Common mistakes in energy reporting that make me groan

A conversation on Twitter recently reminded me of how often poor reporting of energy gets units mixed up - or is misleading in other ways. Here are some examples of my personal pet peeves published online : confusing power and energy, reporting power but not energy for storage, and reporting capacity but not energy for generation. I have included examples from mainstream news, trade magazines and even the IEA. If I have quoted your publication, rest assured you are not alone in making these simple, but irritating (and sometimes confusing) errors.

1) Confusing power and energy

People get kW (power) and kWh (energy) mixed up. This is incredibly common - but every time I pick up this mistake I groan inwardly and wonder about the accuracy of the rest of the article. Did the writer understand the story or just mis-transcribe from someone else? The intended meaning is usually clear but sometimes not. 

Energy is what you use when you draw power for a time. For example, an LED light bulb (0.006 kW) on for 24 hours uses the same energy as your kettle (3.0 kW) uses in 173 seconds. 

I see lots of these errors when talking about charging batteries. Just remember, you charge them with energy. In this example the writer should have said kWh per hour or just kW: The battery charges at a rate of 7.2kW per hour, and can fill from empty in two and a half hours at max speed. 2021 BENTLEY BENTAYGA PLUG-IN HYBRID DETAILS REVEALED (Sunday Times Driving)

Here is an example which uses kW per hour and kW incorrectly in the same sentence! The tariff should definitely be in cents/kWh not kW but I am not totally sure about the kW per hour - this could be for capacity (kW) rather than kWh. In any case kW per hour does not make sense in this context. The total cost of electricity is estimated at 21cents/kW per hour for every kW sold, while the tariff remains unchanged at EDL since 1994 at 9.5 cents/kW. How long will the lights stay on? (Executive magazine)

There are contexts when kW/hour does make sense - such as when considering balancing supply and demand on the grid. When the demand increases rapidly we have to bring more generating capacity online to match it. This is called the ramp rate and is measured in GW/hour.

2) Reporting power but not energy for storage.

When considering battery storage both power and energy are important. You need to know how much power you can get and for how long, and also how fast you can charge. Depending on what it is for, you might need minutes (e.g. for short term stabilising on the national grid), hours, or days. 

Here is an example from the Guardian: NSW coal-fired power plant chosen as site of Australia's biggest battery (Jan 2021) : Analysts say Origin’s plan for 700MW battery at Lake Macquarie Eraring power station and Neoen’s proposal for a 500MW storage project near Lithgow will accelerate the state’s energy transition

Buried in the text, it becomes clear that Origin's battery can supply for 4 hours, which is intended as reliable supply during peak times: He said the potential early closure of coal-fired power stations would create a need for dispatchable capacity during a four-hour window from about 4pm to 8pm that could be met by batteries.

As might be expected, Energy Storage News consistently reports both, for example: ..., a 1MW / 5.1MWh project awarded to technology provider FlexGen is expected to be the biggest of its kind in the US state of Kansas. Kansas’ biggest battery storage project awarded to FlexGen by community energy group (Energy Storage News). However I am still wondering if that is the biggest in energy or in capacity. 

In case you are wondering, 1 MW is not very big. The largest installed and running battery in the UK as of November last year was 50 MW/75 MWh (Thurcroft, S. Yorks) and the largest in the world is 230 MW/230 MWh (Gateway in California) [2]

3) Reporting capacity rather than energy generation - which makes solar look good

Energy generating capacity is a measure of how much power it will generate when running at maximum capacity. This is reasonable for a gas power station but utterly misleading for intermittent renewables, where you need to take into account the average capacity factor. For example, you might have 4 kWp solar panels on your roof. If they ran at full capacity all the time you would get 35 MWh in a year which is about 10 times your typical demand. However in practice you will probably only get about 3.5 MWh because averaged over the whole year it generates at 10% capacity (typically in UK).

Reporting in this way tends to make solar power look much better than it is because it has a very low capacity factor - globally this is about 10-20% (depending on location, angles and shading). Wind is also affected although not quite so badly. The best offshore wind sites achieve 50% or even more and 40% is not unusual for onshore wind [1]. So you need 2-4 times as much solar capacity as wind to generate the same energy.

Here is an example of misleading reporting from Renewables 2020 (IEA) - Solar projects now offer some of the lowest-cost electricity in history. Overall, renewables are set to account for 95% of the net increase in global power capacity through 2025.... Solar PV alone accounts for 60% of all renewable capacity additions through 2025, and wind provides another 30%.

With globally typical capacity factors of 15% for solar and 40% for wind, this would mean that of the new capacity, wind will actually generate 30% more energy than solar (see chart). Also if renewables account for 95% of the net increase in capacity, that probably means 80% of the increase in energy supplied - which is still nice but does not sound quite as good as 95%.

The IEA reported that 60% of the increase in generating capacity to 2025 would be solar and 30% would be wind. Assuming typical capacity factors, that means the wind capacity will generate a third more energy than the solar. This chart assumes the following capacity factors: 60% for 'other', 40% for wind and 15% for solar, based on data from [1]

So now I've got that off my chest - let's resolve to do better in 2021.

[1] Average annual capacity factors by technology, 2018 (IEA) 

[2] UK’s largest battery storage project at 640MWh gets go ahead from government (Energy Storage News) Nov 2020