Solar Array swa sizing

[pc1966]

I may have really over-estimated the average solar power, but equally I have probably underestimated future electricity costs as well but a large amount!

 

[marconi]

I may have really over-estimated the average solar power, but equally I have probably underestimated future electricity costs as well but a large amount!

There are on line estimating tools which can produce annual and month by month figures for pv generation.

I use this one:

PVGIS (PV-GIS) - https://photovoltaic-software.com/pv-softwares-calculators/online-free-photovoltaic-software/pvgis

 

[pc1966]

There are on line estimating tools which can produce annual and month by month figures for pv generation.

I have seen some tools such as that and they offer you the expected annual power. But the problem here is the I2R losses depend on the current squared, so the shape of the illumination waveform has an impact on the relationship between average solar generation and average cable loss.

E.g. if we get 10% of the panel capacity on average (roughly what I just saw on one site for south England) that could be:

100% generation for 10% of the time, so I2 term is 0.1 * (17.4)^2 = 302.7
50% generation for 20% of the time, so I2 term is 0.2 * (17.4 / 2)^2 = 15.1
25% generation for 40% of the time, so I2 term is 0.4 * (17.4 / 4)^2 = 7.6

Due to Earth's rotation the average daylight per year is 50% so 10% panel capacity averaged per year really means 20% average illumination per day, so probably 30-40% sunny cloud-free time and allowing for sine-shaped illumination arc over the day, etc. (but of course that varies summer/winter, and so on).

Above I had assumed 100% time at 1/4 power = 18.9 for I2 term so loss cost maybe high by 20-50% or something.

 

[pc1966]

I think the TL;DR answer is a 1% VD goal is probably close to the most economic cable choice!

 

[marconi]

The trick of course is to connect the panels to achieve the highest off load pv array voltage - to keep current lower for a given power- while being mindful of the inverter input voltage limit and staying under the Uo/U voltage spec of swa cable noting that for example 600/1000V are ac rms figures so higher figures for dc by a factor of sqrt2 Thus 850/1400V dc.

Uo voltage between live conductor and earth.
U voltage between live conductors.

In practice, peak panel power output is normally at a voltage whigh is between 70 and 80 % of the no current open circuit voltage not at 50% I mentioned in my opening post. I tend to do pessimistic calculations so it performs better than expected rather than worse. Real panel voltage/current/irradiation curves tend to allow thinner rather than thicker cables cos the IsquaredR energy loss is lower Than the 50% assumption would calculate.

 

[marconi]

PMP = Maximum power point.