THE OFF GRID SOLUTION
Most people interested in solar usually would like an "Off-Grid" solution. "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
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.
These totals we can calculate the size of the inverter and the maximum drain that the batteries need to withstand.
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.
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 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 12-20 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 12-20 year life span. Using more than 20% will shorten the life span. Eg. If the battery is discharged to 80% DOD (20% remaining) 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 could last up to 20 years. If these batteries are badly maintained and drained to much each day, they may last as little as three 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.
NB!!! ONE CANNOT USE 3KW BATTERIES WITH A 5KW INVERTER (AS ADVERTISED BY INSTALLERS ON FACEBOOK). AT THIS TIME THERE IS NOT A SINGLE 3KW BATTERY BANK ON THE MARKET THAT CAN HANDLE A CONSTANT DRAIN OF 5KW. GENERAL RULE OF THUMB - BATTERY SIZE SHOULD BE DOUBLE INVERTER SIZE.
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 may possibly generate 12KW/H
20 x 200W = 4KW - from 09:00 - 11:00 we may possibly generate 8KW/H
20 x 200W = 4KW - from 13:00 - 15:00 we may possibly generate 8KW/H
20 x 100W = 2KW - from 08:00 - 09:00 we may possibly generate 2KW/H
20x 100W = 2KW - from 15:00 - 16:00 we may possibly 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 (x3 to compensate for overcast days and maximum battery life) R420,000.00
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
MINIMUM (THEORETICAL) COST - AROUND R319,000.00
MINIMUM (PRACTICAL) COST - AROUND R519,000.00
THIS DOES NOT INCLUDE HIGH LOAD EQUIPMENT LIKE GEYSERS OR OVENS. As a rule of thumb the above (especially batteries) 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 "THEORETICAL" SYSTEM IN THE ABOVE EXAMPLE WILL NOT GENERATE SUFICIENT ENERGY TO POWER THE HOME DURING THAT TIME.
In reality there are numerous options, much cheaper and smaller, to choose from, please contact us for a custom designed system that will suite your home's needs.