Lighting Ratio 6:4; Cooking Ratio 6:1

In trying to explain to my brother why Solar Electric Cooking might have reached a tipping point, I found a new way to reframe the problem. My brother has no background in energy, and came at it as a generally educated UK citizen. When I explained that the current emphasis was on lighting, he began to ask more detail about the energy content and how much energy the battery will need to store. In struggling to explain without using detailed numbers, I realised something I hadn’t realised before.  There is a considerable ratio advantage!

When solar lighting is installed, we are told that for Africa we should use a 6 hours mean figure of sunshine to generate the energy.  Taking this 6 hours then, solar lighting system design is such that it assumes the light will be consumed over 4 hours. I had to explain that most home solar lighting is around the equator, and unlike the UK that means that it gets dark consistently at around 6pm, and that most people go to bed by nine or ten. So in sizing the system, the ratio of energy generation to energy consumption is approximately 6:4 – 6 hours of a 20 Watt panel, might give four hours of six 5 watt bulb (if there were no losses – yes, yes, I know thats not the case, but for illustration only…….).

However when we are talking cooking, we only want to cook for an hour (or at least that is a key assumption we have made). That means that the ratio of energy generation to energy consumption is 6:1. In our case we have said that we want 500 Watts stove initially, (but eventually 1 kW). At 500W that implies a 80 Watt panel (again ignoring for the moment any losses).

This may all seem a crazy way to explain things for an engineer, but for my brother the idea struck home. He could see how people who were currently focused on trying to generate 4 hours consumption at 30 W (and therefore installing 20 W panels) might implicitly expect a cooking system at 500 Watt to require solar panels at 330W. But by talking about the difference in ratio, my brother could see that it doesn’t require 16 times the panel size but only 4 times the panel size.

For an engineer they will think in terms of kWhrs and energy (Joules), however for the lay person this reframing might make a difference

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