Calculate solar panel output

[SolTex]

yes a 10.8kw system should cover my 13mw usage but i can not generate 10.8kw as my inverters will only out put 9k

I would suggest taking J.P.M.'s suggestion and running your numbers in the PVWatts calculator. Try inputting your present system and see what you get. You may be suprised to find that regardless of what the fast-talking sales person said, you were in fact sold a system that will meet your requirements. If, OTOH, if the calculator says your present system falls short, you can run it again with 4 more panels and another inverter and see how that comes out.

You don't mention your geographic location or any specifics about your installation. Just as a comparison, I put what numbers you have stated up against our system. We live in West TX where the annual solar radiation numbers are very favorable. Below in the little chart I compare my system to yours:

Annual consumption: (mine) 26,000 kWh (yours) 13,000 kWh
System DC size: (mine) 15,200 W (yours) 10,800 W
System AC size: (mine) 11,800 W (yours) 9,000 W
DC/AC Ratio: (mine) 1.29 (yours) 1.20

You can see that my annual consumption runs about twice what you consume. Yet your system (AC rating) is about 75% the size of mine, so under suitable conditions should have no problem supporting your annual consumption.

When I plugged my system into PVWatts (BEFORE we signed the contract) it predicted we would offset our usage 110% over the course of a year. So far, after a year of operation, we are running well ahead of the prediction. I think the calculator can produce some pretty reliable results, as long as the inputs are correct and accurate.

[J.P.M.]

Here the standard for commercial systems is the inverter max
output KW, and a DC:AC ratio to indicate over paneled. I see
the panels as intended to keep the inverter busy, sometimes under
conditions where the DC total can never be achived in operation,
so I use the same standard. Somtimes panels have multiple
orientations, allowing serious power to flow for more hours and
achieve more energy, more evenly, with a given size inverter.
Bruce Roe

I believe I understand what you're writing, and that makes sense for commercial systems, but for residential systems it makes little sense to me to oversize an inverter beyond what the design calls for and probably something a bit less than the STC rating of what's driving the inverter.

Rather than seeing the purpose of the panels as keeping the inverter busy, I see the inverter's purpose as an interface and power conditioner between the panels and the grid, at least for a grid tie system, with the arrays sized to the design duty and the inverter(s) sized to meet and handle what the array(s) produce in the safest, most efficient and most cost-effective manner.

Arrays with multiple panel orientations can indeed produce a more even power output over a day but will do so at the expense of less overall power output per installed STC kW over the course of that day.

In terms of annual system output, multiple array orientations always mean more installed STC kW capacity will be required than for a single optimally oriented array.
Want examples ? Do a PVWatts run on a 2 STC kW array for any location and find the orientation that maximizes output. Then, find any 2 orientations for 1 STC kW arrays for the same location that provide greater output.

Q.E.D.

And while we both know that a single orientation array will mean more inverter capacity to handle the higher peak output, and so likely at (some) greater inverter cost, it will also mean lower panel and associated installation cost.

My educated guess is that an incremental panel STC W costs more than an incremental inverter capacity W, particularly when construction costs such as wiring/framing and construction costs are considered.

While there may be some exceptions such as max. kW output rates to the grid type of restraints, and maybe some other considerations, besides common sense, I've only to look at the number of single orientation large solar farms vs. the number of multiple array orientation farms for confirmation that a lot of solar farm designers that use PV panels seem to agree with me that given the choice, single array orientation is more cost effective and probably more practical.

[alfie1111]

You were, in all likelihood, told something close to reality.
Do not confuse PV system rated power (in STC kilowatts) with PV system annual energy production (kilowatt-hours).
Depending on your location, a 10.8 STC kW PV system may very well generate 13,000 kWh/yr. of electrical energy +/- maybe 5%to 10% or so per year depending on the weather.
Do a PVWatts run with accurate input that correctly describes your system to model what a 28 panel * 455 STC W/panel = 12.74 STC kW system will produce in your location.

yes a 10.8kw system should cover my 13mw usage but i can not generate 10.8kw as my inverters will only out put 9k

[J.P.M.]

We use Net Metering here so what I put out to the grid I can take back as needed

Back to what SolTex wrote, be careful about system sizing. Often, folks think they can turn a PV system into an unlimited revenue generator beyond electric bill offset. That's probably not so.

While it's true that net metering can be a good deal for PV owners, the concept of net metering was never meant to produce a lot of excess revenue or be a cash cow for residential PV system owners.

Check the rules you're under carefully, including the fine print that applies to your net metering agreement.

Many such agreements or at least most of them have limitations on the value of excess generation that a NEM customer sends back to the grid.

Sometimes all of the energy that's sent back to the POCO - regardless of customer usage quantity - is reimbursed by the POCO at the same rate as what the POCO charges.

However, and usually, POCO payments per kWh for excess generation sent to the POCO in excess of usage is reimbursed at a much lower rate than the retail rate the POCO charges its customers.

[J.P.M.]

So I was told I would need 10,800k of power to cover my electricity usage in the year(I used 13mw) then they should have installed 28 panels and 7 inverters which would be 12,700w?

You were, in all likelihood, told something close to reality.
Do not confuse PV system rated power (in STC kilowatts) with PV system annual energy production (kilowatt-hours).
Depending on your location, a 10.8 STC kW PV system may very well generate 13,000 kWh/yr. of electrical energy +/- maybe 5%to 10% or so per year depending on the weather.
Do a PVWatts run with accurate input that correctly describes your system to model what a 28 panel * 455 STC W/panel = 12.74 STC kW system will produce in your location.

[alfie1111]

We use Net Metering here so what I put out to the grid I can take back as needed

[SolTex]

So I was told I would need 10,800k of power to cover my electricity usage in the year(I used 13mw) then they should have installed 28 panels and 7 inverters which would be 12,700w?

Now you are getting into the economics of sizing a solar system. How "big" you want your system depends on what happens to your excess production. Does your utility provider pay you (or give you credits) for exported energy, or do you send it to the grid and get nothing in return? In some cases you are better off only covering say perhaps 80% of your average annual consumption, so that you can actually use almost every kilowatt that you generate, and don't spend money on solar capacity that is wasted exporting to the grid.

[alfie1111]

So I was told I would need 10,800k of power to cover my electricity usage in the year(I used 13mw) then they should have installed 28 panels and 7 inverters which would be 12,700w?

[bcroe]

Here the standard for commercial systems is the inverter max
output KW, and a DC:AC ratio to indicate over paneled. I see
the panels as intended to keep the inverter busy, sometimes under
conditions where the DC total can never be achived in operation,
so I use the same standard. Somtimes panels have multiple
orientations, allowing serious power to flow for more hours and
achieve more energy, more evenly, with a given size inverter.
Bruce Roe

[J.P.M.]

Since before residential PV became popular, PV panels and systems have been sized and designed for the required duty (say, in kWh/yr. of system output), and generally described in terms of the total STC output of the panels, not inverter capacity.

Inverter size is indeed the end limiting factor in terms of maximum system output, but there is more to system (and inverter) design than just that peak output.

Inverters are also sized and designed to meet the required system duty, including safety, operational and cost considerations.

Similar to the prime mover (the ICE or electric motor) in a vehicle, and for a lot of reasons, a PV panel or array will commonly and only rarely achieve it's STC rating in use.
By something of an analogy, the engine in your car rarely, if ever, operates at its rated horsepower or kW output.

Just like HP ratings of ICEs the STC rating of a panel can be thought of as an output under standard test conditions.
Because panel will almost never operate at its STC output, the maximum (combined) rating of all the system inverters is often and usually designed to be less than the STC rating of the panels.
That is, if a system has, say,10, 300 STC W panels making it a 3 kW STC system, it has always and generally been referred to as a 3 kW system.
If in a sunny climate and equator facing, the best inverter choice(s) for that 3 STC kW array may be close to 3 kW, but probably closer to, maybe 2.5 kW or so.
If that same array is east or west facing, a smaller system output can probably be expected and so a smaller inverter (less than 2.5 kW) may be a better choice for both design and cost considerations.

Describing systems that way (in terms of STC W of installed panel capacity) also seems to make price comparisons and system economics easier or at least more straight forward.
Multiple systems for describing system sizes can be troublesome for lots of reasons, one big one being that such multiplicity of system descriptions is a ready-made playground for vendors to make already confused and solar ignorant makes more prone to getting screwed.

In the interest of consistency in describing PV systems, it would seem to me that keeping the convention of describing system size by the STC wattage of the panels will avoid confusion, particularly for system pricing and pricing comparisons.

Systems should be sized and designed to meet the required duty and design parameters.
Panels and inverter sizes are related but probably not identical. Total inverter capacity is usually but not always smaller than total STC panel size.
Systems ought to be described using the total STC panel size.
The way system sizes are commonly and currently described doesn't need changing.
Doing so will only screw things up and make the waters muddier than they already are.

Seems to me that describing a PV system in terms of inverter capacity is a bit like describing how fast a vehicle can go in terms of the tires.

[SolTex]

It consist of 24 Longi 455w panels and 6 APsystem QS1A inverters rated at 1500w continuous

Despite the fact that you were (in my opinion) misled as to the actual "size" of your system, it is a "well proportioned" system by current design guidelines. Conventional wisdom says that the ILR (Inverter Loading Ratio) of a system should be around 1.25 or close to it. The ILR, also called the DC/AC ratio, is commonly calculated by dividing the total STC rating of the panels (in DC watts) by the total continuous output rating of the inverter(s) (in AC watts). In your case 10,920 divided by 9000 = 1.21 which is a very good number. Your system should provide excellent output, for many years, in a wide range of environmental conditions.

[SolTex]

When I see people describe roof top systems, has always been by amount of panels, and rarely the inverter size is included.

Which is exactly the problem with how people are "sold" on solar systems. Either deliberately, or because of ignorance, the salesman touts the STC rating in DC watts when selling the system. Later, when the customer learns that their costly system will actually only output (at best) the max continuous AC rating of the inverter(s) then they are understandably angry/disappointed/frustrated.

[chrisski]

I have an Rv and I would say a 10.8 kw of panels and 9 kw of inverters. When I see people describe roof top systems, has always been by amount of panels, and rarely the inverter size is included.

[alfie1111]

I was just wanting to clarify that, I was pretty sure what the answere would be, I have a 10800w system that I was sold at $2.75 per watt, so $29,700 for the system. It consist of 24 Longi 455w panels and 6 APsystem QS1A inverters rated at 1500w continuis(I have seen 1596w max) .I figure that by using inverter calculations I was sold a 9k system. Correct me if i'm wrong

[jflorey2]

I have 4x455w panels but my inverters only put out 1500w is this a 1500w system or a 1720w system?

First off, 455W is STC DC which you will almost never get. It means 100% full sun (~1000 watts/sq m) directly on the panel when the panel is 25C and perfectly clean. That almost never happens. In general a panel will give you 70% to 80% of its rating in full sun depending mainly on temperature.

Second your inverter cannot output more than it is rated for. If you have those 455 watt panels actually giving their STC rating (which again is rare) you will see 1820 watts - but the inverter will limit at ~1500 watts. (Assuming this is a grid tie inverter; if you have a charge controller going into a battery slightly different rules apply.)