Domestic Energy Saving

This page will answer a number of questions that domestic users have. 1. Which devices use lots of Energy? 2. The conservation of energy 3. How much energy does a geyser timer save? 4. Is Power Factor Correction an option for domestic? 5. Can a Geyser blanket save Energy? 6. Should I replace incandescent lights? 7. Are Flat Panels or Evacuated Tubes Better Solar Collectors? 8. Is a Heat pump a viable option? 9. What are my Solar water heater options? 10. Solar Electrical Panels for Water Heating 11. What should I look for in a Off-Grid or Load Shedding Proposal?


Is the Powerwall as good as they say?

Which devices use lots of Energy?

When we talk about electricity, we refer to energy in Kilowatt-hour abbreviated as KWh. KWh implies a Kilo (that is 1000) Watts of Power for one hour. Note the use of power and energy. Think of Power as the speed at which you use Energy, so Power multiplied by the time in hours is the total Energy you use in KWh. NB-NOTE: It is Kilowatt-hour, i.e.: power multiplied by the time in hours, not kilowatt per hour which would be power divided by time, i.e.: the rate of change of power, which in this context is a meaningless concept. If your contractor talks about kilowatt per hour, run, he is an idiot. In terms of cost, a KWh from Eskom costs over a rand if you are lucky (closer to R 1.80 for most people, but let’s use R 1.00 in our examples to keep it simple). So if your pool pump uses 1.1 KW and you run it for 4 hours per day, it will cost you about R 4.40 per day (for 4.4 KWh). If you want to conserve energy, you have to think of how much power each appliance uses, and the length of time you use it. Some devices, for example a kettle use 2 KW but we only use it for 3 minutes at a time, so boiling a Kettle takes 0.1 KWh to boil, that is 10 cent. The worst device at home is a tumble dryer. It uses about 2.8 KW and you run it for say 40 Minutes. That would be 1.87 KWh or R 1.87. Notice that all these amounts are small, the problem is they add up. Lights is a good example, lets say you have a room with 6 halogen down lights and you switch them on for 5 hours per day. That would be 50 Watt x 5 hours x 6 lights = 1.5 KWh or R 1.50. Now do that in 4 rooms for 30 days then it becomes R 180.00 per month. I get irritated if people tell us to unplug things like cell phone chargers. It’s a waste of advertising time, which can be used much better. My charger uses just under R 0.13 per month if it is left plugged in all the time. So, in general: devices that are used to heat anything use the most power and devices that have electric motors are next. Then there are lights, which use less power, but stay on for long and lastly electronic devices. For example: a 42 inch LCD TV plus a DSTV decoder left plugged in for 18 hours while you don’t use it, costs R 13.5 per month. “Tech Talk” - Sometimes you see people referring to KVA (Kilo-Volt-Ampere) instead of kW. Watts is Volts times Amps, so why use VA? It gets complicated so settle for this. If you have devices that work on magnetic principles such as electric motors, then you have power that simply oscillates in and out of the device, without being used, but it does result in a bigger current flow. VA includes this oscillating power and the power being used, so you need say 15% more Volt-Ampere than Watt. Else think of it as the same. Eskom does not like this, because the extra current causes bigger losses in their networks.

The conservation of energy

When thinking about energy, we always have to remember the law of conservation of energy. It essentially says: “The energy that goes into, and the energy that comes out of a system, must be the same”.

How much energy does a geyser timer save?

People are advertising as much as 30% of your monthly electricity bill. The answer is “very little” and probably less that R 20.00 per month. Remember, energy in and out is the same, and for a geyser, it’s easier to understand how much energy goes out, so let’s look at that. Energy goes out of a geyser in one of two ways. 1. You withdraw hot water from the geyser and replace it with cold inflowing water. 2. A geyser has heat losses. A timer switches the geyser of, so if the water in the geyser loses heat, then the geyser does not restore the heat while the timer is off. This simply postpones the heating process and leaves the geyser colder until the timer comes on. The water therefore remains colder for a longer period. So the question is now, how does that save energy? You want water from your taps at a certain temperature and you mix the hot a cold water to get what you want. So a hotter or colder geyser water simply means you will use less or more water from the geyser in the mix. That saves nothing. So the answer must lie in the heat losses. Heat loss from the geyser is proportional to the temperature difference between the geyser water and the ambient temperature. So a warmer geyser loses more energy, and that is where you save. But how much? New energy efficiency Class B geysers in South Africa must comply to a SABS standard that implies that they do not loose more energy than 1.44 KWh per day. That is R 2.45 per day or about R 75.00 per month. If your geyser is left at say 45 degrees instead of 55 degrees and ambient is 15 degrees average, then the temperature difference is 30 degrees instead of 40 degrees, which means losses will be 25% or less than R 20.00 per month. If your timer switches on twice a day then the temperature will be lower for say 6 hours total. Now the R 20.00 becomes R 5.00 per month. But: If you go on holiday and switch the geyser off, then the geyser cools down to ambient temperature, so all the losses goes away while you are gone. So why does Eskom want you to put in a timer so dearly? Because you can postpone the reheating to periods when electricity demand is not what high. That helps them. Warning: Some guys are selling geyser profilers as wonderful devices that will save enormous amounts of power. As far as I can establish, these are simply timers and if so, refer to my explanation above. Else there is magic out there that I don’t know about and they could not explain it when I asked.

Is Power Factor Correction an Option for domestic?

Some people also call this a Power Optimizer Corrector (POC). Electrical motors use Resistive and Inductive power. The inductive power results from current which simply oscillates between between the supply and the load so there is current but no power being used. Eskom measures and charges for the power so you cannot save anything. Eskom does charge industrial users for the inductive power (kVAr), so power factor correction does work in industrial applications.

Can a Geyser blanket save Energy?

Yes. That is a very good idea. Also insulate the water inlet and outlet for the first two meters from the geyser with 25mm thick (so called R-1) insulation. By blanket I mean geyser blankets that are at least 50 mm thick, so two blankets will be 100 mm thick. Be careful of those thin geyser blankets sold at some hardware stores, that is rubbish. A well installed blanket will save more than a third of heat losses. Problem is that it’s not a nice job and blankets are cheap, so no one wants to do it. Contractors make more money out of installing timers and they don’t have to get into the roof. That’s why everyone punts timers and not geyser blankets. With the new SABS specified energy efficiency Class B geysers, a blanked has a smaller effect. New geysers loose about half of the energy by comparison to older Class D energy efficiency geysers.

Should I replace filament (incandescent) lights?

Yes, and if you have halogen spotlights (low of high voltage does not matter) you should do it immediately. The bottom line is that Neon or LED lights save 80% - 90% electricity while generating the same amount of light. But beware, Neon lights are often more efficient that LED lights, so don’t replace Neon lights with LED lights without understanding the efficiencies. Contractors love to recommend spotlights because they charge per light to install them. With spotlights, they put in one switch and route one cable to the ceiling. Then they simply loop from spotlight to spotlight and make 6 times the money per room. Nice. With lights you have to become a label reader. Light output is measured in Lumens and to get the efficiency of a light you have to divide the Lumens by the Wattage of the bulb. Neon tubes produce up to 70 Lumen per Watt. LEDs vary but many are about 45 Lumen per Watt and much more expensive for the same wattage. LEDs are however getting better and some can be up to 80 Lumen per Watt. Read the label, and if it does not state the light output in Lumens, don’t buy it. Especially with LEDs you have to consider how much light you will have in the room. Replacing a 50 Watt halogen light with a 4 Watt LED also means that you could be replacing a 700 Lumen light with a 300 Lumen light. That may not be enough light for the room. You should not have put in spot lights, in the first place.

Are Flat Panels or Evacuated Tubes Better Solar Collectors?

There are 2 factors that impact the efficiency of vacuum tubes by comparison to flat panels. The total collector area of vacuum tubes are much less that the panel area because some of the space are taken up by the glass tube vacuum area and the gaps between the tubes. This makes them less efficient. The glass tubes create a vacuum around the hot areas which reduces their losses. This makes them more efficient. So the relative efficiency depends on the difference between the hot water and the ambient temperature. In summary, the efficiency of vacuum tubes are better in cold areas such as the European winter, and flat panels are better for South Africa’s warm conditions. The graph below shows a comparison. Winter freezing and resulting burst pipes is however a major factor in the Northern Cape and Freestate areas. With flat panels the water is exposed to the cold whereas with vacuum tubes it is not. We have seen a burst pipe ratio of 150 plat plate systems to 1 vacuum tube system during cold winters.

Is a Heat pump a viable option?

Short answer. Probably, but buy a brand with good support and understand the limitations. I see many very old air conditioners that still run, but they are good brands that I recognise. Heat pumps and air conditioners use one and the same mechanism so a good heat pump will last as long as a good air conditioner. The key is, don’t buy from Joe (Pty) Ltd who imported a container and is now flogging heat pumps. The most important aspect to understand is the Coefficient of Performance (COP), which is essentially the energy gain that you get. It is mostly quoted as a number, but it is a variable which depends on the source and supply temperatures. The efficiency gets lower as the ambient (source) temperature goes lower and as the water (delivery) temperature gets hotter. “Tech Talk” - Remember the fundamental law of conservation of energy? I just told you that heat pumps can deliver more heat energy to the water than the electric energy going in, so where is the magic? This is because pumping heat from one place to another takes less energy than converting energy from any other form of energy to heat. Nicolas Carnot published his theory on heat engines in 1824 and that became known as the Carnot cycle, which is what happens is all heat pumps. It was at the time when steam engine efficiencies were improving and everyone wanted to know how high the efficiency can get. He defined the ideal heat engine cycle and proved that the maximum amount of energy available if heat flows from a high to a low temperature is the temperature difference divided by the high temperature. So, if you have steam at 200 degrees Celsius and ambient is 20, then the maximum efficiency that any heat engine can theoretically attain is 38%. (By the way, temperature is measured in Kelvin which is the absolute temperature measure and nominally 273.15 degrees more than Celsius.) Fortunately, if you want to pump heat from 20 degrees Celsius to 200 degrees, then the opposite is true. In theory, you only need 38% of the heat energy you are pumping. Normally this gain factor will be quoted as the Co-efficient of Performance (or COP) equal to 1 divided by 38% = 2.63. Remember the heat is taken from one place to another, not generated. So if a heat pump has to pump ambient heat from the outside air to the geyser water and let’s say the air temperature is 10 degrees Celsius and the geyser is 50 degrees Celsius, then the COP should be over eight. So if the COP is only four, then the efficiency of the pump is actually 50%. Note the following, as the difference between the water and ambient temperature increase, the theoretical COP goes down. Eskom would prefer you to heat water at night when demand is low, but that is also when the ambient temperature is low. Another factor is that heat pumps and air conditioners have a limited working temperature range and if ambient goes too low the efficiency drops. So, the efficiency is never the same, and if it is very cold outside, it can even get close to one. Then you simply bought an expensive heater element.

What are my Solar water heater options?

There are a number of options to choose from when selecting a Solar water heating system. The choice depends on your situation and one cannot simply say that a certain option is always better than another one. There are however a few factors to consider and which you should discuss with your contractor. Active (Pumped) v.s. Passive (Thermosiphon). For Thermosiphon the tank has to be physically higher than the solar collector. Often this means the tank is mounted on top of the roof and you must decide if you like that. It is the simplest solution which is why it is popular for low cost housing. Active systems use a pump and a controller to ensure that water only circulates when the collector is warmer that the tank. This makes the system more expensive but tanks can be mounted any where. It is often a good option for retrofit systems or if you don’t like the look of a tank on top of the roof. For Direct or Open Loop systems the potable water circulate through the collectors. The problem is that these systems have a continuous fresh supply of oxygen and calcium in the water, which causes rust and scaling. There is also no overheat or freeze protection. For Indirect or Closed Loop systems a heat exchanger is used and a Glycol antifreeze mixture is pumped through the collector. This means there will be no scaling or rust and the system offers freeze protection. For bigger systems, e.g.: for large or guest houses one has to start considering other factors. Stratification, for example, is employed in some large systems to maintain different temperature layers in the tank. That means that hot water is available quicker in the morning.

Solar Electrical Power Panels for Water Heating

There are some innovative ideas and new systems that use Solar Power panels to generate electricity and then use that to heat water. Consider this. Solar power panels cost about R 12.00 per watt and Solar Thermal panels about R 4.00 per watt. So, thermal panels are three times cheaper than power panels. But, a heat pump has a COP of three, so if the power is fed to a Heat pump then the panels cost the same for a given amount of water heated. Now consider this scenario. The family bath/shower in the evening and use the hot water. The geyser has to heat up (at least partially) for the morning. There is no sun so the electric element is used to heat the water. Now the family uses the morning hot water, and leaves for work or school. The geyser is cold and heats up using electricity. By 10:00 the geyser is hot, and the sun is up. The solar system is at it’s peak for the next 5 hours, but no-one is home and the geyser is hot, so it’s all wasted. This is referred to as the solar factor. i.e.: How much of the heating is powered by the solar system? Very few people (read installers) understand the solar factor and the SABS has no regulations concerning the control system to improve the solar factor. Hence, most solar thermal systems are a total waste. So, ask yourself, if I rather buy a heat pump. Then use solar power panels to reduce my electricity use, I achieve the same, without the solar system losses due to the solar factor. I also don’t have freeze problems. I can then add batteries and protect myself against load shedding. If you want to get smart. Measure the power from the solar power system, and if it’s enough, start the heat pump and heat water while you have more solar power than you use. For this to work, you do need a hybrid controller with additional switching logic. All in all, it will cost more for the total package, but you do get a lot more. P.S.: There are ways to improve the solar factor. Set the electrical element to about 45 degrees, and use a timer to heat the geyser from about 14:00. If the sun has not heated the water by then, it’s a very cloudy day, so you may as well use the electrical element.

What should I look for in an Off-Grid or Load Shedding Proposal?

Before you even get proposals, reduce your power usage. e.g.: Get energy saving lights, change your stove to gas, use solar or gas or heat pump water heating, etc. All space heating should be gas or fire (often we are talking about bush locations). Air- conditioning is a difficult one, but there are low energy solutions that use much less energy that compressor systems. Before you get proposals, understand the problem you are trying to solve. If you have access to Eskom power, then that is always the best option. Let’s focus on two types of problems: No Eskom power, calling for an off-grid solution. Unreliable Eskom power, calling for a load shedding solution. If you consider the architecture of an off-grid solution it actually consist of two very separate sub-systems. The 1st one is a combination of a battery backup system which could be supported with a generator. The 2nd sub-system is a generation system, which would be Solar or Wind or Biomass etc. For a load shedding solution you need power backup when the electricity fails. Sizing this is critical and there are many factors that you have to consider. Unfortunately I cannot describe all the design considerations here, but if you get proposals, look for some critical data to compare: What is the maximum KVA (and KW) rating of the system? How much energy (in KWh) can the batteries deliver? Deep cycle batteries should not be discharged to below 50%. So find out what the batteries nominal capacity (that’s the 100% capacity) is. Divide that by 2 to get the usable capacity. Then find out how many cycles the manufacturer guarantees at 50% discharge (referred to as 50% Depth of Discharge). Estimate how many days it will take to get that number of cycles and convert to years. The above two parameters should be more than your low energy devices and continuous power requirements. For the high energy devices, such as washing machines etc. I would still use a generator. The generator should be started and stopped automatic, and the inverter should be able to carry the load while the generator starts. If you have no Eskom grid power or extremely unreliable power, you will need a larger system, with more solar and battery capacity. Again consider running some devices (high-power, short-time devices) on a generator and the continuous and lower power devices on solar energy. Estimate the power needed per day in KWh. Make sure the power delivered by the power generation system delivers that. For solar power for example, work on 5 hours sunshine per day, so the total panel rating should be the total energy in KWh divide by 5. There are other factors, such as “Days Autonomy”, “Battery discharge rate” and many other to consider, but if you understand the above, you will at least be able to make some comparison between proposals.

Is the Powerwall™ as good as they say ?

Tesla (Elon Musk) is a smooth salesman and have been portraying the Powerwall as a new invention that is key to going off the grid. At least, that is how the average South African (especially the technology disadvantaged one’s) understands it. So, what lies beneath all the hype? The technology is Li-Ion, the same as your cellphone battery. The same technology has been available in solar batteries for years from very credible european manufacturers but, given the cost and people’s experience with cellphone batteries beyond two years, no one considers them viable for home backup solutions. They work well in cars, where the discharge rate, combined with deep cycle capability is important. That is however not a requirement for home systems. It is interresting to note that the 10kWh Powerwall has already been withdrawn from the market with no announcements. So, once you understand that it is simply a battery as used in cellphones and electric cars that is now available as a home battery, the question is: How does it compare to other home battery systems? Take a 200 Ah, 48v deep cycle lead-acid battery bank, which is a very polular offering for home backup systems. This will consist of four 12v batteries with a 200 Ah capacity each and the four together cost of about 30% of the Powerwall. The lead-acid bank provides a usable cycle capacity of 4.8 kWh vs. the Powerwall’s 6.4 kWh. So that is 75% of a Powerwall’s capacity but 2½ times more cost effective. If you now double the number of batteries, you have 150% of the Powerwall’s usable capacity at about 60% of the price. Somehow the belief that you can go off grid with a Powerwall was created. That is true, but only if your current electricity bill is less than R 200.00 per month. In terms of the life cycle. Assume two power failures per week over 10 years. That would total 1,040 cycles. Both technologies can do that if you buy good lead acid batteries. If you use it off-grid, you will have to replace the lead-acid before 10 years. See it as two installments, 30% upfront and 30% after five years, vs. 100% upfront. In conclusion, it is just another (very expensive) battery. Battery technology is currently developing at a huge pace. Below is an example of the type of articles that appear weekly. Click below to see a (real) new discovery: cycles-is-discovered-by-accident-107106.html
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