Oversizing the system

[solarix]

If your POCO requires you to be on a time-of-use rate plan, you may need to over produce in order to compensate. In our area, we have to design for 130% to 140% of actual usage in order to break even on the billing. Plus almost everyone uses more power as time goes on - all I ever hear is "more power, more power". Plus, the solar array does degrade slightly over time, although current stats are proving it to be less than 1/2% per year. So with solar PV panels being so inexpensive now, go as big on the array as you find affordable. Then put in an inverter that is as big as the largest array your roof can hold. The next person with a large family that lives in your home will love you. There is very little difference in price between the smallest and largest inverters. An oversize inverter will last longer than a smaller one operating at its max. Anymore, we almost exclusively install 7.7kW inverters (max size for a 200Amp service) for everyone.

[Sami Najjar]

I will, thank you very much

[peakbagger]

I agree, dont waste your money until you have run PV Watts. Its easy to use and gives conservative numbers as long as you are accurate in taking into account local conditions like shade from landscaping and shadows from the structure.

[bob-n]

Sami, If you have a few minutes, you may want to try running PVWatts and see how it compares with 1580kWh/kW. It is an on-line tool with weather data for the whole world. You can use it with all defaults or include as many details as you like. For example, if your panels are facing south-east instead of south, it accounts for that. Also, if your panels are facing upwards at 30 degrees or 25 degrees, it accounts for that.

[Sami Najjar]

Thank you very much Bob. And for the 1580kWh/kW, it is the Peak Sun Hours multiplied by the losses factor of 0.8 and by 365 days (for annual calculation), this is how it is done here in my region.

[bob-n]

Sami, Welcome to the forum.

I'm not sure where the 1580 kWh/kW rate came from. Perhaps it is a specific model for your region and installation. If you want to calculate the correct ratio for your specific situation, many of us like a web tool called PVWatts. It does much better than a "rule of thumb".

Without knowing more, I would assume that the ratio is based on inverter AC output, but that's a guess.

Assuming that you want one string inverter, you can them it in various sizes. For example, SMA makes the Sunny Boy inverter in 3kW, 3.8kW, 5kW, 6kW, 7kW, and 7.7kW. Another brand, Fronius, makes the Primo in 3.8kW, 5kW, 6kW, 7.6kW, 8.2kW 10kW and even larger sizes.

An alternative is to put one inverter on each panel. These are called microinverters. That way, you can expand at any time, just by adding another panel and another microinverter. For the same power level, microinverters can be more expensive than the SMA inverter, but not by much. Some people like microinverters for their ability to work well when the panels get partial shade.

As for the size inverter to buy, we use a ratio of somewhere around 90%. If you are using 16 400-watt panels, you will get 6400 watts peak, so a 5700 watt inverter is close to optimum. A larger inverter will work well, but cost more money. A smaller inverter will put out significantly less, so you will be wasting panel capacity.

Although the inverter may seem undersized, it will only result in less-than-peak power for a short period of time around noon. The rest of the time, the panels will produce less than rated power and the inverter will handle it fine. That slight loss of power will be ~10% for a few minutes, so really just ~1% of total production. We consider this optimum because you are saving more than 1% of the installation cost.

One last option you may want to consider is QS1 quad microinverters from a company called APsystems. These have the cost advantages of string inverters and the flexibility and shading advantages of single microinverters. They save money by putting four independent inverters in one box. I have no vested interest in this product, but have used it successfully at my home.

Some people on this discussion group don't like microinverters because they require placing the electronics on the hot roof. String inverters are normally installed on the ground, where it is cooler, but may require optimizers or other electronic devices on the roof, so you don't avoid the heat problem either way.

I mentioned a few products here. If any interest you, use a search engine to find the manufacturer's website and learn more about them. Each has advantages and...

[Sami Najjar]

Hi, I'm trying to size an on-grid PV system, I have an electricity bill of 9000 kWh/yr and the annual specific solar yield is 1580 kWh/kW. I have a couple of questions.
1) By dividing the two values I will obtain a 5.7kW value, does this number refer to the DC power of the panels or the output AC power I need to cover the electricity bill?
2) I'm considering oversizing the system by 20%, there is an inverter with 5.7kW, so is it better to oversize the system up to 6.8kW DC power and use this inverter?or use a 4.8kW inverter and keep the DC power at 5.7kW?