Don’t plot renewables and nuclear capacity on the same graph

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I write repeatedly about my number two pet peeve: the continued use of renewables capacity figures to show the rate at which renewable energy is growing. This is problematic for a number of reasons. First, it lumps wind, solar, and hydroelectricity into one category despite the fact that they all have very different capacity factors.

As a result it makes it seem that solar is growing more quickly than it really is in relation to wind and hydro. But more importantly, looking only at capacity makes it seem as if renewables are growing much more quickly than they really are in relation to fossil fuels.

A good example of the use (and abuse) of capacity figures come from Friends of the Earth. Here is how they describe solar in Germany:

In Germany, the amount of solar power installed grew from 2 Megawatts (MW) in 1990 to 37 Gigawatts (GW) in 2014 – 37,000 Megawatts.
That’s equivalent to nearly half of the UK’s entire power generating capacity. On one sunny Sunday in May 2014, Germany’s solar panels and other renewables provided almost 75% of total national electricity demand. [My italics]
It is hard to imagine a more misleading string of statistics. The above numbers will lead the average reader to imagine that solar power in Germany makes up the majority of supply and is the equivalent of half of Britain’s grid. Yet, last year Germany got 34.9 TWh of electricity from solar panels, whereas Britain generated a total of 335 TWh of electricity from all of its power plants. Instead of being the equivalent of half of Britain’s electricity grid, German solar is the equivalent of 10% of it.
This is what happens when you install solar panels in high latitude and cloudy countries: innumerate environmentalists get confused.
A more common issue is the continued plotting of the growth of renewables and other forms of electricity on a single graph using capacity only. The following graph of future additions to global electricity generation appeared in a recent Bloomberg article:
This fairytale figure is misleading in numerous ways. Solar capacity additions appear to be much greater than fossil fuel additions over the next 25 years, yet in generation terms they will clearly be lower.
The graph, if it is to be meaningful to the average person should not be in terms of capacity, but generation.
Let me show this further by comparing solar and nuclear in Germany in the two graphs below. On the left we have the commonly used capacity graph. On the right is a graph showing actual generation from nuclear and solar. The one on the left tells a very different story the to the one on the right. The reason is simple. German’s nuclear power plants produce around 9 times more electricity per unit capacity than their solar panels do.
The above graph should make it clear. Capacity only graphs are misleading. Now, if only we could pass a law to ban them.

3 thoughts on “Don’t plot renewables and nuclear capacity on the same graph

    Bill Schutt said:
    July 6, 2015 at 12:51 pm

    It’s a pity we don’t have a objective mechanism to ban bull shit. It would completely silence both the climate-change-denialists and the anti-nukes.


    Mark Miller said:
    July 11, 2015 at 12:59 am


    It’s been a few years since I had to manage the instantaneous generation (watts or energy) of our PV system (6.12 kW STS DC rated, 5.22 kW AC rated) to our load over time kWh (or power). Nothing brings home the difference between the two concepts like having to pay your utility company (during the highest CASIO system wide demand ever seen) $.30+ kWh for any power you send to the grid due to programming issues with our digital TOU meter and how the meter data was managed by the billing department of our utility. A PG&E line technician had previously been out to our place to determine what our various loads were so it wasn’t rocket science to figure out how to match our generation output curve between noon and 6 PM.

    Thank goodness I wasn’t really having to deal with instantaneous loads as I would of tripped more than a few fuses. I was just trying to match my generation to our load on average over 6 hours. Lots of spikes in kW per second occurred throughout the 6 hours as the amperage to start up my well pump, HVAC systems, hot water tank, electric dryer, etc. is higher than what it takes to run the equipment over time. I would of just turned my inverter off if I had to deal with increments of time less than an hour or so. If I had the knowhow of a team of electrical engineers from NASA, like the guys who were able to figure out how to get the crew of Apollo 13 home, I might shorten that time frame.

    And yes a few of the sales folks who provided us quotes a decade ago for some self-generation used STS capacity rating (DC, potential max output of a PV system in a lab with perfect pitch and insolation) vs the closer to real world output of a PV system (the AC output through the inverter to the grid interconnector) to sell us their solution. They only way to compare one firms project proposal to another’s was to convert all the quotes into power (kWh) units per month/year. As we are on a Time of Use meter we had to make a few SWAG’s about day to day variation in both our demand for power (kWh) and our expected generation as well as our cost to use a kWh from the grid at peak times was about 400% more than what it would have been if we could shift our demand to off peak times.

    I agree that a capacity value is nice to have but without a capacity factor for the specific time frame being evaluated (hour, day, month, year) it isn’t sufficient to take any action (i.e. to use more, or less, power at any given time to optimize costs, CO2 per kWh, work flow, transmission congestion, etc.., or to determine which source of carbon neutral generation should be given priority as far as getting approved to get into the loading order).


    Bill said:
    July 12, 2015 at 1:29 pm

    For the generation graph you should use GW*years for the energy unit. This would make the relationship with capacity much more obvious.


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