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How to size a PV array


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Hi all

 

Based on the very helpful advice on this forum, I have decided on a system. I will be getting a Sunsynk 5kW hybrid inverter and I have already bought a PylonTech US3000C on a Black Friday special.  I am happy that this part of the system is adequately spec'd for my needs.

 

How do I size the PV array? I cannot seem to find much guidance on this, other than get 5kW of panels to match the 5kW inverter.  Is this all there is to it? Does it need to be "overspec'd" to cater for weather/seasons? I don't want to to go overboard and then have too much surplus that I cannot use, spending more than required. Under spec'ing results in reduced potential savings. How does one find the balance? 

 

I am in Pretoria. My roof is 45 degrees, facing NNW. Shade is not a problem at all, at any time of the day.

 

Any advice on how to go about sizing the array?

 

Cheers

Craig

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Basic random thoughts that can hopefully get you started. Start with assessing your electricity needs. How many units (kWh) do you use per day, throughout the week or over a month? How much in winter vs summer? At what times of the day? If you don't have it measured, track back through the last year's bills. How is it likely to change in the future? Can you optimise any household consumption before buying panels?

Consider the aesthetics of upgrading with newer panels in the future that may not match the look, specs, size or age of what you're getting now.

Will you be going off-grid, in which case more is better.  Will you be grid-tied, in which case, don't exceed municipal requirements for your home's supply.

Dark cloudy days can reduce output by 80% or more. Can't really put up enough panels to compensate for that.

Read the manual to confirm the maximum power & voltage limits your inverter can handle and do not exceed it. Consider the maximum input on each MPPT individually and consider a panel size that will divide evenly into that number. Assuming the panel price is within reason. 

Your 45 degree roof angle means you're more or less optimised to generate maximum power in Winter, but will sacrifice summertime generation. Try to find an online solar calculator to tell you by how much, then get more panels to compensate if needed. 

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Thanks for your response Vassen.

 

Thanks for the max PV size the inverter can handle. I suppose that is the ultimate (but costly) objective.

 

Objective of system: Cost reduction, minimise electricity bill.  I do not want to go off grid, I am happy to use Eskom to supplement.

The wife wanted loadshedding backup, which is why I got the battery. I am undecided on whether to cycle the battery daily.  It's just enough (or maybe just short) to get us through the night on essentials only. We'll get another battery when funds allow, short term objective was to keep lights on for loadshedding.

Peak loads, unmanaged, isn't more than 7kW. Generally we are well under 5kW. I am looking into the deep hole of home automation to explore ways of moving peak load to peak PV generation time. Peak loads include geyser, pool pump and dishwasher, all can be programmed to run when optimal. The wife is open to managing peak loads when the system is installed (check the monitor before putting the kettle on kind of thing). Idle essential load is <250W.

Daily average consumption is in the region of 20kWh. Likely to start increasing as the baby gets older.

I have the analogue meter 🙂 Not sure if I can count on spinning it backwards though, I am not sure on the technicalities/legalities on doing that in Pretoria. I am open to it, if its possible.

I am in agreement in principle to start off and add more if necessary. But it adds to the cost to have to add more later (extra delivery, installation etc.).

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10 minutes ago, GreenFields said:

Basic random thoughts that can hopefully get you started. Start with assessing your electricity needs. How many units (kWh) do you use per day, throughout the week or over a month? How much in winter vs summer? At what times of the day? If you don't have it measured, track back through the last year's bills. How is it likely to change in the future? Can you optimise any household consumption before buying panels?

Consider the aesthetics of upgrading with newer panels in the future that may not match the look, specs, size or age of what you're getting now.

Will you be going off-grid, in which case more is better.  Will you be grid-tied, in which case, don't exceed municipal requirements for your home's supply.

Dark cloudy days can reduce output by 80% or more. Can't really put up enough panels to compensate for that.

Read the manual to confirm the maximum power & voltage limits your inverter can handle and do not exceed it. Consider the maximum input on each MPPT individually and consider a panel size that will divide evenly into that number. Assuming the panel price is within reason. 

Your 45 degree roof angle means you're more or less optimised to generate maximum power in Winter, but will sacrifice summertime generation. Try to find an online solar calculator to tell you by how much, then get more panels to compensate if needed. 

Thanks GreenFields, good points.

 

Refer previous post for my consumption and peak stats.

Consumption is optimised as far as the wife will allow 🙂 Short of putting motion sensor lights in the bathroom, I think I have done all I can. One additional thing I will consider is to change the geyser element from 3kW to 2kW, if necessary. Pool could maybe run a little shorter.

The system will be grid-tied. Where can I find the municipal maximum supply requirements? Could it be more than 6.5kW, which is the max the inverter can take?

I found the following calculator https://solecsa.co.za/sizing-tool/  With my data, it suggests a 5kW array. I am open to looking at any other calculators you can recommend.

The next big question will be, what size panels 😂

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36 minutes ago, Craigm said:

a 5kW array

This is a decent sized array, not too big not too small. This would serve your needs. You could also add to it later if need be.

 

37 minutes ago, Craigm said:

what size panels

The most cost effective size is around 335w, but if you plan to install them yourself beware, the 72 cell panels are a little heavy. They also tend to catch a lot of wind when you try to install them. Somehow whenever I install panels, the wind magically starts to blow harder than usual... 😂 60 cell panels are a little lighter and catch a little less wind, but you will need to install a few more of them. 

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You can work on the following rule-of-thumb.

Your PV will generate between 4-6 times your PV installed over the year, usually 4 in Winter and 6 in Summer. So you're talking 20kWh to 30kWh max on a cloud free, cool day.

With your pitch of roof, you will probably get very good generation in Winter and not to good in summer.

Remember that the 20kWh is produced from dawn to dusk, so you will not get 5kWx4hrs, you will get a normal bell curve with max generation between 11am - 2 pm in Winter. In summer from 10am to 3pm...See my PV generation profile. Clouds and rain etc. will give you the spikes, tree shade will eat into it as well.

This production is with 6kW installed PV, where only 4.8kW is generated and then if there is additional load or battery needs charging it will go to 6kW briefly.

 

Capture1.PNG

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11 hours ago, FixAMess said:

You can work on the following rule-of-thumb.

Your PV will generate between 4-6 times your PV installed over the year, usually 4 in Winter and 6 in Summer. So you're talking 20kWh to 30kWh max on a cloud free, cool day.

With your pitch of roof, you will probably get very good generation in Winter and not to good in summer.

Remember that the 20kWh is produced from dawn to dusk, so you will not get 5kWx4hrs, you will get a normal bell curve with max generation between 11am - 2 pm in Winter. In summer from 10am to 3pm...See my PV generation profile. Clouds and rain etc. will give you the spikes, tree shade will eat into it as well.

This production is with 6kW installed PV, where only 4.8kW is generated and then if there is additional load or battery needs charging it will go to 6kW briefly.

 

Capture1.PNG

Thank you FixAMess, this is excellent information! 

I appreciate your efforts.

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13 hours ago, Vassen said:

If you run out of hot water, then for me it’s better to install a second geyser inline and use the pv power to heat the water and store it for use after hours. 

 

If you can, I suggest you get a cheaper energy meter like a Ellie’s efergy. It’s not super accurate but will give you a fairly good idea of your loads at different times of day / night. This will help you plan better and also help you to be more efficient. 

I agree on the hot water - I will look into this in future. Same story with the stove and moving to gas.

 

I have an efergy metre - it has been great to be able to analyse my data.  In my previous place, I analysed my geyser consumption when considering a solar geyser. By simulating the scenarios, I was able to determine that I would save approximately 30% just with a geyserwise, which meant that that solar geyser capital would take a lot longer to pay off  than everyone promises. At the end of the day, I saved that 30% anticipated and the geyserwise paid itself off in 3 months! Well worth the investment and effort. Data is gold!

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