Thought it would be cool to extract some interesting notes & share here. Please correct if you spot any mistakes.

This is a good book because it gives you numbers.

It's important to understand the quantities of things around you. I think that there's a sort of deep understanding that you can have when you know the exact figures (or ranges) for many physical phenomena. This is something I hint at in being good at the basics. Smil's book is a good introduction to such thinking.

Energy in the biosphere

On celestial matters:

• If the Earth didn't have a "buffering" mechanism for absorbing solar radiation, and it re-radiated all the solar radiation back immediately (black body radiator), the planet would stay at -18°C. So, all water would be permanently frozen. The mean surface temperature is 15°C, which is all caused by greenhouse gases buffering the radiation. Buffers = life!
• The power of the sun is 3.9x10^26 W. So, each second that the sun shines, it sends 13 orders of magnitude more energy than all fuels and all primary electricity on Earth in a given year.
• Solar flow on topmost atmosphere is 1376W/m^2, or a little more than 1 kW per meter squared. However, due to the Earth being round, in any given second, some parts have more light, some fewer (towards the poles), and some none (nighttime); the average is 342W/m^2. Then the light goes through a bunch of filtering mechanisms, such as the ozone layer, different cloud layers, and so on. So, roughly half of the radiation that hit the uppermost atmosphere hits the surface; averaging it out over the entire surface of the earth, that's around 170W/m^2.
  • This adds up to 2.7x10^24 J every year, around 5000 thousand times more than the world spends each year.
• About two thirds of global albedo is due to cloud tops.
• Many equatorial places are poorly insolated due to large amounts of clouds preventing the sun from accessing the earth. So, "tropical" doesn't necessarily mean "good for placement of pV systems".
• All the inbound radiation is eventually re-radiated back to space, but taking into account the "buffering" effects mentioned above. For example, 95% percent of surface level re-radiation doesn't end up in space directly, but rather gets buffered by the atmosphere. The most dominant greenhouse gas is by far water vapor. The Earth is essentially wrapped in a water cocoon, which protects it from getting how too fast, and getting cold too fast.
• Planetary water cycles moves around 585,000 km^3 of water every year. Around 46 PW of power is required to vaporize this water; this is half of total insolation.
• Geothermal power is comparatively small; about 44TW (~90mW/m^2 on average).

On plants:

• External biodiversity obscures the fact that there are very few metabolic pathways in organisms. That is, energy comes into the biome only through autotrophs: either phototrophs converting solar energy into mass, or chemotrophs converting inorganic compounds into mass. Literally everything else which is alive is downstream of these two producers.
• Photosynthesis reminder:

  6CO2 + 6H2O = C6H12O6 + 6O2
  

• Energy efficiency of photosynthesis is quite low.
• Chlorophylls have narrow absorption bands: 420-450 nm and 630-690 nm. Meaning that all the light outside of these bands is useless for photosynthesis. For reference, the human eye can see from 380 to 750 nm. So, around 370 nm worth of radiation. Plants can convert around 60 nm worth, so around 6 times less than what the human eye can see (and much much less if we count in the other radiation except visible light).
• Maximum theoretical efficiency is around 4% of insolation, but in practice, the global continental average is 0.33%; the average for the entire biosphere is 0.2%. In other words, if 500 photons hit the surface, only one of those photons gets converted into plant mass.
• Despite the inefficiency, continents produce around 120 billion tonnes of plant mass a year; oceans around 110 billion tonnes. Different forests produce different amounts, but e.g. continental forests produce around 0.5 kg to 2.5 kg per m^2 every year. These rates are roughly equal for grown crops.

Energy in human history

• Human caloric needs are roughly around 2-3 thousand kcal a day. There are differences across age, gender, size, as well as level of activity. The basal metabolic rate: 50-80 W for females, 60-90 W for males (again, given average sizes).
• Energy in food: pure fat gives 39 MJ/kg, protein 23 MJ/kg, carbs 17 MJ/kg. However, the body processes carbohydrates primarily, and the protein comes last (at least for energy needs).
• Total amount of energy available at higher trophic levels is reduced. For example, all the prey in the world combined has less energy than all the plant food of that prey. This explains historical dominance of vegetarianism in foraging societies (besides, meat is difficult to get, in addition to not being as abundant as plants). The energy return for foraging (the amount of joules from foraged food vs. the amount of joules expended for foraging) is around 10x. So for each joule of energy expended foraging, you'd, historically, get around 10x more, sometimes 30x more.
• Agricultural efficiency in ancient Egypt was 1 person per hectare (the cultivation of one hectare of land would energetically meet the needs of one person). That number grew very slow; some areas reached up to 5 persons per hectare. Europe (pre-industrial) never went above 2 persons per hectare.
  • There are two reasons for this: growing non-improved crops (where the conversion of energy is inefficient, and relatively little is converted into usable food, and the rest is converted into inedible phytomass) and lack of nitrogen.
  • And an additional reason: it's energetically very costly to perform agricultural manual labor (tilling a hectare of land takes up much much more than foraging a hectare of forest).
• Humans can work at around 60-80W; draft animals yield 300-500W (and also don't need specialized feed, as they can eat grass).
• Dry wood has around 16 MJ/kg. That's pretty low energy density. Compare:
  • Coal: 24 MJ/kg
  • Diesel: 45 MJ/kg
  • Gasoline: 46 MJ/kg
  • Natural gas: 55 MJ/kg
• The low energy density of wood is not a problem if the source is dense (and you can harvest a lot from a small acreage), but that's not the case: there's usually no more than 200t per hectare (20 kg/m^2), the yield is 300 MJ/m^2. Since it takes 50-100 years to return to pre-harvest state, you need to divide the harvested energy by the time to regenerate. For example, dividing by 50 years, you get 0.2 W/m^2 (of sustainable wood energy).
• Traditional charcoaling gives 0.04 W/m^2 (again, by taking sustainable harvesting figures). Pretty wasteful.
• Energy needs in pre-industrial cities: 10-30 watts per m2 (that's for heating, cooking, metallurgy, and so on). This means that if they relied solely on wood, cities would need forests 50-150 times their size nearby (for sustainable harvesting).

Energy in the modern world

• Every civilization is fundamentally solar: fossil fuels are essentially batteries for solar energy. The difference between pre-industrial and industrial civilizations is whether they're locally solar (recent phytomass) or remotely solar (old phytomass=fossil fuels).
• Coals are basically just plants in swamps, subjected to high temperatures and pressures for up to 350 million years. Many of these plants are still around, in smaller form. For example, mosses up to 30 meters in height vs. today's moss.
• "Peak oil" is unjustifiably pessimistic; there is little chance of hitting a supermassive oil field like Saudi Arabia's Ghawar field, but the total oil reserves are still pretty large.
• A third of the world's electricity is not generated by the combustion of fossil fuels.

Energy in everyday life

• The Mediterranean diet has switched to much more meat, fish, butter, and cheese; originally it was much more heavy on bread, fruit, potatoes, and olive oil. The true (original) Mediterranean diet now survives only among the rural elderly population.
• American walls (wood frame, drywall, insulation) provide around 4x more insulation than sturdier European brick walls.
• Dark roofs get up to 50°C hotter than the air temperature in summer; white or light roofs are just 10°C hotter.
• One lumen of US electric light is three orders of magnitude more affordable in 2015 than in 1900.
• "Phantom loads" (vampire power; use of electricity even when everything is turned off) is around 50 W per household in the US; combined, that's more electricity than Singapore's use in a year.
• Energy costs of various items, in GJ/t (for reference, one tonne of crude oil is 42 GJ/t):
  • excavating sand: 0.1 GJ/t
  • quarrying stone: 1 GJ/t
  • extracting construction wood: 1.5-3 GJ/t
  • cement: 3.5 GJ/t
  • adding steel to concrete makes it 3x more expensive
  • insulation: 10 GJ/t
  • a three-bedroom house costs around 500 GJ (around 12 tonnes of crude oil)
  • wheat, corn, fruits: 4 GJ/t
  • rice: 10 GJ/t
  • peppers, tomatoes, greenhouse veggies: 40 GJ/t
  • wheat contains four times as much energy as was used to produce it; greenhouse tomatoes can have up to 50x less energy than what was used to produce them
    • however, food's total energy cost is dominated by packaging & transport