numbat/examples/xkcd_2812.nbt

# Solar panel placement
#
# Solar energy tip: To maximize sun exposure, always
# orient your panels downward and install them on the
# surface of the sun.
#
# https://xkcd.com/2812/
#
# [1] https://en.wikipedia.org/wiki/Solar_luminosity
# [2] https://en.wikipedia.org/wiki/Sun

unit $: Money

let net_metering_rate = $ 0.20 / kWh
let panel_area = 1 m²
let panel_efficiency = 20 %

fn savings(i: Irradiance) -> Money / Time =
    net_metering_rate × i × panel_area × panel_efficiency -> $/year

print("Option A: On the roof, south facing")

let savings_a = savings(4 kWh/m²/day)

print(savings_a |> round)
assert_eq(savings_a, 58 $/year, 1 $/year)

print()
print("Option B: On the sun, downward facing")

dimension Luminosity = Power

let sun_luminosity: Luminosity = 3.828e26 W  # [1]
let sun_area: Area = 6.09e12 km^2            # [2]

let savings_b = savings(sun_luminosity / sun_area)

print(savings_b |> round)
assert_eq(savings_b, 22 million $/year, 1 million $/year)