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Well, I'd agree that you need to take into account all the relevant design goals and site conditions including the use of process economics when designing the system.
For the Saraha, those might include, but certainly not be limited to cell operating temps. as f(ambient air temps., wind vectors, irradiance). Most any decent design models will include such relevant design parameters as required model inputs.
Then, if you want, and after all the design iterations are complete and a design is pretty well firmed up taking all chosen, relevant design variables into account, if you want a DC/AC ratio, divide the chosen inverter size(s) by the STC rating of the array for all the good it will do.
The DC/AC ratio is an output of the design process, not an input. It comes out as a result of the design process.
IMO, it's unfortunate that PVWatts uses a DC/AC ratio at all rather than simply have the inverter size as an input, but it is what it is.
Most of my design work now uses SAM but if you're using PVWatts:
Get a theoretical modeled array size based on the annual design duty in kWh/yr. (including any intentional oversizing for whatever reason).
Then, get a practical modeled array size based on available module STC sizes and stringing requirements that's slightly larger than the theoretical array size and use an inverter that's the same wattage size as the module sized array.
Then, iterate the practical modeled array by increasing the DC/AC ratio (that is, by decreasing the inverter size) until the modeled system output matches the required design output.
Then, since inverters come in stepped sizes, bump the inverter size upward to the next available size.
Doing it this way, you'll probably clip some but"
- You'll meet the required design duty.
- And,have a good chance of getting the smallest array that'll meet the required duty.
- And, the smallest inverter which will probably be smaller than the array STC size.
- And, the combination will also most likely have the highest probability of being the most cost effective for the duty and the type/manufacturer chosen.
Then, you can divide the array STC size by the inverter electrical size and get a DC/AC ratio if you want, but at that point, it'll be academic, like who cares ?
For the Saraha, those might include, but certainly not be limited to cell operating temps. as f(ambient air temps., wind vectors, irradiance). Most any decent design models will include such relevant design parameters as required model inputs.
Then, if you want, and after all the design iterations are complete and a design is pretty well firmed up taking all chosen, relevant design variables into account, if you want a DC/AC ratio, divide the chosen inverter size(s) by the STC rating of the array for all the good it will do.
The DC/AC ratio is an output of the design process, not an input. It comes out as a result of the design process.
IMO, it's unfortunate that PVWatts uses a DC/AC ratio at all rather than simply have the inverter size as an input, but it is what it is.
Most of my design work now uses SAM but if you're using PVWatts:
Get a theoretical modeled array size based on the annual design duty in kWh/yr. (including any intentional oversizing for whatever reason).
Then, get a practical modeled array size based on available module STC sizes and stringing requirements that's slightly larger than the theoretical array size and use an inverter that's the same wattage size as the module sized array.
Then, iterate the practical modeled array by increasing the DC/AC ratio (that is, by decreasing the inverter size) until the modeled system output matches the required design output.
Then, since inverters come in stepped sizes, bump the inverter size upward to the next available size.
Doing it this way, you'll probably clip some but"
- You'll meet the required design duty.
- And,have a good chance of getting the smallest array that'll meet the required duty.
- And, the smallest inverter which will probably be smaller than the array STC size.
- And, the combination will also most likely have the highest probability of being the most cost effective for the duty and the type/manufacturer chosen.
Then, you can divide the array STC size by the inverter electrical size and get a DC/AC ratio if you want, but at that point, it'll be academic, like who cares ?
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