Capacity, Design & Back-up Supply (Author: The Economist)
A real life true journey of a South African family to go off-grid on sustainable energy supply and usage.
How big must my solar system be was probably the first question that we needed to answer. Every family will have its own needs, its own budget and its own unique circumstances. The following factors played a large role in our decision making about the size of the system:
1. Economic viability: We calculated the expected cash payments which we will probably make over the next ten years to pay for electricity supply. We realised that it is an estimation so we required a large positive margin for error. Our calculations showed that we would pay away approximately R400,000 for electricity over the next 10 years. Next we calculated our expected electricity needs and found that we had to provide for about 15kwh daily use in high summer to about 30kwh use in high winter. These usage levels were achieved only after converting our electricity utilisation to high levels of efficiency (see our article here). We then obtained quotes and with some negotiation were able to install a system which will meet our needs at R200,000 which left us with a 100% positive margin for error. Understanding the technical requirements for calculating interest we did an easy to use calculator for establishing the economic value available to purchase a solar system (Viability Model, which we shared with you here).
2. Managing electricity consumption: The next factor which we initially underestimated but have since come to fully appreciate is how and when we use electricity.
(i) The first major impact is night and day. Seemingly obvious but the full design and financial impact is not immediately visible. With solar you will have electricity generation during the day only and will rely on batteries to get you through the night. It follows that with solar you want to schedule as much electricity consumption as possible during the day and as little as possible during the night. Yet, the supply of charge to the batteries must not be affected. Electricity hogs simply cannot run at night as the battery supply required will break most budgets. So no more tumble dryer at night, no conventional oven after dark (we successfully use a convection oven and pressure cooker), no electric stoves (we use induction plates) or electric heaters (we use gas heaters only) at night.
(ii) The second major impact is electricity generation time zones. A solar system has high electricity generation time zones (middle of the day) and low electricity generation time zones (about two hours each early morning and late afternoon). In deep summer the high generation zone runs from about 07h30 to 17h30 (about 10 hours) and in deep winter the high generation zone runs only from about 09h30 to 15h00 (only about 5.5 hours). For more details see our article on seasonal changes here. It is best to restrict the household electricity load during low generation periods. In the early morning hours, electricity is needed to fill the batteries after having depleted them overnight and the late afternoon generation again must be mostly used to ensure that the batteries are fully charged to capacity for overnight needs. Most solar systems will be producing excess electricity during the high generation time zone so the best time to run energy hogs such as the heat pump, the oven, the tumble dryer, etc. is in the high generation time zone. Note how you will have 10 hours in summer to run the tumble dryer but only 5 hours in deep winter, which will require you to spread your washing over days in winter rather than having a single washing day. We are blessed as our family business is on the same premises as our residence so we can manage electricity consumption 24/7 to schedule our use. The scheduling impact on the solar system is – the higher your ability to schedule usage, the lower your need for batteries, to a lesser extent panels for generation and back-up supply.
(iii) The third major impact is the occurrence of overcast conditions, how often do they occur and how long does these conditions generally last. Solar electricity generation falls dramatically during even only partial overcast conditions and electricity generation is negligible during rainy or dark overcast conditions. You batteries must tide you over during rainy and overcast conditions. The fact is, unless you install budget breaking battery back-up and excessive power generation, you will deplete your batteries sooner rather than later and will have a solar load shedding event. Each household will have to find their own balance between having battery reservoirs and alternative electricity back-up solutions. We have been using the municipal electricity as back-up during our transition from municipal supply to solar supply. There simply is no need to rush the process. It is best to allow the transition to take place at a leisurely pace as the household adjust to the dynamics of the solar supply. We have experimented with solutions to bridge the electricity supply gap of overcast and rainy conditions and have found that we can generally re-schedule the use of the tumble dryer and oven without too much difficulty, can use gas for cooking and have invested in a pure sinewave generator to supply electricity to charge batteries when they are depleted. These solutions are economical and cost effective. We will only relinquish the municipal electricity supply once we have thoroughly tested these solutions. We want to go fully off-grid as Joburg Power charge us a basic R605 monthly fee for electricity supply even if we use no electricity and the value of that monthly fee is significant over time (the municipality frequently increases this charge). Thus our back-up alternatives are tested for economic viability against the cost of municipal supply back-up. Not all municipalities have this flat fee cost structure so if yours does not have a flat fee then it is the best and most economical to use the municipality electricity as your back-up electricity supply. See more hereunder in Back-up Supply.
Using a solar electricity supply system requires a balance between economics, utility and household use management. I am reminded of the wonderful maximising of an available resource which I observed on a trip to India many years ago.
Designing your “off-grid” solar system is not nearly as complicated as you think. Yes, there are some technical parts better left to your supplier but you can take charge of the process. You have 4 basic components in your system; solar panels, an inverter(s), a battery bank and back-up. Terminology such as “off-grid” often deceives us into rigid thinking when flexible intelligent planning would serve us better. For us, “off-grid” simply means that we are substantially independent in our electricity supply with an economically viable back-up supply in place. The best and by far the most convenient back-up supply is still the municipal supply if one can obtain it cost effectively. Alternative back-up supply should only be considered when municipal back-up supply is prohibitively expensive such as the Joburg Power flat charge.
The four components functions are:
(i) Solar panels generates the electricity and for design purposes must generate sufficient supply to effectively meet the needs of your household. We, for example, know that our general peak need (excluding extremes) is during the winter at around 30kwh per 24 hour period. Thus we know that we need solar panels which will produce around 30kwh during available daylight in deep winter. Our panels are on this specification with no spare capacity in deep winter. It follows that we have significant spare capacity in deep summer as the available daylight hours and more intense radiation in summer almost doubles our generating capacity.
(ii) The inverters receives DC electricity from the solar panels and distributes it to charge the batteries and as AC electricity to the household. The inverters must also supply sufficient electricity to cope with electricity use and with electricity spikes which most appliances generate mostly when they switch on. We have a maximum limit of 8,000 kwh (8 mwh). It translates to an upper limit of “drawing” no more than 8,000 kwh electricity at any point in time including all spikes. Our household needs seldom exceed 6 kwh (usually only when the heat pump, tumble dryer, oven and dishwasher all run at the same time).
(iii) The batteries are your reservoir power to most importantly bridge the night and thereafter to bridge overcast or other extreme conditions. The better you manage your use of electricity combined with a good back-up system the less your need for batteries. Many countries have stable municipal electricity power as back-up and an ability to sell excess electricity back to the municipality making it possible for a solar system to be designed without battery banks or other back-up. We do not have a viable similar system in place so we have to add batteries at least and alternative back-up electricity where municipal back-up is not cost effective. It is not cost effective to provide battery back-up for all eventualities as battery banks are expensive and “eventualities” can be managed with suitable back-up.
(iv) The back-up system or systems is the final component in your system. Do not even bother with back-up alternatives if your municipality does not have a flat fee system, it is best to use the municipality as your back-up supply.
The whole question of back-up electricity supply seldom get any attention yet it is a defining aspect of any solar system implemented. As is almost always the case it is the rands and cents which matters.
Joburg Power charge us a flat fee made up of a Service charge of R105.29, a Network charge of R424.86 and VAT on both of R74.22 for a total flat fee of R604.37 payable irrespective of how much electricity we use unless we disconnect the electricity. Then they charge us on a sliding scale for electricity used, usually around R1 per kwh. So if we were living in a municipality where the only electricity charge was the cost per kwh then using the municipal electricity at R1 per kwh would have been very inexpensive.
The law of averages between a fixed cost (the flat fee) and a variable cost (the R1 per kwh) for us comes into play. We will pay R604.37 plus R1 if we only use 1kwh making the cost of that 1kwh equal to R605.37 which is extremely expensive. The truth is that we are using about 20-30kwh of back-up municipal electricity per month presently and we are expecting it to average even less per month when calculated over a whole year. That means, for us, that the municipal back-up cost per kwh using say 20kwh per month will be (R604.37+R20)/20=R31.22 per kwh. At R31.22 per kwh it is still very expensive to use Joburg Power as electricity back-up.
A 2.6kwa pure sinewave generator suitable for our needs generates 1kwh of electricity at a cost of around R10.35 in fuel consumption. Thus the variable cost for 20kwh per month will be R207. We will need to pay for the generator and we will need to provide for maintenance costs which in our case amounts to a flat cost of R70 per month which brings the generator back-up option cost to R277 per month vs the municipal back-up cost at R624.37. The difference of R347.37 is worth about R54,000 over ten years at an investment rate of only 5% pa so it’s economical for us to rather use a generator back-up supplemented by gas cooking when needed.
The final take-away:
There are two general design alternatives for a household which can manage its electricity use within a given inverter capacity of say 8,000kwh.
• Municipal back-up. If you have the municipality as a reliable, viable and cost effective back-up supplier then you really do not need any batteries at all. You only need to provide limited battery back-up where you only wish to have an electricity reservoir for unreliable municipal supply such as load shedding for the best economic balance between inexpensive solar power during best supply periods and least expensive back-up supply. You would then only install batteries to bridge the load shedding events and nights but rely on municipal back-up to bridge the overcast days. You should be able to recover the cost of a municipal backup system with battery backing for about 33% of average daily use within 5 years and it is the most cost effective system for most South African households where the municipality does not charge a flat fee. Your system must be capable of automatic switching between municipal supply and solar system supply (including batteries) which in most systems are standard.
• Generator (pure sinewave) and gas cooking back-up. The householder will have to provide a greater battery reservoir and alternative back-up solutions in those cases where the municipality imposes a flat fee. Here the householder will have to balance the cost of providing batteries and the need to provide for eventualities. To cover 5 rainy day events one would have to have a battery bank large enough to allow for a 5 day electricity reservoir. The cost benefit calculation is skewed in favour of using an alternative back-up as these extended overcast events in most South African locations are relatively rare. We have chosen a battery reservoir sufficient to bridge 2 days of conservative electricity use in overcast conditions. Those rate events where we have dark overcast conditions for more than 2 days will require running the generator to charge batteries.
The majority of the South African regions are summer rainfall areas and of these only the coastal areas have extended overcast and rainy conditions regularly. Overcast and rainy conditions in winter is a challenge for a solar system as the limited daylight hours and overcast conditions will stress any reservoir system which probably makes it best in those areas to stick with municipal back-up.
Notes by The Shroom:
The article had been written before we added an additional battery bank to the system (we now have 3x 24 sets) and since the final installation in October 2015 we have not used any municipal electricity and therefore are essentially off-grid! We have not yet pulled the plug on the municipal electricity as we would like to see the system performace during the winter and thereafter would make the final decision on turning off the municipal electricity entirely. Updates will be posted!
Our system has been designed and installed with the assistance of Jurie Venter, cellphone 083 557 6031 and email email@example.com .
Part 1: How to go off grid permanently (The System Set-up)
Part 2: Living with Solar in Gauteng, South Africa (Batteries)
Part 3: Solar Power in South Africa - Solar & Seasonality (Solar in winter)
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