Solar design

[Isetech]

Thanks to Eskom, it looks like we are already off to a bumper new year.

If we are already on stage 3 load shedding and power outages caused by faulty infrastructure are lasting 12-24hrs, and the large companies are still on holiday. It looks like we are in for a dismal start to 2024. Not all doom and gloom for some of us, the calls have already started. Yesterday we started our first new project for the year, and the calls are coming in by the day.

The good news for customers, the prices for new equipment hasn't looked better since we started doing installs in 2022. An example, we selling lithium batteries R5-6000 cheaper than we were in 2022. Panels prices are the lowest I have ever seen them.

[Isetech]

So lets move onto the thread heading, design.

For most the budget controls the size of the system design, not the required power per installation, it is just a reality. We would all love to have a 16 kw system with a bunch of batteries and panels on the roof.

I get the most requests for small 3.5 kw systems, which we current dont take on, one day if work quiets down we might.

We install the odd 5 kw with a 5 kwh battery and a couple of panels, but the most common request is for 8 kw inverters, they pretty much take care of the bulk of power required during load shedding.

To give you a very simple breakdown of what to expect.

5 kw inverter (for now we only install Sunsynk) 5 kwh battery and a max of 14 x 460 watt panels (you can use 550 watt, if you shuffle the design a bit)

A 5 kw inverter will give you a max of 22 amps on the essential side of the DB. Pass through current, distance and heat must be taken into consideration when doing the calculations. We see too many people suing 2.5 mm twin or 2.5 mmsq 4 core surfix. The cable will get hot and because most people think installing systems in the garage is required, you have these long runs through the hot ceiling space, in conduit under ground etc.

The more power being used during load shedding the faster the battery discharge, a quick calculation would give you an idea of the discharge time ( I am not going to go into that today)

When stage 6 load shedding kicked in, most people just wanted a 2 hour backup solution, so an inverter and battery was requested, then they got the bill and realized the additional cost on the bill at the end of the month. People started using a basic kit, 5 kw, with a 5 kwh battery and 6 460 watt panels.

However, people realized that they can not only charge the battery, but if the system was designed properly, they could use the power to feedback in to the non essential part of the DB and even back into the grid.

Now people want more panels, so they have increased the capacity to 14 panels.

Then winter arrived and noticed the capacity dropped significantly, so a few more panels were added to the installation.

[Isetech]

The 5 kw inverter (6500 watt power input) is a very basic and easy design, with 2 MPPT trackers which can connect 2 strings (1+1)

Then we move up to the 8 kw.

The 8 kw (10400 watt max Dc input) gets alight more difficult, because there are also 2 MPPT trackers, however they can connect 2+2 strings. This is where some people get a little confused.

Lets look at how you would decide on the size and amount of panels.

If you have a max of 10400W max Dc input power and you use 550 watt panels which is most commonly used, then you would take the max power 10400 and divide it by 550 which would give you around 18 modules.

Some people use lower wattage panels, lime 360 watt which would give mean you could install at least 28 modules. Some people add more to compensate for winter, I have seen as many as 32 panels installed.

The power is not that important, why, because the inverter will automatically cap the power to 10400W, or you can reduce the cap as required.

What is important, the VOC and the current rating of the panel.

You have to look at the VOC of the panel and inverter.

Lets look at a JA solar panel 550 watt panel with a VOC of 49.9 V and current (imp) of 13.11

10400/550=18 modules

If you create 2 strings of 9 panels

550*9=4950W each string
9modules*49.9VOC = 449V (within the 500 V range)
2strings*13.1amps(imp)=26.1amps (at the limit but should be ok) (PV input current 26Adc per string)

Or you could create 4 strings, because the MPPT can take 2+2

What you could do is add a 2 more panels and split the 20 panels into 4 strings, made up of 5 modules per string.

550*5=2700W x 4 strings = 11000W (which the inverter would cap or you could reduce in summer and increase in winter)
5modules*49.9VOC=249.5VOC (ideal would be 370V, but 249.5 is above the startup voltage 125V and within the 500V range)
4 strings*13.1amps(imp)=26.1 amps

When I started designing these systems, this got me thinking, I wasnt sure if the VOC was per MPPT or per string.

[Isetech]

If you look at a 12kw 3 phase inverter, it would get a little more confusing if you dont understand how the MPPT and strings work.

12kw 3 phase inverter has a max DC input of 15600W

160-800VDC range
160VDC startup

MPPT x 2
but not 1+1 or 2+2 , instead it has a 2+1

What does that mean if you use 550 watt panels

15600W/550=28 modules
You can spit it into 3 strings made up of 2+1
2strings of 9modules each + 1 string with 10modules
Current would be 26A+13A, so we know a 550 watt module would be fine because they have a current rating of 13.1 amps (13+13)+13

What you would need to be aware of is the startup voltage, which is a little higher @160V

Because the voltage range is 160-800V, you could put as many as 16 panels on 1 string 49.9VOC*16modules=798.4VOC (it would be at the limit, so I wouldnt recommend doing it)

IF use 360 watt panels, you could install 43 panels (14 per string)

14modules*49.9VOC=698.6VOC

Something to consider, why would you install more lower wattage panels (the price is per watt) over higher wattage panels, less equipment and less labour cost