What are the known / alleged problems against using energy as currency? What are the known (and alleged) problems of using Joules (i.e. a measure of energy/work) as a currency? I tried to find such idea in Google (searched terms like "technocracy", since someone told me that technocracy had such idea), but I'm not clear about where to search in.

(Edit : asked in contrast to fiduciary/trust/faith-based currencies)

(Edit 2 : found this link, but i'm not quite an expert in eco to judge the possible problems, not exposed in the article)

(Edit 3 : This one is my fault since I asked a not-so-clear question, becuase my lack of knowledge. I have to clarify that this proposal is not mine, and I want to just study the concept. In this way, many principles I should enumerate are:


Energy taken into account is the "seizable" energy, like solar or fossil.
Fossil energy can be stored. Fossil industry would be credited for that.
Solar energy can be captured, transformed, and injected into the power network to be credited for that [as currently is, in many countries, right now], and have such energy redeemed later; the only difference with the current situation is that [the modern proposals support that] the economy could be based in such energy, instead the vice-versa case.
The exchange act would not involve energy directly, since a wallet of that type could be quite dangerous to carry in the ... ¿pocket?.


In this sense, I think useful answers have already been provided to cover the expressed points, althought I don't want to consider the answer as just-closed, but narrow the extent of the issues I want to understand about).
tl;dr: Energy is one of the worst ideas for a currency that anyone's had.
It's not fungible, it's not a store of value, and has absolutely no scarcity. Let's look at the details.
Commodity currencies are problematic
jmbjera has listed core problems with the gold standard, and with commodity currencies generally: its supply is managed not by a democratically-elected government, but by one industry, so you remove a hugely useful tool from the policy-makers' toolbox; and you expose yourself to very high short-term volatility.
But even amongst commodities, energy has unique issues that make it worse than useless as a currency.
Energy is ubiquitous and bountiful
There's absolutely enormous quantities of it lying around everywhere. One kilogramme of anything contains mind-boggling amounts of energy - $E=mc^2$, right? c, the speed of light, is huge. Squaring it makes it a lot bigger (when we're in SI units). $3\times10^8m/s$ squared becomes $9\times10^{16}$ - that's how many joules of energy in 1kg of matter. Any matter. Soil, platinum, flesh and blood, it doesn't matter, it's all the same amount of energy: 90 Petajoules; aka 25 Terawatt-hours. That's a few weeks worth of electricity supply in Britain. So an energy currency means one kilogramme of sand is equivalent value to sufficient electricity to power that G7 country for a few weeks.
Additional energy arrives on Earth every second of the day
So not only is there immense quantities of energy lying around for free, but vast amounts arrive every second from our friendly neighbourhood fusion reactor, the sun. Irradiation on the earth is about 130 Petawatts. That's about four orders of magnitude more than human energy consumption. Or, to put it another way, sunlight on one ten-thousandth of the Earth's surface brings energy in at about the same average rate as global energy demand.
Energy's value varies hugely in time and space
But it gets worse, as far as currencies are concerned. See, energy's value differs constantly in time, in space and in form. You must have noticed that wind and sunlight are free - no one's charging you for them - and they're energy. Whereas you have to pay for natural gas. You probably pay even more, joule for joule, for diesel or petrol (gasoline). And you will pay a lot more for electricity. On top of that, one unit of electricity has a very different price depending on where and when you buy it, and the quantity you're buying it in.
Energy's form drives its economic value. 1 Joule is rarely worth 1 Joule
Electricity is a really high quality vector of energy: it can do huge amounts of work. But you can only use it in the instant it's generated (ok, you can store it briefly in a capacitor, but that will cost you, too). Heat is a really low quality vector of energy: and the closer the temperature is to ambient temperature, the less work it can do. i.e. the less economic value it has. Now, we've got enough problems with variable marginal values of money as it is, without this scale of problem. The other thing is, if you're storing your currency of energy as heat, it's losing its value every second as it cools off. Yet, at the same time, cooling has economic value, in aircon and in refrigeration. So that's a lack of energy that has economic value.
Many energy transformations add economic value but consume energy
Refining crude oil costs you energy and other resources. And the end product is more valuable, which is why people do it, and make a profit at it. But that would be absurd if energy were the currency.
On sophistry
I've seen the argument for an energy currency before. Usually, when you dig deeper, they're not really talking about an energy currency. They're actually trying to sell something. Typically, but not always, fossil gas. So their proposed energy currency turns out to be something very like gas futures contracts (which have the usual commodity-currency problems: control of supply lies with one industry, not democratically-elected governments; and the whole economy is exposed to high short-term volatility). Now, as we know, we've got far more gas than we can possibly burn (see my colleagues' paper in Nature on this), and the gas industry is starting to get nervous. And so it should. This desparate bid for a &quot;gas currency&quot; is just one more bit of special pleading and rent-seeking from what must become a dying industry.