Designing and building an energy efficient SMART HOME

"The swimming pool has its own Internet address"

Why do the project

  • We started this project in 2014 when larger solar panels were just becoming available
  • Since that time , solar panels have decreased in cost to 20% of what we were paying when we started
  • Electricity purchased from the city council had been increasing at about 15% per annum.
  • The systerm is based on grid-tie inverters with no battery storage. The grid-tie inverters and some of the load is controlled by a computer to minimise purchases of electricity from the municipality.
  • When we started, payback was expected in 20 years. Nowdays payback is less than three years for a do it yourself builder.
  • It is a project where you can start small and add slowly as you get more understanding and can afford more panels.
  • With modern technology it requires no modification of your house wiring.
  • We have many appliances running off the solar energy during the day, from geysers, swimming pool, bore hole pump, lawnmower, hedge cutter, lathe, milling machine,air compressor and as well as all the household appliances such as fridges, freezer, kettle, microwave, toaster, stove, DSTV, CCTV and computer networks.
  • As we add more capacity we have spare energy and don't feel guilty about burning up the planet when we run appliances.
  • The life of the technology is more than 25 years with virtually no annual maintenance costs.
  • We have a lot of automation. When we turn on a kettle in the kitchen, the swimming pool shuts off to free power automatically.
  • Solar array mounted on outbuilding roof
    Solar array mounted on outbuilding roof
    Four banks of three 250watt panels each mounted in an antitheft frame and each frame feeding a grid tie inverter typically at 24 volts DC

    What we have learnt

  • Do not use batteries - they do not solve a problem, they make the project uneconomic and result in high annual running costs.
  • Start small, and add incrementally.
  • Install power meters to measure your energy profile so that you can understand the problem and know when you are succeeding.
  • Have multiple small grid-tie inverters each fed from their own array so that you can control output power from the system. (You need multiple inverters as it is not possible to instruct an inverter to only deliver 50% power for example. It is designed to convert all available solar power to mains power. Hence you switch off inverters when you want less power.)
  • Use 24 volt and 12 volt networks rather than high voltage networks.
  • Wire 24 volt panels in parrallel so that shadows falling on some of the panels do not switch off the entire system.
  • You cannot go off the grid completely but you can reduce your power consumption dramatically.
  • Move energy consuming devices with timers into daytime operation when solar power is available - such as geysers, swimming pool, borehole, lawnmowers
  • It is uneconomic to sell power back to the city council, rather switch off spare capacity if over generating.
  • City Council power meters do not measure direction of power flow - so if you over generate and feed it back into the grid you are paying for the oversupply at current electricity rates.
  • You can control certain loads in your house to use spare power or free power for other devices so as to minimise city council supply.(Geysers, borehole, swimming pool)
  • Graph of power control done via a Raspberrypi processor.
    Graph of power control done via a Raspberrypi processor.
    Blue is solar energy,
    Red is extra power from city council - overgenerating when goes below zero axis (shown as dark blue)
    Purple is total power used (solar and city council)
    Numbers 1,2,3 refer to control of individual grid tie inverters to shut off part of network- 0 is all grid ties are working
    Letter P indicates swimming pool motor control (Red is ON and blue is off)
    Khaki graph uses right hand axis and shows instantaneous voltage from city council


  • Although you will buy a panel rated at 250 watts, in Johannesburg you can expect to only get 60% of that power from the panel. This is due to the specifications being for an ideal 1kW/sq meter test in ther factory whereas the power falling on the panel is impacted by impurities in the atmosphere, operating temperature, orientation to the sun and other factors.
  • We have 15 of 250 watt panels working through seven grid tie inverters. This network delivers 2.9kW at peak midday and gives 15kwH of energy in winter and 18kwH in summer.
  • On a bad overcast day we can expect 3kwH
  • Johannesburg is in the southern hemisphere and we have winter in June. In summer we get rain storms, but longer days and so more sunlit hours.
  • Our shortfall in being energy independant is about 14 kwH per day.(We have a very big house with lots of computer systems).
  • The angle of the track of the sun changes by 45 degrees from summer to winter
  • Solar power generated on one day in winter
    Solar power generated on one day in winter

    Solar power generated over the month of June 2019
    Solar power generated over the month of April 2023 (294KwH)

    Typical profile of electricity supply from council
    Typical profile of electricity supply from council.

    Project phases
  • Start by getting a power meter on your incoming supply to measure the profile of your usage.
  • Buy some panels and a grid tie inverter to get a system running that can reduce your base load.
  • Add more panels and grid tie inverters and control the gridtie inverters using a timer
  • Add a power measuring module that can determine direction of power flow to your distribution board.
  • Add lots more panels and grid tie inverters as well as a control system to switch off grid tie inverters when generating surplus power.
  • Use Sonoffs to computer control loads such as swimming pools and geysers to load the time period when spare power is available from the solar system.

  • Solar panels
  • Solar panels used in solar systems usually are developed for 12 volt OR 24 volt OR 48 volt systems. (The 48 volt systems are needed for high voltage chargers for lithium battery systems where they are wired in series to give voltages from 140-200 volts DC)
  • A standardised size has become common practice so that they can easily be built into large arrays without having gaps between the panels. Size is 1650x992 millimeters.
  • Power delivered from these standardised panels are between 250 watts and 330 watts at a test factory energy density of 1 kilowatt/sq meter. (48 volt panels can deliver 400 watts/panel but these are used in series for charging lithium batteries).
  • A panel used in a 24 volt system will have 60 individual cells that are wired in series using up the total surface area of the panel. Each cell in full sun develops 0.5 volts and the 60 develop 37 volts when no load is applied.
  • Sun shining on each cell converts photons to electrons which flow though the connections of each cell into the adjacent cell and eventually to the connectors on the panel.
  • Typical specifications of the panel will give
    • Open Circuit voltage=37.5 volts
    • Short circuit current=9.12 amps
    • Optimum operating voltage=30.4 volts
    • Optimum operating current=8.6 amps
  • The panels have a 3.2 mm thick layer of specialised tempered glass protecting the cells and allowing the maximum transfer of sunlight to the cells.
  • The panels are supplied in a rigid aluminium frame which has mounting holes for fastening the panels which is 40 millimeters high.
  • The efficiency of converting light to electricity is typically 16%.
  • The performance of the panel, because all the cells are wired in series, is dependant on the performance of the worst cell in the chain. Placing one cell in shade while the rest are in full sun will result in the performance of the panel dropping to 5 %, that is a 95% loss of performance.
  • When collecting energy from multiple panels, wire the panels in parrallel so that even it there is shade on one panel, the others can supply full current.
  • The life of a panel is expected to be about 25 years by which time the performance will have decreased to 80% of the performance when new.
  • A lot of these panels have been made. Just one solar farm in South Africa is reported to contain 500 000 of these panels.
  • For 12 volt systems there are 30 cells, which are twice as large physically than the 60 cell systems and deliver double the current and half the voltages. They are the same size physically.
  • Pricing is specified in Rands/watt.When we started in 2014 we were paying R20/watt which dropped to R4.00 per watt for many years. (In 2023 there has been a rush to put up solar systems and the shortage of panels has caused the price to rise to R8 to R10/watt.)
  • Virtually all panels are made in China. China has large solar farms and is reported to have nearly installed capacity that is equal to three times the energy consumption of the United Kingdom. Some panels are made in India.

  • Grid Tie inverters
  • A grid tie inverter is an amazing invention that takes DC energy from solar panels or a battery and converts it to mains energy IN EXACT PHASE with the mains energy being supplied by the city council. We need this type of inverter technology as if the phase between the two supplies should differ, there will be a bang and the inverters circuits will be blown away.
  • This is a very different type of inverter to that used when you are off grid and can generate your own electricity with any phase as you are the only source of energy for your appliance.
  • The grid tie inverter has to deal with a wide operating range of voltage on the DC side as different light conditions result in variations of input condition, as these changes occur suddenly as a cloud drifts to obstruct the sun.
  • A grid tie inverter for wind energy is similar to the solar one but has different response times to cater for varying wind.
  • A grid tie inverter continuously monitors the mains energy and switches off within a fraction of a second if it senses a change in phase or loss of power making the system safe.
  • As the grid tie inverter is made to supply additional power to a mains system in exact phase with the mains, it cannot be used for off grid applications.
  • The process of operation of a grid tie inverter is that it will initially monitor the mains energy to determine the frequency and phase of the mains voltage. It phase locks its timing onto these values and continues to monitor them to detect any variation.
  • On the DC side it chops the incoming DC power to a high frequency AC power waveform and converts the AC waveform to about 400 volts which it rectifies and stores on a capacitor. It is this ability to convert the incoming voltage to 400 volts that allows it to vary its contribution to the mains to cater for variations in supply.
  • Another section of the inverter converts the 400 volts DC to 220 volts AC in exact phase with the incoming mains.
  • By increasing and decreasing the 400 volt on the interim capacitor, it can vary the contribution of power supplied to the mains system, by raising the output voltage slowly above that of the municipal supply so that your appliances are drawing power from the grid tie inverter rather than the mains. It uses this method to adjust the power supplied such that the solar panels are delivering their optimum power available at their optimum voltage and current for the light conditions that are then prevailing.
  • The Chinese have developed a wonderful version of the grid tie inverter that is ideal for small solar systems. It is designed to take a few panels that are mounted close together, convert the energy to mains voltage and feed the energy directly into the wiring system via a conventional wallplug, not needing any change to the house wiring. The systems are designed to operate on 24volt solar arrays or 12 volt solar arrays converting up to 600 watts of energy per unit, and allowing many units to operate together in parrallel allowing many panels of different voltage ranges to convert sunlight to mains energy.
  • As each of these inverters manage directly those panels that are connected to that specific inverter and convert their energy into mains energy, it allows different solar panel voltage systems and manufacturing technologies to be combined in a single system to get optimum performance out of all panels in the system.
  • Our system has panels from three different technologies, some at 12 volts, some at 24 volts, some with standard size panels and some with small panels, all combining via seven grid tie inverters to get optimum power from the sun.
  • Typical specifications of the 24 volt versions of the inverters are
    • Power 600watts
    • DC input range - 22-60VDC
    • MPPT Voltage - 24-48V (Voltage range over which it will maximise power transfer from solar panels)
    • DC Max current - 50 amps
    • AC Max output power - 650 watts
    • AC output voltage range - 230VAC(190-260VAC)
    • Power Factor >97.5%
    • Stable Efficiency/220V >84%
    • Protection Islanding; Short-circuit; reverse connection; Low Voltage; Over Voltage; Over temperature Protection
    • Work Temperature -25 to +65 degree C

  • These types of grid tie inverters are relatively cheap and have a long operating life.
  • A grid tie inverter is a sophisticated instrument and is designed to operate at maximum efficiency. It is not suitable generally to be controlled to say operate at only 50% power. When one has an array of grid tie inverters, power control can be achieved by switching off individual inverters to limit generation.

  • Connecting a grid tie inverter to mains
    Connecting a grid tie inverter to mains.

    Power measurement systems
  • A solar energy project is about converting your energy usage from council supplied power to power supplied to you directly from the sun.
  • To know if you are succeeding, you need to measure your usage at present and your usage at future stages.
  • To measure power, you need to get a power meter and you need to keep records.
  • Measuring power in electrical terms, requires the meter to measure the voltage at the appliance and the current flowing into the appliance. Power is voltage X current.
  • There are two ways to measure the current. One is to break one of the conductors and have the current flow through a current meter, and the other is to connect a magnetic sensor around the neutral wire and detect the magnetic field caused by the flow of the current in the neutal conductor. The second method is preferred for a solar system as it causes no modification to the wiring of the house and does not need an electrician to do the installation.
  • There are a number of power measuring systems that can be plugged into a mains outlet and measure power usage for a single appliance. In fact some of the Sonoff switches (see later) that can be used to control devices report back the power usage of the device they are controlling.
  • We have been monitoring our usage for about six years and we started using an OWL meter made by RADIANT. This had magnetic sensors for either single or three phase which sent data via a radio link to a display meter that showed instant readings and it accumulated readings over time.
    OWL type power meter
    Remote display for measuring power - OWL type

  • A company called Efergy in the United Kingdom has developed a range of devices specifically for monitoring your power usage. This includes the measuring devices as well as software and cloud services that let you get a deeper understanding of your power usage. It allows you to monitor power usage via a display on your desk, via a web browser from anywhere in the world, and via your cell phone.

    Like the OWL system, the system measures the electrical current component of the power calculation using a magnetic sensor that is clipped around the neutral wire of the incoming power cable not needing any electrical connection to the power system. Data from the sensor is sent via a radio link to the desktop display and to an optional hub interface connected to your WiFi internet connection which enables the data to be stored and processed by Efergy on the cloud, hence giving you private access to your data via a web browser or your cell phone from a remote location.

    Efergy type power meter
    Desktop remote display for measuring power - Efergy type

    The graph below shows data presented from measurments stored on the cloud via an Efergy Hub.This shows power bought from the Municipality over 24 hours with the energy needs supplementing the solar power during the day. High spikes are appliances such as geysers (4kilowatt elements).


    Efergy presentation of data via cloud using data gathered from an Efergy hub
    Efergy presentation of data via cloud using data gathered from an Efergy hub

  • Basic calculations

  • The electricity you buy from the city council, is detailed on your municipal account. The unit of electricity is the kilowatt hour (KwH). This unit represents a kilowatt of electricity that is supplied for one hour.
  • A kilowatt is 1000 watts and a watt is the voltage times the current. If 10 amps flow from a 24 volt panel into a grid tie inverter, that is equivalent to 240 watts or 0.24Kilowatts.
  • A typical kettle uses 2 kilowatts of electricity to heat the water. At 220 volts it results in 9 amps of electricity flowing from the city council. From a 12 volt battery it would require 160 amps flowing which is more than used by the starter motor of a large car and would need cabling that is 10 millimeters in diameter.
  • A 200 litre geyser in the typical house has a 4 kilowatt heater to heat the water and takes 12KwH of electricity to heat to operating temperature from cold.
  • Another important electrical formula is the effect of resistance. If 10 amps is flowing through a length of wire that has a resistance of one ohm, the voltage drop will be 10 volts - and the loss of power will be 100 watts. This is from ohms law which states voltage=current X resistance and power loss is current x current x resistance (current squared x resistance). This means you want good quality wires bringing the power from the panels to the grid tie inverter. We use 6 millimeter diameter electrical cable.
  • Batteries - not useful
  • Batteries are an expensive part of the solar system and make the project un-economic.
  • Batteries introduce maintenance issues and have limited lifespan meaning they need to be replaced periodically.
  • People want batteries to enable them to go off the grid - but due to weather conditions you cannot count on solar 365 days a year, meaning you always have to be connected to the grid even if you are not buying power regularly.
  • Motors in fridges, freezers and pumps have a surge current on startup and need access to a stable supply for a short period - which requires overdesign of an off grid system.
  • A small battery system used in conjunction with the solar panels provides a successful off grid system for key appliances during loss of power due to load shedding etc.
  • To give you an example of the implications of a battery system for off grid. Assuming the weather is perfect and we needed to add to our existing solar system to convert it to an off grid system.
    • We need 27kwH per day of energy from the storage system
    • A 100AH battery at 12 volts stores 1.2KwH of energy - We need 23 such batteries.
    • A Python lithium battery system (such as promoted by Tesla) rated 2.4KwH - We need 12 such batteries - costing R144000
    • An additional 27KwH of solar energy is needed to charge the batteries during daytime - requiring a further 28 solar panels and charging equipment for the batteries. Cost about R120000.
  • By adding extra solar panels to the current system and by moving loads into daytime, one could reduce the 27KwH requirement slightly saving on batteries.
  • If you do need to use a battery to store energy, use lead crystal batteries. These have been developed for the solar industry. They are similar to a car battery, but have a life of up to 25 years and are virtually industructible - allowing you to over-charge or completely drain the battery without any damage. The batteries are extremely heavy, weighing twice as much as the same size car battery. They also have no dangerous chemicals and emit no vapours when operating.
  • Track record.
    Built a solar system in 2014 and have generated approx. 33000kwh from a system costing approx R30000. Have a 21.6 sq meter system of 21 panels and 7 mini inverters controlled by a computer to throttle down power when not needed.System can generate up to 3kwatts peak and maintain at this capacity if needed. Generate about 15kwh per day. Have no batteries and use municipal power to deal with peaks such as geyser, stove and motor startup. Generate about 60% of our daily needs from solar. Author is an Electrical Engineer educated at Wits University in the 1970s.

    Getting help and advice
    This description is provided to show how easy it is to get going. I am not selling anything, just encouraging users to take the first step. You can contact me for help and advice from Mike at Tel +27 72 992 6040

    1 May 2023

    Also see an article on Cost effective solar systems for South African conditions
    Also see an article on battery storage systems
    Also see an article on Solar water heating - solar geysers
    Also see an article on Measuring systems for solar systems
    Also see an article on Gridtie inverters -Converting solar energy
    Also see an article on Stand alone inverters -Converting battery energy
    Also see an article on Energy equivalents