How to determine solar panels needed

[jitprocurement]

Good day and greetings from South Africa.

This is my very first post so please do not be too hard on me.

I come from the generator business and have some 15 plus years of experience. We currently experience load-shedding in South Africa on almost a daily basis which should continue for the next 3 years. Currently we are not allowed to feed any electricity generated by ourselves back to the grid. It seems that this can change within the next 60 days or so. We have decided to offer something else to customers than generators. In the hope of grid-tie systems to be allowed we want to enter this market. I have read and learned a lot on this forum the past 48 hours. I have read on how to calculate the size of the battery pack here http://www.solarpaneltalk.com/showth...p-Why-Tutorial and also on how to size the inverter vs battery pack here http://www.solarpaneltalk.com/showth...-Size-Tutorial
The next thing I need to learn is the sizing of the solar panels. It is probably straight forward but I want to learn how to do it.

I am off the opinion that to become totally disconnected from the grid, will be even more costly in SA than in the US.
In SA I think that we have 2 possible options when speaking about grid-tie. But, please correct me where I am wrong. this is all very new for me. I am used to measure the current drawn at a peak and size a generator for that. I have realised that with this stuff everything is about Watts. For this example, we are talking about households. We use 230V single phase with 50Hz as the frequency. Please correct me if I am matching the supplies wrong. For this argument both options are connected to the national grid. A customer asked me to quote on a 3000W system.
Option 1:
* No batteries (he does not want batteries now as it is very expensive.)
* 3 000W grid-tie inverter.
* He has a small 2.6kVA generator
Under normal, daily use: Please help me calculate the required panels needed to match a 3 000W grid-tie inverter and at what voltage? I understand that 24V is better than 24V and 36V better than 24V and 48V better than 36V.
When there is load shedding and their time to not have electricity, the 2.6kVA generator is there. I assume this has to be connected through a changeover switch which totally isolates the generator from the solar panels & mains electricity supply?

Option 2:
* 3 000W grid-tie inverter
* Solar Panels calculated as above
* Any other suggestions?

Apologies for all the questions but I trust that this will be very easy for you to get me started with.

For another customer, I need to put a 20A circuit breaker on solar. Will it be pretty much the same?

Thank you very much so long for your help and directions.

Best Regards,
Deon

[Mike90250]

Welcome.

First, you can build a much smaller cost system if you center it around a 12 hour battery bank, and an inverter/charger. Solar is not needed unless you have outages lasting longer than 12 hours.

So most of the "hybrid" inverters (They "pass through" Grid power, can recharge batteries from grid or generator, and Invert) can be configured to Invert, but not to sell back to the grid. Or change the programming and you can sell to the grid. Also will generally require electrical inspection/permit. Mist Hybrids have internal transfer switches, but only up to about 50A ac.

If you add solar, you are adding panels, racks, charge controller, and circuitry protection (fuses/breakers).
In a city, you also get building inspections for the added roof load.

Flooded batteries are the least expensive, but require monthly maintenance. AGM batteries have much less maintenance, and have shorter lifetimes.
Both types may require special construction to provide safety if there is a mishap, or spill. Both do require venting of explosive fumes, even sealed batteries can leak or catch fire (Boeing Dreamliner)

When you add batteries, you will not break even, the electrical sell back cannot offset the replacement expenses of the batteries every 4-10 years.

[Sunking]

You do not need solar to do what you want, just a UPS sized to carry you through outages and use commercial power to recharge the batteries. Much less expensive and way more effective than solar.

Rectifier > Battery > Inverter

[jitprocurement]

Thank you to the both of you for your help. It makes more sense to me now.

What do I for the guy who has a small generator and does not want batteries and insists on using panels?

Can you please help me with the formula on how to calculate on how many panels to power a 3 000W inverter?

Thank you so long.
Deon

[Mike90250]

There is no way to power an inverter for a house, without batteries to stabilize the DC.

Grid tie systems work by pumping all the solar power to the grid, if a cloud comes by, they slow down. If it was connected to your house, the lowering voltage will fry your appliances.

And if the grid fails at night, PV only will not help a bit (or on a cloudy day either)

A Giant UPS with inverter/batteries/charger and recharge when the grid returns, is the way to go, till everyone has one, and the huge surge from recharging zillions of UPS batteries will knock the grid down again !

[inetdog]

Thank you to the both of you for your help. It makes more sense to me now.

What do I for the guy who has a small generator and does not want batteries and insists on using panels?

Can you please help me with the formula on how to calculate on how many panels to power a 3 000W inverter?

Thank you so long.
Deon

1. Solar panels without batteries are not going to do much good at all. Off grid inverters are designed to take power from the stiff voltage and almost unlimited instantaneous current from batteries, not the uncertain and current limited power direct from the panels.
While grid tied systems are designed to work directly from the panels, but require grid power to be present to operate.
2. A 3000W inverter with 24V input would require about 125 amps. If you can find panels with a Vmp of about 24V and a total current output of 140A (divide 140 by Imp to get number of panels) then you would have about the right amount of power. But only near noon on a perfectly clear summer day. But the inverter would not work (see #1)
3. If you go with a battery system, first figure out what your actual load over a 24 hour period, regardless of the size inverter you will be using. Then figure out the maximum wattage that will actually be on at one time, with allowances for surge for motor starting. That will tell you how large a battery bank you need and also how much panel power you will need on a winter day with only a few equivalent sun hours.

For a much better organized version of all of this, with details, just read the sticky threads in the off-grid section.