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Posts: 1
Reply with quote  #1 
Hello. My first time here and my first PV installation. I have three questions about inverter sizing. Is this too many?

I get the over and under clocking concept of inverter sizing. By how much I might want to underclock I'm not sure. It makes such a difference which output is used to calculate and which is being used is not made clear in articles I've been reading so far.

1. Shouldn't the NOCT rather than the STC output be used in calculating inverter size?

2. A salesman is pitching me a 7.7kW inverter for a 6.6kW STC, 4.88kW NOCT array. Does that seem right? If I expand it to 7.9 STC, 5.86 NOCT, is that about right?

3. Am I looking at this correctly? Seems to me an overly large inverter penalizes you even more early and late in the day as the discrepancy between inverter input and capacity is greater. As I said, my first installation. Maybe I have this all wrong.

My greatest daytime load is summer A/C in Santa Fe NM, 35.7N.

Thanks so much in advance for any advice!

Rick H Parker

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Posts: 808
Reply with quote  #2 
1. NOCT and STC is used to benchmark Photovoltaic cells. Neither has much to do with inverters.
2. Inverters are sized to meet your peak power requirements. Photovoltaic arrays are sized to meet your energy requirements.
3. Yes you are looking at this all wrong. Inverters do not harvest solar energy, Photovoltaic arrays harvest solar energy.

Will this system have battery storage or is it a net metering "storage".

Research the difference between energy and power.

Rick H Parker
Kansas, USA
Electronics Engineering Technologist

Posts: 4
Reply with quote  #3 
I agree with Rick. 
Looking for special design or custom made solar panel, Hovall can help!!!

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Posts: 2,925
Reply with quote  #4 
Rick is right. First is this a grid-tied or battery-based system? The requirements are different. If grid tied your inverter only needs to handle the output of the array. Battery based inverters have to handle the maximum load.

Solar panels produce more at LOW temperatures, and less in high, so the efficiency loss of the larger inverter in the heat may be less than the losses of an undersized inverter in the cold, when your panels are really cranking.

Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay

Posts: 174
Reply with quote  #5 
Clocking??? What do you mean? You don't change the frequency of an inverter's output!

Presumably you are referring to under or oversizing the output capacity of the inverter. Undersizing is pointless while oversizing is wise, it prevents the inverter having to work at it's theoretical limit, thus generating less heat, and less chance of stressing components & breakdown over time.

Many items, motors for example may have a rated current draw of x, but actually draw 2 or 3 times x on initial start up. Hard on an inverter sized for x. If you can afford it,  I'd size an inverter at least 50% more than the calculated load. It will run cooler and last longer.

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