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How much mains power does it take to charge an EV battery from empty to full? For motorists who are off-grid, what size of solar power inverter of fossil-fuel generator would be necessary?
Like cars themselves, electric vehicle (EV) batteries come in different types and sizes and offer different mileage ranges, too. Smaller units need battery capacity of about 40 kWh, average cars need about 60 kWh, and bigger units (like the Ford Mustang) take 100 kWh or more. Watch this space for frequent technical advances.
Electric appliances are indifferent to whether the power they receive comes from the mains grid, or solar panels, or a generator as long as the voltage and frequency are within the correct (and narrow) range centred on 240 volts and 50 hertz. Fast or slow, the required quantum is the same.
The delivery capacity of the mains/grid is not an issue for charging a single EV. Whether the charging is a trickle or very rapid depends on the charger system used between switchboard supply and the vehicle.
The main fuses in single phase domestic systems are about 60 Amps (multiply that by 240 to estimate the watts). Commercial users increase that by using multiple phases.
Generators and solar powered inverters can range from very small (half a kW) to industrially enormous. With a six kW generator a large house will have to be “aware” but rarely have to ration use of fridges, lights, television, computer, and intermittent hair-drier, washing machines, toasters, kettles, microwaves etc.
Just don’t turn them all on at once! Generators can be wired into domestic switchboards in a way that either supplies all the circuits in the house, or just one or more selected (essential?) circuits such as lights, telly, device chargers, and the fridge/freezer.
Domestic solar systems range from 1KW inverters (as a “light” back-up in mains blackouts, often with just one 100 Ah battery) to more than 10KW for those who run constantly on solar, with or without a mains supply or generator as back-up.
An inverter max of three kW is modest, five kW is average, and eight kW offers a comfortable margin for most family homes. Size comes with a price ranging from Sh50,000 to nearly a million.
Commercial users have back-up generators of 25kW and upwards, and/or battery banks of several dozen (and much bigger chequebooks).
The capacity of the inverter has to be balanced with the solar panels that collect the energy from the sun and the capacity of the batteries which store that energy as a reserve when heavy clouds blow in or the sun goes down.
The potential range of panels and batteries is unlimited (other than by space and cost), and that technology is also evolving at pace.
The current norm is panels of 400W each (at about Sh30 per watt) arranged in an array… however many are sufficient to collect enough sun to meet the demands of the day and keep the batteries fully charged to see you through the night. About a dozen panels is a benchmark for an active family home.
Batteries are the most expensive part of the system, and if you are off grid without a generator, you need a lot of them for assured round-the-clock power supply. Again, a dozen is the “comfortable” norm (nominally about 200 Ampere hours each).
The design calculations are quite complicated, even for those familiar with the relationship between volts and amps and watts. There are energy losses of around 30 percent between the panels and the batteries, and between the inverter and your light bulbs.
And to protect the current design of batteries, the system will automatically shut them off when they are discharged by more than half their nominal capacity (so you need twice as many as you might expect).
Then there is the question of how many hours of bright sunshine you get on most days and/or on average (solar panel ratings are their maximum at noon on the equator). Their theoretical “capture” is unlikely to be anywhere near max for even six hours per day.
All of this could become key wisdom to many more people with the introduction of electric vehicles, and it is one of the reasons I think that hybrid vehicles will lead the changeover in Kenya, and why petrol stations are still a very long way from being a thing of the past.
In the longer term, we might be looking at several million electric vehicles adding a load of at least 20 gigawatts (!) to the national grid.
Electricity might be cheaper than petrol or diesel, but whatever its source it is not free. EV owners will probably keep their batteries topped up almost every day, but if that works out like a full charge once a week that will mean an additional 300 Kenya Power “units” per month. That is also the rate at which an investment in solar charging will be recovered.
Current technology estimates the design life of batteries and panels to be “about 10 years”. If they cost about a million shillings to install, that is a depreciation/replacement rate of about Sh10,000 per month.
You need to be saving at least that on your Kenya Power bill to make solar viable on purely economic grounds. Reliability of supply is a separate factor.
While you do the maths, which is always likely to be marginal between one option and another (business and political economics will make sure of that).
In a save-the-planet context, bear in mind the sources of mains electricity (coal, oil, hydro, geothermal, wind, tide…) and the energy consumed in the manufacture of panels and batteries (and cars).
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