Solar basics you need to know before you buy…!
Wh = Watt hour – the unit in which power is measured. Units consumed are shown on your electrical bill in kwh. 1000 Wh ( watt hours ) = 1 kWh ( kilowatt hour )
kWp = kilowatt Peak – maximum demand or peak load, this refers to electricity consumed when all items are on at once. This is not how many kW you used in and hour but the peak consumption for that moment.
Important note when calculating off grid Inverter size.
Ah = Amp Hour – battery available autonomy over a certain period of hours, we calculate it back to kWh to size your systems backup time for use.
100Ah x 12v battery = 1 200Wh x 50% D.O.D ( Depth of discharge ) = 600 Wh capability.
Pv = Photovoltaic panels – form part of a solar PV system, the Pv solar panels create DC energy from sunlight.
DC = Direct current – Electrical current mostly found in batteries / Pv panels. DC current is inverted to AC current for use in homes or businesses, both can be calculated in wh / kwh.
AC = Alternating current – Electrical current mostly found in home appliances or electric motors. AC current is used in homes or businesses and are the units you pay for in kwh.
How do Pv panels work?
Pv panels convert solar energy (sun radiation) into an electrical DC current and then a solar Inverter converts this to alternating current (AC), which can then be utilized as kw electricity in your home or business. This is not thermal heating like solar geysers but radiation transfer to electrical current for household or business use.
Pv panels work even when it’s overcast. Clouds do not stop the solar UV rays. Power production from Photovoltaic Solar panels actually work most efficiently in cooler temperatures. Completely overcast rainy days will have an impact but so will hot sunny days.
How many kwh can Pv panels array generate?
Location: The amount of radiation per square meter (kwh) will depend on where the system is installed. This may vary in different countries due to the amounts of annual sunshine & depend upon latitude and other environmental items to be taken into account. The table below gives examples of kwh per square meter generation for 1 kWp of PV panels installed.
Bloemfontein 1 kw Pv Array x 5.5 hours per day = 5.5kwh production x 365 days per year = 2007kwh
Sun hours vs 24 hours day cycle – Bloemfontein
5.5 hours effective sun 09:00-15:00 > Daily Load
18.5 hours in-effective sun 15:00-09:00 > Nightly load (Batteries)
How long do Pv panels last ?
If you keep the panels clean they should last in excess of 30 years. The efficiency of Pv producing power over 20 years will reduce by roughly 1% per year. 20 years – 20% = 80%. Therefore a new 100watt panel – 20% reduction in life cycle = 80 watt output in year 20.
Batteries if maintained will last their full cycles as indicated on the data sheet. When you buy batteries see cycles on depth of discharge on the chart (D.O.D). On 2v flooded lead acid batteries normally 50% D.O.D = 2500-3000 Cycles, that’s 10-12 years. Every battery type differs, don’t let someone mislead you with low battery quotes, “bargains”, it will cost you more in the long run.
PV systems are inherently very low-maintenance;
- Wash the panels every 2-3 months when it’s not hot ( before sun rise / after sunset ), to remove dust or any other dirt.
- Grid tie has no further maintenance.
- Off-Grid – Check battery water levels every month and test batteries every 2nd month. Replace faulty batteries if required.
Sizing and Placement of system?
Pv Systems should be mounted correctly and angled north facing to optimize the amount of sun they receive to maximize efficiency. The optimum generation of electricity will depend on how close to north facing the panels are angled and will vary during season changes. The best average all year round angle for Bloemfontein is 25 – 35 degrees North.
Bloemfontein 1 kw Pv Array x 5.5 hours per day = 5.5kwh production x 365 days per year = 2007kwh (units )
Solar companies can quote you only if they know your kilowatt hour (kWh) consumption as well as your peak load on demand to calculate the right system and Inverter size to suit your requirements.
kWh is on your electricity bill. (units you use )
The units that you use per month. To make the right choice you have to know how many average kwh you use in a 24 hour period. It is best to record 3 mid-winter months as well as a 3 mid-summer months to determine a good average consumption. Total the 6 months kwh and divide it by 6 to get an average per month. Divide the average month kwh by 30 days to get an average kwh usage per day. This should be 90% accurate. If you use 30kwh per day add 15-20% to the system design to be safe.
It’s not a matter of how big your house is… every household differs…
- A house for example consumes 1080 kWh a month, an average of 36 kWh per 24 Hour cycle, average use of 1.5kw per hour.
- The next person may use 800 kWh a month, an average of 27 kWh per 24 Hour cycle, average use of 1.12kw per hour.
- The following person may use 600 kWh a month, an average of 20 kWh per 24 Hour cycle, average use of 0.830kw per hour.
The solar design differs between these three homes…. These homes are exactly the same size in square meter-age but the first house has more equipment on the load than the other two. House size does not have the impact on kwh, the home owner himself does.
To deliver the kWh per 24 Hour period you need to install the correct solar equipment in order to meet the kWh requirements you need. Kwp PV on roof x 6h a day = kwh production a day.
Example: Bloemfontein 6kwp Pv x 5.5 hours per day = 33kwh production per day.
Don’t let someone mislead you with low solar quotes, “bargains”. This will only result in under performing installations… The kWh you need = price you pay = Kwh production a day.
If kwh is not available you can install a temporary data logger to establish the kwh usage.
Operation, specification & parameters:
Important note: Battery / off grid systems – You have to know the capacity of the system and you have to stay in the parameters of that system. Do not overload the system, do not use more kwh per day than what the system is designed for.
Example: Inverter capacity is 4000w continuous but it all depends on the quantity of PV panels & batteries to support the kwh load that you want to use. The system is designed to produce 40kwh as indicated in a 24 hour cycle. 40kwh / 24 hour period = 1.66kw per hour average base load.
Do not exceed daily kWh usage of 40kwh and do not exceed average base load of 1.66kw per hour.
The Inverter may peak at 4000 watt (4kw) for short periods with use of a hairdryer or a kettle but the total hour usage must not exceed 1.66kwh. If you use more kW per hour average than what it is designed for the system it will have a shortfall and will shut down or transfer back to grid if grid is available.
Bad weather day’s production can be as low as 50% or lower and if it’s overcast with rain it can have as low as 0% production. To make the right choice you have to know how many average kWh you use in a 24 hour period.
Stay with in the parameters that the system is designed for and plan internal load shedding on bad weather days if there is no grid or generator backup.
Different system voltages… 12v / 24v or 48v ?
This refers to the input voltage from the battery bank. The main consideration is that at higher voltages the current is less so that you can use smaller wires between your solar panel array and your battery bank. Of course, when you decide on a system voltage and want to expand in future, the solar panels, Inverter, and battery bank all need to use the same voltage and this is where the problem starts.
You limit yourself by building a small 12v / 24v system. This method works but comes at a price. If you want to start with a few lights for a few hours you need 1 kW / 12v Inverter and a few batteries with PV and a charge controller, but…..
What’s the future goal of your whole system?
- Inverter: For now you need an Inverter – 1kW, 12v – Larger systems operate at 48v, 12v / 24v systems are limited on controller or Inverter size, if you start off wrong, how do you correct it?
- Charge controllers are 12v, 20–30 amp Pwm – Large systems need 48v, 60–100 amp MPPT charge controllers, you start wrong how do you correct it?
- PV – Large systems voltage on Pv panels are 36-44vdc to match MPPT charge controller, you start wrong with 18-24vdc Pv panels due to 12/24v system. Very important: Different Pv panel sizes do not match each other in system design, you start wrong how do you correct it?
- Batteries – buying deep cycle 12v 102ah batteries last only 350 cycles +- 1-2 years. Large systems need 48v, 2v flooded lead acid battery cells last 10-12 years. 40% more expensive than normal 12v deep cycles but you don’t have to buy it 5-8 times in 10-12 years…
If you want to expand, buy the right Inverter, Pv, charge controller and batteries, there are no shortcut answers.
The choice of battery type is not a simple decision with many different factors to take into account but we would always recommend that a comparison is made using the considerations and looking at the total cost over the life cycle of the battery system and not simply choosing the lowest initial cost option which in many cases may be more expensive in the long run.
The choice of a battery is one of the most critical decisions that needs to be made when designing a grid-backup or enhanced self-consumption “off grid” solar PV system.
The main types of batteries commonly chosen for solar PV systems are Lead Acid / Lithium Ion / AGM – with various different specific types like gel / pure lead batteries.
10 Year storage or float life design is not the actual cycle life of the battery… If a battery is drained 50% every day how many cycle would it gave over time ?
50% depth of discharge (D.O.D.) = 1200 cycle / 365 days a year = 3.25 years life time and not 10 years like most Gel batteries indicate…
The table below gives a summary comparison of the key attributes of some of the most used different battery technologies.
|Gel / Agm||Lead Acid 2v Cells||Lithium Ion|
|Total Storage Capacity
An individual 12v gel battery will typically have a gross storage capacity of 100Ah – 250Ah. They are connected in series for a higher voltage and/or in parallel for greater capacity at the same voltage. A typical gel pack suitable for a residential grid-backup solution will be in the range of 500–1200ah (12kwh-28kwh) depending on the length of time required to operate off-grid and the total power of the loads to be supported.
An individual 2v lead-acid battery will typically have a gross storage capacity of 500Ah – 1660Ah. They are connected in series for a higher voltage and/or in parallel for greater capacity at the same voltage. A typical lead-acid pack suitable for a residential grid-backup solution will be in the range of 500–1200ah (12kwh-28kwh) depending on the length of time required to operate off-grid and the total power of the loads to be supported.
Lithium Ion battery packs typically are supplied as self-contained units with a built-in battery management system (BMS). Gross capacities vary from about 2kWh up to <kWh depending on the model and manufacturer. Some models may be connected in parallel, others may be extended with expansion packs and all need to be fully supported by the software in the battery charger/inverter chosen.
|Daily Usable Capacity
There is a close relationship between the amount of the total battery capacity that is used each day and the life of the battery as expressed by the number of cycles and typically it is recommended to only discharge gel battery down to about 50-80% of the total capacity of a gel battery, this if referred to as a Depth of Discharge (D.O.D). This makes the storage capacity available for daily use only 50-80% of the gross storage capacity.
There is a close relationship between the amount of the total battery capacity that is used each day and the life of the battery as expressed by the number of cycles and typically it is recommended to only discharge a lead-acid battery down to about 50% of the total capacity of a lead-acid battery, this if referred to as a Depth of Discharge (D.O.D). This makes the storage capacity available for daily use only 50% of the gross storage capacity.
Most lithium-ion batteries can be used daily down to about 90% of their gross storage capacity with little or no impact on their lifetime in terms of number of cycles. This makes the storage capacity available for daily 90% of the gross storage capacity.
|Full Cycle Efficiency
Gel batteries tend to get less efficient the nearer to full capacity they reach which either results in a low full cycle efficiency of less than 90% if they are re-charged near to their full capacity or designing the system to only use about 90% of their full capacity in order to maximize their efficiency.
Lead-acid batteries tend to get less efficient the nearer to full capacity they reach which either results in a low full cycle efficiency of less than 80% if they are re-charged near to their full capacity or designing the system to only use about 80% of their full capacity in order to maximize their efficiency.
Most lithium-ion batteries have a full cycle efficiency around 95% even for a cycle from their full depth of discharge up to full capacity making them ideally suitable for daily use applications like solar PV systems which need to use most or all of their retained energy in the evening/night and charge up again fully during the day.
The number of cycles that a gel battery can be used for is directly related to the amount of energy charged and discharged in each cycle. With a system configured to utilize 50% of the gross storage capacity of a daily basis a typical lead-acid battery will have a lifetime of 1000-1500 cycles. Allowing for some degradation over the life of the battery a useful lifespan of about 5 years in a well designed system may be expected.
The number of cycles that a lead-acid battery can be used for is directly related to the amount of energy charged and discharged in each cycle. With a system configured to utilize 50% of the gross storage capacity of a daily basis a typical lead-acid battery will have a lifetime of 2,500-3000 cycles. Allowing for some degradation over the life of the battery a useful lifespan of about 10-12 years in a well designed system may be expected.
A good quality lithium-ion battery may have a lifetime of 5,000 – 7,000 cycles which is considerably more than 12-15 years of normal usage. The built-in battery management system will ensure that the battery condition is always maintained in optimum condition and a full 10+ year life may be expected.
The initial investment cost of a gel battery will be relatively cheap when expressed as Rand per kWh of gross capacity but all comparisons should always be done a Rand per kWh of usable capacity which makes a gel battery twice as expensive as it may initially appear.
The initial investment cost of a lead-acid battery will be relatively cheap when expressed as Rand per kWh of gross capacity.
The initial investment cost of a lithium-ion battery may be 2.5 – 3 times more expensive per kWh of gross capacity compared to a similar sized lead-acid battery but when comparing the Rand per kWh of usable capacity the difference will be typically about 1.5 times as expensive. The lithium-ion battery will however last longer than lead-acid or gel, so over a 12 year period the lithium-ion will almost seem a cheaper option with no need to renew the battery after 5-10 years.
A gel battery may weigh between 50kg and 100kg per battery so a typically 12kWh-28kWh domestic battery pack may weight in excess of 500-1000kg which may cause difficulty in locating a large battery pack in a residential property as a strong floor will be required. However it only require 1 square meter.
A lead-acid battery may weigh between 50kg and 100kg per cell so a typically 12kWh-28kWh domestic battery pack may weight in excess of 500-1000kg which may cause difficulty in locating a large battery pack in a residential property as a strong floor will be required. However it only require 1 square meter.
A good quality lithium-ion battery pack will typically weigh between 10kg and 15kg per kWh of usable capacity so considerably less than a equivalent lead-acid pack but a typically residential battery pack will still weigh 75kg – 100kg requiring some consideration as to where to place it.
|Charge / Discharge Power
Most gel batteries can be charged and discharged relatively rapidly and when connected in parallel the total charge / discharge rate is in effect increased. In a typical solar PV system a gel battery pack may be charged and discharged in 2 – 3 hours with a peak discharge rate much higher for short period of times.
Most lead-acid batteries can be charged and discharged relatively rapidly and when connected in parallel the total charge / discharge rate is in effect increased. In a typical solar PV system a lead-acid battery pack may be charged and discharged in 2 – 3 hours with a peak discharge rate much higher for short period of times.
Most lithium-ion batteries have a relatively restricted charge / discharge rate often needing 3 – 4 hours to charge and a maximum discharge rate of between 1kW and 2kW for a typical residential system. A system utilizing lithium-ion batteries therefore needs to be designed to take care to only connect essential loads to the circuit that will be powered from the battery pack. Only certain Inverters / charge controllers may be used on lithium-ion batteries
Gel batteries are significantly impacted by the ambient temperature but not as much as lead acid batteries and an increase from 20c to 30c can result in a 15% reduction in the lifetime as defined by the number of cycles and a 50% reduction in the lifetime as defined in years.
Lead-acid batteries are significantly impacted by the ambient temperature and an increase from 20c to 30c can result in a 25% reduction in the lifetime as defined by the number of cycles and a 50% reduction in the lifetime as defined in years.
Lithium Ion is less impacted by moderate temperature changes and ambient temperatures in the range of 15 – 30 degrees centigrade will not significant impact the lifetime nor performance of the battery.
|Battery Operating Time||The next critical decision is to decide the number of hours that the system needs to power the essential loads for. Typically a planned grid outage due to load shedding will last for 4 – 6 hours whereas a failure due to a grid fault will typically last for between 1 and 24 hours. The decision on how many hours to allow for is largely driven by the budget available as the cost of the battery pack will be directly related to its size and its size will be directly related to the number of hours chosen. Usually a system will be sized to support the essential loads for between 12 – 24 hours.|
|Space Available||Especially when choosing a lead-acid battery the space available to hold the installed battery and the strength of the floor may be a consideration that imposes a limit on the maximum size of the battery that can be installed. Large racks can be constructed to limit space. With a Li-Ion battery this is unlikely to be a major concern as a Li-Ion battery will be much smaller and lighter than a similar usable capacity of lead-acid battery. Gel batteries consist more the circumstances as lead acid batteries.|
|Charging Time and Rate||The battery will be charged from the surplus energy available from the PV system, this is the difference between the energy generated by the solar PV system and that used by the loads during the daylight hours. It is therefore important to ensure that the battery can be fully recharged during a typical day of sunlight, especially in the winter months. A battery pack which is too large relative to the PV system will not get fully recharged and therefore not be fully available to provide power in the event of a grid failure. If all Pv energy is consumed by loads batteries will also not charge. Stay in parameters of system design.|
|Maximum Depth of Discharge||Each battery pack will have a recommended maximum depth of discharge, e.g. lead-acid might be 50% and Lithium Ion might be 90%. Having determined the total energy required to be generated from the battery pack with the equation : ‘essential loads energy in 24 hours divided by 24 multiplied by the required battery operating time’ then the gross battery capacity needs to be determined by dividing by the recommended DOD.
10 000W / 48v system = 209ah needed x 2 (50% DOD) = 417ah bank
|Total battery bank
417ah x 48v system = 20 000w x 50% DOD = 10 000w capacity in bank
The choice of battery type is ultimately up to the customer but from experience, we like to choose lead-acid 2v cells. These are the most affordable, value for money and have been tried and tested over the years. We would like to think that the future will surprise us with a totally new storage concept, but only time will tell.
What to consider before installing a solar system ?
Is the location suitable for a PV system installation?
You will need to make sure that you have space, whether the system is ground mounted or roof mounted. Remember sky lights, chimneys, and hips on the roof will all determine how many solar panels you can install and they will also have a shade impact.
Is the roof structurally sound, and will installing PV compromise its structural integrity?
It goes without saying that solar should never be installed on a roof that is structurally unsound, or if installing solar would make it unsound. It is also not advised to install on a roof that has asbestos or thatch. If in doubt, you should source an expert to survey your roof.
How suitable is the property in terms of positioning?
To make the most of the sun in South Africa, it is best to install on roofs that face north. You can also mount facing north west or north east however this is less effective. Some roofs have been installed east and west but its not as effective as facing north.
What is the optimum angle to install on?
This is measured on a case by case basis, depending on the location. To be most efficient all year round, solar panels should be mounted at an angle of between 25 – 35 degrees which is a good winter / summer average. Panels should never be installed lying flat. Dust and other debris can’t wash off with rain if its horizontal.
Is the installation suitable for self-consumption using batteries or grid back up?
When it comes to utilizing energy in the most effective way possible, many are looking to battery products, either to supplement their own self-consumption, or as a reliable source of energy in the event of a grid failure. Any system using a battery source needs to be of a certain size in kwh in order to benefit the user. Some smaller systems would not be able to support an integrated battery and may fail within an hour or two.
What’s the legal requirements?
Any system will not present a problem as long as you don’t feed back to the grid. Grid tie feedback is in some areas legal with approved applications.
Grid tie with feedback in non legal areas…. Well to be honest. This is a nightmare. Illegally connected systems may be liable to a fine or grid disconnection. Some areas like Port Elisabeth, Cape Town, Gauteng and Durban as far as we know allow grid tie but the rest do not.
SARS – Tax break on solar
As from 1 January 2016, Section 12b of the Income Tax Act (South Africa) was amended from a three-year (50% – 30% – 20%) accelerated depreciation allowance on renewable energy to an even quicker depreciation allowance of ONE year (100%).
This accelerated depreciation allowance came about from a proposal in the 2015 draft Taxation Laws Amendment Bill that the definition of solar energy be amended to distinguish between photo-voltaic solar energy of more than 1 megawatt, photo-voltaic solar energy of less than 1 megawatt and concentrated solar energy. The amended Section 12b provision now provides for an accelerated capital allowance of 100% in the first year, in respect of photo-voltaic solar energy of less than 1 megawatt.
The reason for the change is to accelerate and incentivise the development of smaller photo-voltaic solar energy projects, as it has a low impact on water and environmental consumption. This is also intended to help address the energy shortages facing South Africa in a more environmentally friendly way.
Section 12B of the Income Tax Act No. 58 of 1962, as amended (the ‘Act’), provides for a capital allowance for movable assets used in the production of renewable energy. More specifically, it allows for a deduction equal to 100% basis in respect of any plant or machinery brought into use in a year of assessment for the first time and used in a process of manufacture or any other process which is of a similar nature. It is important to note that the allowance is only available if the asset is brought into use for the first time by the taxpayer. In other words, the allowance is not limited to new or unused assets. The wording merely prevents the taxpayer from claiming the section 12B allowance twice on the same asset.
What does this mean for your company? Currently, company tax in South-Africa is 28%. With this incentive, you can deduct the value of your new solar power system as a depreciation expense from your companies’ profits. This means that your companies income tax liability will be decreased by the same value as the value of the installed solar system. This reduction can also be carried over to the next financial year as a deferred tax asset. In effect it is the same as getting a 28% discount on the price of your solar system!
With this incentive, it makes the financial model of a commercial roof-top, grid-tied, solar power system look very attractive.
Please confirm with your accountant how it can work for your business.
More info here: SARS guide to Taxation in South Africa 2015/2016.