Nikolai Kardashev (1932-2019), a Russian radioastronomer, was involved in the early days of SETI. To set constraints on what we might observe from Communicative Civilizations defined different levels of energy access and utilization – the Kardashev Scale as it is called now.
His original paper, Transmission of Information by Extraterrestrial Civilizations (1964), described the levels as:
(I) Utilizing 4 x 1019 ergs per second.
(II) Utilizing 4 x 1033 ergs per second, equivalent to the output of the Sun.
(III) Utilizing 4 x 1044 ergs per second, equivalent to 1011 solar units. The Milky Way radiates about 30 billion solar units.
NB: For those who grew up using SI, 1 watt = 107 ergs. The erg is from the old astrophysical c.g.s system of units – centimetres/grams/seconds. One unit of force (a dyne) applied over one unit of distance (a centimetre) giving one unit of work/energy (an erg).
Kardashev meant the First Level as being at the same level as ourselves – using about 4E+19 ergs per second (4 terawatts in S.I.), which was a valid estimate for 1964. About 1 kW per capita. That’s risen overall to about 2.6 kW per capita in the present day.
Since the original paper, other researchers have expanded the meaning of Level I to be equivalent to the energy resources available to the whole planet, which is dominated by the Sun’s input. Earth intercepts 174,000 TW (terawatts) from the Sun, absorbing 122,000 TW of that, reflecting the rest back into space. A significant amount is absorbed into the oceans and atmosphere, which is released as a net flow of wind and thermal energy from the equatorial zones to the Poles, to dissipate as a net heat loss to space. About 5,000 TW (5 petawatts) is radiated from both Poles from memory of my old USQ Climatology course notes.
Humans presently utilize about 20 TW of various energy sources, most being directly or indirectly based on Solar Energy. Fossil fuels are stored solar energy we’ve dug up, while wind and solar are directly powered by the Sun’s input. [Global Energy Reference: IEA Data & Statistics ]
If human Civilization peaks at 10 billion people, then at present usage levels we max out at 26 TW or so. Just 0.05 W/m^2. Kardashev Level I, if equivalent to Net Solar input, means 240 W/m^2. So either 4,800 times more people or 4,800 times more energy release per person to reach K-I. The problem is that whatever we use *must* be radiated away as waste heat eventually. Earth is already suffering an excess of about 4 watts per square metre thanks to global warming. Replacing all the energy usage with wind and solar at present day levels would have a negligible effect and the global warming issue would fairly quickly disappear as we go into negative CO2 levels – i.e. no longer producing CO2 for energy or any other industrial process, allowing reforestation to draw down the excess.
Clearly a Kardashev I Civilization needs to be Space-Based to utilize the 122 petawatts Earth receives from the Sun. So what does that look like?
Let’s imagine access to Space on a Grand Scale. If all the issues of Space Elevators can be overcome, then the energy to lift a mass from the Equatorial Space Port to GSO is 48 MJ/kg. The 122 petawatts of Sunshine represents lifting to space over 2.5 million tons per second! 80 trillion tons of freight per annum. As the 2020 total Sea-Freight traffic was 11 billion tons, 80,000 billion tons launched to Geo-Synchronous Orbit does seem excessive.
However what if the destination is further afield? Imagine freighters to Mars travelling at 150 km/s, needing about 225 times the energy of the Space Elevator. Thus total freight traffic of 356 billion tons.
What if we go faster? A 1 gee trip to Mars when it’s at its maximum distance from Earth requires a delta-vee of 12,400 km/s. Using beamed power to the rocket, then total energy expenditure per kg of vehicle at its maximum efficiency is 120 TJ per kg. That’s a power output of 95 MW per kg of vehicle over 1.265 million seconds trip-time. About 1.3 million tons of space traffic can be supported between Earth and Mars per annum at such power levels. When Earth and Mars are at their closest the delta-vee drops to 4,600 km/s and the specific power needed is 36 MW/kg. Alternatively lower accelerations can be used, allowing more cargo for the same energy expenditure. As trip-time is inversely proportional to the square root of the acceleration, the maximum amount of cargo that can be shipped per year per TW.year probably has an optimal acceleration somewhat lower than 1 gee. But we will reach Kardashev Level I when we’re shipping a modest amount of cargo between the planets at high speeds.