Scenario: You are working from home and you promised the client the document will be sent to him before, 6 p.m. You were chasing this client for months now and when he says email me the document you know he wants to buy. He will however buy from anyone who comes first. He needs a “quick supplier”, he says. You have to work hard here so you spend the whole day typing it up. At 5.45p.m the document is almost ready to send. You checked for errors and the document is good to go, but before you hit SEND your desktop computer switches OFF, in-fact everything switches OFF and you hear your wife’s snapping in the kitchen, she had her sadza on the stove. Electricity has gone and it’s probably coming back at 10p.m. And you remember Jimmy is chasing that client as well.
Power outages are rampant in our country. Often a 5hr blackout has resulted in the major loss of revenue. Phone operators claim they lose 20% revenue in unmade calls. With this in mind we decided to give our readers out there a step by step guide of how you can run your house completely off solar, is it possible? Yes but requires smart thinking. However if you are reading this you are already smart since you are interested in solar power.
Solar equipment usually has 4 components, the solar panels, the battery bank, charge controllers and the inverter:
- The solar panel is responsible for converting solar energy to electrical.
- The battery bank stores energy for use during the night
- The charge controller regulates energy flow into batteries, hence protects the batteries from too much power
- The Inverter converts 12V DC power to 220V AC power. Usually household appliances use AC power.
Calculating how much equipment we actually need requires three stages, namely: Power Assessment, Power optimisation, Final calculations
“Power assessment” are two lengthy words that mean: Let’s figure out how much energy you need to run your house. This obviously has to be known for us to proceed. Your requirements may not be similar to your next door neighbour’s since you have an extra home theatre and your son has a computer in his room. There are three ways of figuring out how much energy you are using:
- If ZESA has installed a prepaid meter at your home that means you now buy units. What we need to do here is figure out how many units you use per month. If you are anything like my boss then you just know you pay $40 to ZESA per month. So let’s convert that $40 dollars to energy:
- Firstly ZESA charges $0.02 per unit for the first 50 units, thus the first 50 units cost (50 x 0.02) which equates to only a dollar. Thus in our $40 electricity bill the first dollar buys the first 50 units.
- On top of the first 50 units ZESA charges the next 250 units with a rate of $1.05 per unit. These units would therefore cost (250 x 1.05) $262.50, if you used them all. But my boss doesn’t pay that much that means he doesn’t use them all and he doesn’t overlap into the third category where ZESA charges $2.95 per unit for the next 700 units.
So how many units in this category does he actually use? The remaining $39 from the first 50 units pays for the next 250unit. In this category he pays for (39/1.5) 37. 14 units
So in total my boss uses 50 + 37.14 = 87.14 units per month. Wow he is so energy efficient.
But we don’t deal with figures in units here. ZESA’s single unit is actually 1kWh worth of energy. So my boss uses 87.14kWh in a month
So that means roughly his daily energy usage is 87.14/30days = 2.9kWh
2. Maybe ZESA hasn’t yet installed a prepaid meter at your home, which means you receive electricity bills at the end of the month. Your units have been clearly laid out for you
This bill shows this client (Mr Boss) used 1300 units. That is 1300kWh. Phew, if I was Mr Boss I’d sweat.
3. The third and more laborious approach. If you have no electricity at all where you are, that means you don’t buy any units and you don’t receive any electricity bills. So how do you calculate the amount of energy you need? Still easy but a bit longish.
Almost every gadget that you have in your house has a power rating or label on it. This may be in the product’s front or at the back.
The amount of energy the product consumes is equal to its power rating times the number of hours the product in actually ON. In the case of a 100W bulb that is switched ON for 6hrs daily. This bulb alone consumes (100W x 6hr) 600Wh worth of energy. The laborious part in this method is going through your whole house noting down each component that uses electricity, its power rating and the number of hours it is switched ON during the day. After this you multiply all power ratings times their hours of use. That leaves you with energy consumption per each gadget. Lastly you add all the energy consumptions to get your total daily consumption.
To make an example I asked my boss about his electrical gadgets and this is the list I came up with (or so he says).
|Gadget||Power rating (W)||Hours of use in a day||Energy consumption|
|2 small plates stove||750 x 2||1||750 x 2 x1 = 1500|
|Microwave||1500||1||1500 x 1 = 1500|
|Refrigerator||250||5||250 x 5 = 1250|
|Television||250||4||250 x 4 = 1000|
|Desktop||250||2||250 x 2 = 500|
|Laptop x 2||150 x 2||1||150 x 2 x 1 = 300|
|Lights x 4||60 x 4||5||60 x 4 x 5 = 1200|
|Total energy consumption||7250 Wh|
The power ratings and hours of use above are rough estimations, don’t sue me if you go by them. However ZESA in their good heart provided us with a more precise list here. Go find out how much power your gadgets use.
Now that our power assessment is done, We know how much energy your equipment is consuming, it’s time for power optimisation
Power optimisation is just a fancy way of saying let’s cut down our current consumption by getting rid of some huge energy “gobblers”. These energy gobblers consist of things like stoves, microwaves, geysers and anything else that has a power rating over 800W. Simply find an alternative for it, e.g. in place of an electric stove use a gas stove, in place of an electric geyser buy a solar one (yes water does boil in them), and in place of a microwave, use a gas oven. These energy gobblers have to be removed otherwise you will cover your whole roof with solar panels. However some energy gobblers are impossible to remove hence we need to content with them, for now. Examples of these are borehole pumps and booster pumps. Well let’s just power them up with solar until someone invents something new.
Replace less efficient products with more efficient ones. A refrigerator will consume more and more energy when it grows older, you have to either service it or replace it. Lights are also an easy target. What’s important for a light is the lumens/watt. This shows how bright it becomes per wat. If you still have tungsten lights, replace them with either energy savers, or lead lights they have more lumens/watt.
Now that we have optimised our consumption, lets calculate how much of the solar equipment you actually need.
Solar obviously converts energy during the day, calculating the amount of energy it consumes is a matter of multiplying its power rating times the number of hours it is exposed in the sun times its efficiency.
Solar’s energy = Solar panel power rating x hours of exposure x solar panel’s efficiency
A fixed solar panel is actually exposed for 6 hours during the day. Yes I admit that a day has more hours than that but when the solar is fixed it doesn’t turn to face the sun hence when the sun is at an angle the solar absorbs less energy than when it faces the sun directly. Clouds are also other factors to consider. So 6 hours is the average time the solar converts energy at a rate equal to its power rating.
Remember my boss uses 2.9kWh per day.
Solar power rating = Energy demand/(Hours of exposure x solar panel’s efficiency)
That means my boss needs 2.9/(6*0.8) = 0.606kW rated solar panel.
This is a 606W solar panel. Of course he can buy one big 606W solar panel or he can use smaller panels with power ratings that add up-to 606W. If he uses smaller ones he needs to join them in parallel. If you don’t know what joining in parallel means click here.
Batteries are measured in Amp hours (Ah). For you to be able to store energy you convert during the day you need batteries that are big enough to contain that energy. So figuring out how many batteries you need is a matter of determining how much energy a single battery can hold. This of course is easy. You just multiply the batteries Amp hours (Ah) times the voltage the battery provides. Many batteries sold in Zimbabwe are 12V batteries. So let’s figure out how many batteries my boss needs to store his energy
Energy storable = Ah x Battery voltage
Ah = Energy storable/Battery voltage
Ah = 2.9kWh/12 = 242Ah
One average sized car battery is 120Ah. That means my boss needs at least 2 car batteries to store his energy
DC to AC converter (Inverter)
This is the unit that converts your stored DC power to AC power that is usable by the household appliances. Its rating has to be equal to the maximum power that needs to be satisfied at a given instance. For example, my boss has all four of his lights ON between 6p.m and 10p.m, during this time he also watches TV, he also plays a slideshow on his laptop while it charges, at the same time his child takes time off from homework and plays on the desktop machine. Thus this for him is the time for maximum power consumption. So his converter needs to be big enough to satisfy this demand at any given time. Inverter’s power rating = the sum of power ratings that can be used any given time.
Inverter’s power rating = Lights + TV power + Laptop + Desktop.
= (60 x 4) + 250 + 150 + 250 = 890 W
I would really just tell him to buy a 1kW inverter, because who knows someone might just decide to switch the microwave ON in addition to the already functional units. Thus it’s safer to buy a converter large enough to power everything at once, but that may be expensive, so this approach is more pocket friendly.
This is the unit that regulates energy as it flows into the batteries. You need it because your batteries might get overcharged or receive too much power at one time. If they break batteries are expensive to replace.
Charge controllers are measured in Amps. How do we determine the amperage (number of amps) of the charge controller that we need. Solar panels come with amperages written either on the packaging or on the back of the panel. The charge controller that you need must have amperage equal to that of the solar panels added together.
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