Personal Energy Usage
Inspired by the posts of UCSD’s Tom Murphy on societal energy consumption (see also The Limits of Growth), I wanted to dig deeper into the implications of our energy consumptions habits at an individual level.
An individual’s energy consumption
How much energy do Americans consume on a per capita basis? The USA had roughly \(3.3\times10^8\) people in it in 20191 and we consumed 100 quadrillion BTUs in 2019 (roughly \(1\times10^{20}\) joules)2. This works out to \(3\times10^{11}\) joules per person in 2019. Given that there are roughly \(3\times10^7\) seconds in a year, we find that each American draws a continuous \(1\times10^4\) watts.
That is to say, our energy consumption is equivalent to every American drawing 10,000 watts at all times. That sounds like a lot!3 Obviously, this varies greatly at the individual level and these numbers include energy from electricity as well as transportation and manufacturing. So while noting the previous caveats, I’ll be using this 10 kW figure as a proxy for the energy required to maintain an American lifestyle for the rest of this post.
As a baseline for comparison, we can also calculate how much energy our bodies consume just by existing. Assuming we need 2000 calories a day to sustain ourselves, we find that humans draw about 100 watts to sustain life4. Thus a modern American lifestyle requires two orders of magnitude more energy than the basal energy requirements of sustaining a person’s life.
Sustaining an individual’s energy consumption with solar
Let’s say I wanted to sustain my lifestyle’s energy consumption by capturing the most available and abundant renewable: solar power. How much land would I need? The solar energy hitting ground level varies considerably depending on your location. Fortunately, the NREL has mapped out the availability of solar power.5
We’ll start with some optimistic numbers. Assuming I have a chunk of land out in the desert southwest. The NREL data suggests that we could expect roughly \(6\text{ }\frac{\text{kWh}}{\text{m}^2}\) per day on average. Our 10 kW constant draw corresponds to 240 kWh per day. If we were able to magically capture all the solar energy we were receiving, then we’d need roughly 40 \(\text{m}^2\) to sustain my 10kW draw. This is a 6.5m\(\times\)6.5m (21ft\(\times\)21ft) patch of land.
Now, how much land does that represent when we make our assumptions more realistic? Modern solar panels vary from 6% to 40% efficiency.6 We’ll use 15% as a ballpark figure for economical mass-produced panels. If we’re only able to capture 15% of the energy hitting our patch of land, then we’ll end up getting roughly \(0.9\text{ }\frac{\text{kWh}}{\text{m}^2}\) per day on average. This means we’ll now need 267\(\text{m}^2\) to satisfy our energy requirements; a 16.5m\(\times\)16.5m (55ft\(\times\)55ft) square.
If we do the math for our basal energy requirements, we find we need about 3\(\text{m}^2\) to get our daily 2000 cal equivalent using mass-produced solar panels.
Sustaining an individual’s energy consumption with low-tech solar
The math above is both incomplete and assumes too much. One of the many ways it’s incomplete is that electricity doesn’t make for a satisfying meal (even 10 kW!). So, if we want to live on our little desert plot, we’ll have to figure out a way to convert our electricity into something more palatable. I’m interested in exploring this further, but for now I’ll just note that this introduces another inefficiency into the system which will make our 2000 cal/day requirement more expensive at the generation stage.
That aside, our analysis also assumes too much. For example, it assumes that the energy consumption of our lifestyle would remain unchanged if we were interested in living off-the-grid in the desert. I suspect, though, that without the infrastructure of society making it easy to consume vast quantities of energy without necessarily realizing it (highways, airplanes, internet shopping anyone?), our 240kWh per day might be overkill for living a full and satisfying life.
We also assume that we will be able to import infrastructure from larger society to serve our needs. However, when we build artificial general intelligence and–oops!–turns out it’s more Terminator than Star Trek, we may not be able to source our solar panels from the wider world. So it’s probably worth checking out the math using nature’s original solar panel: plants.
So let’s see how the math works out using plants.
References
- The feasibility of switchgrass for biofuel production
- Net energy of cellulosic ethanol from switchgrass
TODO
- How much land do we need to cover an individual’s energy needs (optimal vs realistic)?
- Math for storage buffer of energy
- How much land do we need to cover an individual’s energy needs using “low tech” solutions?
- How do we convert energy to satisfy Maslow’s hierarchy (food, water, etc)?
- What would a low-tech sustainable lifestyle look like? How much energy/land would be required?
- Is there a low tech way to create solar panels? Would would a completely off-the-grid existence look like?
- Storage? How much space for storage of energy?
- Culmination: what does a sustainable off-the-grid life look like with AND without modern society surrounding you?
- Plausibility of the argument re offloading energy consumption to other societies via overseas manufacturing etc.
-
I find it less surprising when converted to gallons of gasoline. This 10 kilowatt continuous draw is equivalent to burning 7 gallons of gas a day to sustain each of our energy demands. That I find this formulation less surprising probably suggests that my intuitions around the energy stored in gas are off. ↩
- \[4.184\times10^3\frac{\text{joules}}{\text{cal}} \cdot 2\times10^3\frac{\text{cal}}{\text{day}} = 8.36\times10^6\frac{\text{joules}}{\text{day}}\] \[8.36\times10^6\frac{\text{joules}}{\text{day}} \cdot 8.64\times10^{-4}\frac{\text{day}}{\text{sec}} = 100 \frac{\text{joules}}{\text{sec}} = 100\text{ watts}\]
-
Solar Resource Data, Tools, and Maps (nrel.gov) and Global Horizontal Solar Irradiance (nrel.gov) ↩
