilt. With my latitude (37.45°), 10 degree east/west tilt produced the most power, by far. I lose 16% efficiency vs optimum tilt, but with almost no inter row spacing, I gain about 30% in power with more panels.

- If you tilt, do it in a way that minimizes interrow spacing so you can get an extra row of panels in. Eg. Tilt 2 rows of panels in a single plane, instead of 2 planes, or optimally tilt only the most northerly row (in N. hemi), or tilt rows that have mandatory pathways or setbacks behind them

- don't use microinverters, use optimizers and hybrid or string inverters

- Everyone is using ROI in some way or another. Just that everyone's opinion of ROI acceptability differs depending on what they are trying to achieve.

- I would say use the highest output panels available for the ideal physical size that maximises available roof space coverage

- Tilt is subjective because it depends on how far north and south you are, how willing you are to clean the panels regularly. Low angled tilts attracts dust, bird poop and what nots so need more regular maintenance than steeper angles. I would say optimise the tilt depending on location and opportunity costs.

- Shadows from panel spacing of tilted panels is also an opportunity cost depending on where you are located and thus depending on time of day and length of time the shadows actually affect the panels behind. Microinverters, optimisers and half cut panels can often help to negate this.

- the debate of micros vs op + string is never ending. I sit on the micro side because I have my own reasons and so will others. One cannot just say one is better than the other because it depends on the scenario that the system will be used in.

[fraser]

- Everyone is using ROI in some way or another. Just that everyone's opinion of ROI acceptability differs depending on what they are trying to achieve.

Yeah, there might be some back and forth on choosing between max ROI and max payback, but most solar companies are not optimizing for either. They are pushing overpriced panels with undersized and overpriced inverters.

- I would say use the highest output panels available for the ideal physical size that maximises available roof space coverage

Again, the solar companies aren't optimizing for cost as well, which is a key component of maximizing ROI, so you end up with sub-optimal returns. If you are paying $1/watt for panels, you are overpaying and at least 3/4 of the bids I received included expensive panels.

- Tilt is subjective because it depends on how far north and south you are, how willing you are to clean the panels regularly. Low angled tilts attracts dust, bird poop and what nots so need more regular maintenance than steeper angles. I would say optimise the tilt depending on location and opportunity costs.

This is misleading. Optimizing for tilt angle does produce the max power from a panel, BUT it comes at a huge cost in lost roof space for more panels, especially for flat or low slope roofs. I just tried Toronto Canada on PVwatts, and the loss of 0 deg tilt vs 30-35deg tilt is less than 12% loss, but you'd gain 30-40%+ in roof space. That said, the higher the latitude, the less likely one has a low slope roof.

- Shadows from panel spacing of tilted panels is also an opportunity cost depending on where you are located and thus depending on time of day and length of time the shadows actually affect the panels behind. Microinverters, optimisers and half cut panels can often help to negate this.

If roof space doesn't constrain the number of panels you want to install, then inter row spacing is not much of an issue. But if you use an inter row spacing calculator, you can see how much roof space is needed for inter row spacing. The difference of space needed between 10 and 30 deg tilt is about 40% of the panel size (its actually 50%, but you also gain 10% roof because the area below the panel is 10% less with a 30deg tilt).

- the debate of micros vs op + string is never ending. I sit on the micro side because I have my own reasons and so will others. One cannot just say one is better than the other because it depends on the scenario that the system will be used in.

I was in the microinverter camp initially. But, I found out that IQ8A microinverters, the strongest ones by Enphase, top out at 386w output. If you are using anything over 400w panels, you are losing a substantial amount to clipping. Optimizers can handle up to 700w so you only have to worry about having a large enough string inverter. If you are only using 400w panels, then you are paying 10-15% more in racking, optimizers and installation costs. Not to mention microinverters are much more expensive per watt than optimizers + inverters by about 30-100% (eg. 25 400w panels with microinverters -about $200/panel- vs using 25 optimizers and a single 10k string inverter- about $100-120/panel).

[davidcheok]

I was in the microinverter camp initially. But, I found out that IQ8A microinverters, the strongest ones by Enphase, top out at 386w output. If you are using anything over 400w panels, you are losing a substantial amount to clipping. Optimizers can handle up to 700w so you only have to worry about having a large enough string inverter. If you are only using 400w panels, then you are paying 10-15% more in racking, optimizers and installation costs. Not to mention microinverters are much more expensive per watt than optimizers + inverters by about 30-100% (eg. 25 400w panels with microinverters -about $200/panel- vs using 25 optimizers and a single 10k string inverter- about $100-120/panel).

Im using APS 440W with 600w panels. You should check out their DS3 and QT2 range.

Each inverter links up to 2 panels with separate MPPT. I have 15 inverters with 30 panels. Cost wise.. im sorry but those in the US pay a lot more for solar than some other countries. I paid about $25k BND (19k USD) for 18 kW of panels and 13.1 kW of AC.

IMHO i think all these micro vs optimiser vs string have their pros and cons. We will all have our justifications for them.

[DanS26]

There is a little piece of this conversation that is missing. In the case where the inverter maximum output is limited by either POCO rules or the electrical system in the facility, then oversizing the the panels is preferred since oversizing the inverter(s) is not possible. That allows the inverter(s) to run at maximum capacity for much longer time. ROI on the system is increased without a very expensive panel or POCO supply upgrade.

[Ampster]

I think the problem is the definition of "optimal results". Optimal results should be based on ROI, not on the output per panel or inverter. ......

Optimal results can include some things that can't be quantified easily, such as resiliency. Also while one can make assumptions about the rate of inflation the hedge on inflation is one of those enefits whose optimum result relies on assumptions. In the end, it all depends on where you are standing.

[Ampster]

There is a little piece of this conversation that is missing. In the case where the inverter maximum output is limited by either POCO rules or the electrical system in the facility, then oversizing the the panels is preferred since oversizing the inverter(s) is not possible. That allows the inverter(s) to run at maximum capacity for much longer time. ROI on the system is increased without a very expensive panel or POCO supply upgrade.

That is an important point that often drives the decision process and could lead to a high DC to AC ratio being chosen which may have a lot of clipping. If one were to e distracted by the clipping number one might not see the advantage of running the inverter at maximum capacity for longer and the optimum output for that installation.

[J.P.M.]

That is an important point that often drives the decision process and could lead to a high DC to AC ratio being chosen which may have a lot of clipping. If one were to e distracted by the clipping number one might not see the advantage of running the inverter at maximum capacity for longer.

Ampster: As several posters have observed in one way or another, a good design is usually a balance of design goals.
If one of those goals is to be able to have the lowest long term lowest cost of meeting the electrical needs of a dwelling (or at least < the long term cost of POCO power), that goal may be lost sight of if the cost penalties associated with undersizing an inverter or oversizing an array (or both) are not considered.
It's not about DC/AC ratios. It's about getting a good design that meets the design goals, one big one of which is usually cost effectiveness.

Overpaneling or underinverting are both costly because in doing so either system component is not used to it's optimum (maximum) capacity with the inevitable consequence of a negative impact on system cost effectiveness.
That impact may be unavoidable or part of a balance with other system design goals, but a high DC/AC ratio should be approached with caution and knowledge of its impact on system cost effectiveness.

Dan has brought up one such possible POCO imposed design constraint many residential PV owners and designers deal with. There are others but one reality is that overpaneling and dealing with the clipping can get costly.
If not done carefully, I'd suggest it's possible to design a system that has an LCOE > the LCOE that's bought from the POCO.
That's where process economics comes in.
Unfortunately, that's also about the point that's an order of magnitude or two more than most folks care to get involved.

Bottom line: If meeting the system design duty (load) for the lowest system LCOE is (after safety) the major design goal as it often is, and that goal is to be met in the best way possible in consideration and some balance with the other design goals, it won't matter if the final design is underinverted or overpanelled.

DC/AC ratio is a design outcome, not a design input or driver. I can't even see where it's needed in the design process.

I'd still suggest we might have some discussion about a definition of just what overpanelling an array and undersizing an inverter actually is, starting with the idea that they are not the same.

[davidcheok]

I can't even see where it's needed in the design process.

When you need to take into consideration high ambient temperatures and losses due to variables straying far from STC. Panels located in the sahara will definitely need to adjust the DC/AC ratio to consider the highest panel temperatures it could hit.

[fraser]

Ampster: As several posters have observed in one way or another, a good design is usually a balance of design goals.
If one of those goals is to be able to have the lowest long term lowest cost of meeting the electrical needs of a dwelling (or at least < the long term cost of POCO power), that goal may be lost sight of if the cost penalties associated with undersizing an inverter or oversizing an array (or both) are not considered.
It's not about DC/AC ratios. It's about getting a good design that meets the design goals, one big one of which is usually cost effectiveness.

It seems to me there is too much discussion about optimizing a system or creating a "balance of design", when there are many shortcuts, or rules of thumb, that would dramatically lower upfront costs and would allow for the elimination of electricity payments. So far, I didn't get one bid (out of maybe 20) that was below $2/watt (many were over $3) and none of the bids actually provided 100% of electricity needs. Which means no solar companies are optimizing using cost as a variable to optimize. Frankly, I have a system with double to triple the power and costs less than the systems that were offered by solar companies.

The point about the POCO limiting the inverter is another variable that I didn't have to deal with, but would be important in optimizing design. However I would also point out that if the marginal cost of increasing the inverter isn't substantial, then having a 1:1 DC to AC system would at least ensure that no clipping happens. In my case it was $30 more to have a 16kw AC vs a 15kw AC system.

Why would anyone think that using a 400w panel at $1/watt with microinverters that max out at 368w and cost $.50/w+ is better than using a 540w panel at .58/watt that uses optimizers and a 1:1 inverter that average out to $.30/w ??? Not to mention the 30% savings in racking and labor. I ran the math and for a 10k AC system, it's more than double the cost to use smaller panels and microinverters vs larger panels and optimizers and a string inverter. This doesn't include the additional racking cost plus labor cost of having 27 400w panels and microinverters vs 18 540w panels (both produce ~10kw ac). One could standardize a system around 10 and 13kw and undercut the other solar companies by 20-40%. I know companies offer "kits", but even these don't come close to the cost savings that should be achievable.

[Ampster]

Ampster: As several posters have observed in one way or another, a good design is usually a balance of design goals.
.......

I am in agreement with that statement.

..........
Dan has brought up one such possible POCO imposed design constraint many residential PV owners and designers deal with.

To be clear Dan and I are on the same page. His example is a good way to look at various approaches to analysis.

Bottom line: If meeting the system design duty (load) for the lowest system LCOE is (after safety) the major design goal as it often is, and that goal is to be met in the best way possible in consideration and some balance with the other design goals, it won't matter if the final design is underinverted or overpanelled.

I agree that load is an important goal and the best way to know if that goal can be met with the LCOE is to run several scenarios. In Dan's example, where the size of the inverter is limited, then one would likely run the scenerios of different panel capacities using that same inverter capacity. To use PV Watts to get reasonable estimates of annual production one would have to adjust the assumption in PV Watts and use the DC ratio from each of those design scenarios. Then one could compare the cost of each design and the annual output estimates of those designs. Are we talking about the same process here?

DC/AC ratio is a design outcome, not a design input or driver.

i agree it is an outcome of design choices. What do you see as the disagrement here?

I'd still suggest we might have some discussion about a definition of just what overpanelling an array and undersizing an inverter actually is, starting with the idea that they are not the same.

Feel free to start a new thread with that title or continue the discussion here. This is a topic I enjoy and it may have some benefit to other readers. I would be happy to participate in that discussion.

[Ampster]

When you need to take into consideration high ambient temperatures and losses due to variables straying far from STC. Panels located in the sahara will definitely need to adjust the DC/AC ratio to consider the highest panel temperatures it could hit.

In that example of the Sahara one would essentially be derating the panel output to compensate for lower output due to higher temperatures. I am not sure PV Watts accomodates a temperature derating input, but using the temperature coefficiant one could come up with a derated Wattage figure to input into PV Watts.
I agree that intuitively the result would be that the optimum design might have a higher DC to AC ratio than a system optimized for a cooler environment
A good example is a system which was installed in a home I had in the Wine Country of California six years ago. I did not pay much attention to the details until I started seeing some clipping in March. I found out the DC to AC ratio was 1.5 to 1. It was too late to make changes, but I was able to get the designer to run several scenarios for me. I had assumed that since I was getting clipping in March that it would be worse in June. What I had not figured was temperature. In March the days were cool, averaging 45 F. and in June the average temperatures were 80 F with panel temps abpve that. The June clipping I had feared was not significant and the design was optimal for that particular location. Historically the system typically outperformed the estimates.

[Ampster]

It seems to me there is too much discussion about optimizing a system or creating a "balance of design", when there are many shortcuts, or rules of thumb, that would dramatically lower upfront costs and would allow for the elimination of electricity payments. So far, I didn't get one bid (out of maybe 20) that was below $2/watt (many were over $3) and none of the bids actually provided 100% of electricity needs. Which means no solar companies are optimizing using cost as a variable to optimize. Frankly, I have a system with double to triple the power and costs less than the systems that were offered by solar companies.

You make a good point about designing a system to cover 100% of your needs and doing so at a lower cost than an installer could do it for. That is the joy of DIY.

The point about the POCO limiting the inverter is another variable that I didn't have to deal with, but would be important in optimizing design. However I would also point out that if the marginal cost of increasing the inverter isn't substantial, then having a 1:1 DC to AC system would at least ensure that no clipping happens. In my case it was $30 more to have a 16kw AC vs a 15kw AC system.

I would still use the optimal annual output compared to the cost as the basis of a decision. There is an optimal design whose output would possibly have some clipping but on an annual basis would produce more kWhs than a design which had no clipping. As I have said, clipping is a distraction that ignores the benefits of having the inverter running at capacity for a longer period of time. That is why annual production estimates offer a first order ranking of design scenarios.

Why would anyone think that using a 400w panel at $1/watt with microinverters that max out at 368w and cost $.50/w+ is better than using a 540w panel at .58/watt that uses optimizers and a 1:1 inverter that average out to $.30/w

That is a whole other discusion about the pros and cons of micros versus string inverters. In my most recent DIY install i was committed to micros and I had found a good value in some 435 Watt panels which gave me the gross capacity that I was shooting for. I then decided to run some scenarios to see if I could optimize the cost effectiveness. I looked at IQ7 micros with 290 Watts and 349 Watts in capacity. The larger capacity micros cost $50 more and the small increase in annual ouput had a longer payback than the smaller capacity micros. In that case a DC to AC ratio of 1.49 to 1 was optimal for my location and circumstances.

??? Not to mention the 30% savings in racking and labor. I ran the math and for a 10k AC system, it's more than double the cost to use smaller panels and microinverters vs larger panels and optimizers and a string inverter.....

It is the same discussion whether you are comparing a microinverter system to another microinverter system or two string inverters to each other. Larger panels result in less cost per Watt of racking. In my example above I chose a 435 Watt panel instead of a smaller one. I also did the math to see if the lower cost of the smaller panels would be worth it and ultimatly chose the larger panel because the revenue from the greater production more than compensated for the extra cost of larger panels.