Understanding the true electricity output from a solar panel

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

31 March 2019

Studies have shown that 86 per cent of people will believe any data quoted in a report without questioning or verifying it. I am happy to learn that readers of this column are in the minority 14 per cent that that do check whether the figures given are indeed correct.

Last week, I wrote that the cost of the recently commissioned 168MW (168,000kW) Olkaria-V geothermal power station was Sh135 billion. I then went ahead to state that this works to about Sh80 per watt.

Quite a good number of readers responded to point out that my calculation was way off: the answer is ten times what I wrote. That is Sh800 per watt; not Sh80!

Now, at this point, before some one comes out to challenge the data that I gave in the first paragraph above (the 86 per cent), I must come clean that I made it up!

So, the question of solar versus geothermal electricity becomes even more interesting. Why would one buy a power plant at Sh800 per watt while there is another one going for just Sh200/W?

The answer lies in the number of hours the power plant can generate electricity in a year. Assuming there are no breakdowns, a geothermal plant can run for 24 x 365 = 8,760 hours. Thus, a one-watt installation, generates 8,760 watt-hours (8.76kWh) of electrical energy.

With an average of 8 hours of sunshine per day, Garissa is one of the sunniest (not hottest!) place in Kenya. Thus, a one-watt solar plant there should generate 2.92kWh of electrical energy.

But that’s not all: the rating of solar power panels is based on peak production. That only happens for the short moment when the sun is directly above the panel.

For this reason, some solar plants employ a tracking mechanism that automatically follows the movement of the sun across the sky. However, the cost-versus-benefit of such systems is usually too low to warrant installation.

The net result of the raising and setting of the sun is that, on average, a solar panel effectively produces only half of the rated peak power. Thus, instead of the 2.92kWh from a one-watt panel in Garissa, we get just 1.46kWh.

So, now the choice is between buying a Sh800 plant that has capacity to generate 8.76kWh or a Sh200 one that can produce 1.46kWh per year. Interestingly, the expected lifespans of both are approximately 25 to 30 years.

Which one would you choose?

In closing: I must be clear that I am not opposing the building of solar power plants. My aim is to simply dispel the widely held notion that this is the cheapest source of electricity. It is not! But it remains an important part of the energy mix. In fact, KenGen is planning to build a 45 MW solar plant near its Seven Forks system.