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There is no doubt that solar energy is the answer to South Africa's energy crisis and a means for the home and business owner to keep the lights on. BUT beware, there are a few very important points to remember.


In this article we will briefly  explain as plain and simple possible how solar works, how the system should be designed or specked and what to watch out for.
Solar power is not cheap. Attempting to cut corners or installing the wrong system will cost you much more in the end.

 

NB! The examples, prices, installation, household appliance energy consumption and battery life cycles used in this example are based on averages. The actual usage or costs can be much higher or lower depending on a very wide field of factors including but not limited to, Quality, Manufactures, House Hold Appliance Energy usage ratings, Installation difficulty factors.. In this example it is also assumed that this home has a 30° tiled roof span of minimum 24 meters which faces true North.

A basic off grid solar system consists of Solar panels, Combiner Box, Charge Controller, Inverter and Batteries. It is a rather simple process where the sun energizes a set (string) of panels. The panels are tied together with a combiner box that distributes the power to the Charge Controller. The Charge Controller charges the batteries and the inverter gets power from the batteries. The inverter changes the power to the standard 240VAC that we use in our homes.

 

"OFF-GRID" Now there's a term we hear often, but just how feasible is an off-grid solution? How much will it cost and is it ever truly off-grid?. Yes off-grid is possible but one needs a tremendous amount of money or make some serious life style changes if you want to go "off-grid". One cannot run, electrical ovens, stoves, hot plates, geysers or any other high load household items. Changing these to gas or any alternative will bring the total "off-grid" cost down quite significantly. The term solar assisted should rather be used. Here we use as much possible solar energy and revert back to the grid when needed. To follow is an example of a very moderate "Off-Grid" solution for the low energy consumer. In reality it is still a solar assisted system for the normal home whereby most energy is generated from solar.

 

First we need to calculate our total power requirements:


I) Take an Eskom electricity meter reading every morning at 09:00 and again at 16:00. Do this for a whole week (7 days).

Day                   -         09:00          -         16:00         -      TOTAL  

Tuesday           -        472671       -        472678       -      Day = 7KW  / Night = 9KW

Wednesdays   -        472687       -        472692       -      Day = 5KW  / Night = 12KW

Thursday          -        472705       -        472711       -      Day = 6KW  / Night = 10KW

Friday                -        472721       -        472727       -      Day = 6KW  / Night = 10KW

Saturday           -        472737       -        472748       -      Day = 11Kw / Night = 11KW

Sunday             -        472759       -        472769       -      Day = 10KW / Night = 10KW

Monday            -        472779       -        472785       -       Day = 6KW  / Night = 9KW

Tuesday           -        472794

Clearly the home in the example above does not have a full time domestic worker as power consumption is higher at night than during the day. In homes with full time domestic workers the day and night usage will usually be inverted.

 

II) Calculate your needs:

Day    = 51KW ÷ 7 = 7.2KW average per day

Night  = 71KW ÷ 7 = 10.1KW average per night

As a rule of thumb we do NOT use the average  power consumption but rather the highest  day time and night time figures. In the above example we will be using Day=11KW and Night=12KW. This is our benchmark that we start our calculations from.

 

III)Locate the most power hungry devices in your home and calculate the time of day they are used and how many of them are used at the same time. The idea here is not to calculate the total power consumed but rather to estimate the highest power load at any given time.

Equipment                      -     KW/H     -       Day/Night    

Kettle / Coffee Machine  -     3KW      -         Day/Night  

Clothes Iron                   -     1.5KW   -              Day        

Clothes Washer            -     2KW      -              Day          

Tumble Dryer                -     3KW      -              Day          

Dish Washer                 -     2KW      -               Day          

Hair Dryer                      -    1KW       -          Day/Night    
Electric Lawnmower   -     2.5KW   -               Day          

from the example above we will choose a combination of devices that consume the most power when turned on together. During the day we may use the Kettle, Clothes Iron and Hair Dryer together. At night we may use the kettle and dishwasher together.

Day =  3KW(Kettle) + 1.5KW(Iron) + .8KW(Hair Dryer) = 5.3KW - This equates to the maximum power consumed at any one point during day time.

Night =  3KW(Kettle) + 2KW(Dish Washer) = 5KW                            - This equates to the maximum power consumed at any one point during night time.

 

IV) Calculate how much energy needs to be generated and how much needs to be stored in batteries for night time.

DAY - We need a minimum of 11KW (total from point II) from 09:00 - 16:00 per day BUT we also need to generate at a minimum 5.3KW (Total from point III) at any given time. This adds to the amount of solar panels we need.

NIGHT - We need a minimum of 12KW (Total from point II) usable stored energy per night and we need to supply a minimum of 5KW (total from point III) at any one time during the evening. This adds to the total battery power needed. Our batteries will need to be able to handle a 5KW drain.

 

V) Batteries:

We know we need 12KW/H usable energy  per night and that our batteries need to handle a 5KW current drain. THIS IS WHERE THE TRICKY PARTS STARTS AND WHERE THE INCORRECT CALCULATIONS OR LACK OF KNWOLEDGE WILL COST YOU HUNDREDS OF THOUSANDS OF RANDS.

There are mainly two categories of batteries:

Flooded cells - Lead Acid, AGM, Gel, etc

Lithium - Lithium Ion (is what we will deal with)

Batteries cannot be completely drained, doing so will damage them and shorten their life cycle drastically. For the exact specifications please consult the battery's data sheet. In general high quality lead acid batteries have a 12 Year design life at 20% DOD (80% remaining) if kept at around 25° Celsius. This means that only 20% of the battery's total capacity should be used per day to maintain a 12 year life span. Using more than 20% will shorten the life span. Eg. If the battery is discharged to 80% DOD (80% used) then the battery will only last 2 years. Lithium Ion is more robust and can handle 70% DOD (30% remaining) per day during a 20 year life cycle and are not heavily influenced by minor temperature changes. They will perform without any adverse effects from 15 - 35° Celsius.

We know we need at least 12KW/H usable energy from our batteries per day. Now it's time to work out which batteries an how many.

Flooded Cell Batteries 

60KW/H - 20% = 12KW/H (we need 60KW/H batteries). 600000 (60KW/H) ÷ 48 (battery voltage) = 1250AH batteries. this means we will need.

24 x 12V/200AH = 4800A/H (1200A/H @ 48V) or 57.6KW/H

The average cost of a midrange good quality 12V/200A/H deep cycle battery is around R4,500.00 - R7,000.00 each. At R6,000.00 each we can expect to pay R144,000.00 for the batteries alone. If used as per the specifications above, this battery set can last between 12 and 20 years depending on the battery quality.

Lithium ion batteries

20KW/H - 70% = 14KW/H (We need a 20KW/H Lithium Ion battery)

The average cost of a medium to decent quality 20KW/h Lithium Ion battery is around R140,000.00. If used as per the specs above then this battery bank will last about 20 years. Lithium would be the better choice as it lasts longer, can handle much higher discharge rates, is much more compact and weigh much less than ordinary flooded cell batteries.

 

VI) Inverter:

This is the easy part. We know from point IV that the maximum energy that we will be consuming at any given point is 5.3KW. We need to choose an inverter  that will cover this and a little more. Inverters above 5KW usually come in 8KW (R29,000) and 10KW (R35,000). Since the cost difference between 8KW & 10KW is minimal we will select the 10KW at around R35,000.00. It is very important to purchase a decent quality inverter (pure sign wave) with local support in South Africa. Chinese makes are discontinued at an alarming rate. By the time your chinese inverter's guarantee has expired you will no longer find parts for them.

 

VII) Solar Panels:

We know from point II that we need 11KW/H per day and from point III that we need to supply 5KW at any given time during the day. But we must also take into account the amount of energy needed to charge our battery bank. From point V we know that we may need 14KW/H in total to replenish our batteries during the day. This brings our total energy needed from our panels per day to 25KW/H and must be able to deliver 5KW at any given time (Please note that the batteries will also sustain some of the energy drain during the day when high loads are used). We will use 300W panels in this example. Please remember that a 300W panel will only produce close to 300W from 11:00 - 13:00 if they are perfectly aligned to true North at about 30° angle and in ideal sunny conditions. In this example we will assume that they are perfectly aligned with clear skies.

20 x 300W = 6KW - from 11:00 - 13:00 we wail generate 12KW/H

20 x 200W = 4KW - from 09:00 - 11:00 we will generate 8KW/H

20 x 200W = 4KW - from 13:00 - 15:00 we will generate 8KW/H

20 x 100W = 2KW - from 08:00 - 09:00 we wail generate 2KW/H

20x 100W = 2KW - from 15:00 - 16:00 we will generate 2KW/H

With 20 x 300W panels we can generate around 32KW/H per day during clear skies if the panels are perfectly aligned. 5.5KW Can also easily be used at any given time (as needed in point III). Since we combine panels in strings of 3 we will be using 24 x 300W panels at around R2,000.00 each. Total R48,000.00. NB! Unless you own a very long home with at least 24 meters roof span that faces true North, it won't be possible to fit 24 panels on a single roof. In this case your labour, electrical and panel bracket costs may rise significantly.

 

VIII) Solar Charge Controllers:

Now that we know how many panels we will be installing we need to calculate the amount of charge controllers we need. We use MPPT controllers as they can extract a significantly higher amount from a solar panel than normal charge controllers.

2 x 60A MPPT 48V = 48V x 60A = 3000 / 3KW/H x 2 = 6KW/H Total.

The cost of a 60A MPPT charge controller is around R6,500.00 each. Total R13,000.00.

 

IX) Combiner Boxes:

From point VII we know that we have 24 panels and from point VIII we know that they will be split into 2 groups. To save on installation and wiring costs we tie the panels in series strings of 3 each. So we will use 2 x 4 string combiner boxes with lightning protection at a cost of around R3,500.00 each. total R7,000.00

 

X) Extras:

The extras, as with all things in life is actually much more than one would expect. Aluminum solar panel tiled roof brackets for 6 panels is around R7,000.00. Total R28,000.00 for 24 panels. Tin roof brackets are cheaper. Then we have cabling and cable management systems, battery enclosures, extra DB boards, change over switches and the labour. Installing solar is a tremendous amount of work. A system as in this example will take between 5 and 12 days to complete.

 

XI) Actual ESTIMATED Costs:

Batteries - R140,000.00

Inverter - R35,000.00

Solar Panels - R48,000.00

MPPT Charge Controller - R13,000.00

Roof brackets - R28,000.00

Electrical - R10,000.00

Extras - R5,000.00

Labour - R40,000.00

TOTAL COST - AROUND R319,000.00. THIS DOES NOT INCLUDE HIGH LOAD EQUIPMENT LIKE GEYSERS OR OVENS. As a rule of thumb the above should be multiplied by 3 to qualify as a true Off-Grid solution. Alternatively a complete lifestyle change should be considered when it comes to electricity usage.

 

NB!! PLEASE NOTE THAT SHOULD THERE BE THUNDERSHOWERS FOR A WEEK OR EVEN TWO DAYS THEN THE SYSTEM IN THE ABOVE EXAMPLE WILL NOT GENERATE SUFICIENT ENERGY TO POWER THE HOUSE FOR THAT TIME.

 

In reality there are numerous options to choose from, please contact us for a custom tailor designed system that will suite you home's needs.