Energy: Hours of Power

Let’s do some physics. We need to learn about energy and power to figure out how much hotter the Earth is getting, and how to generate the renewable electricity the world needs to end energy poverty. First, some definitions.

Energy is fuel for everything: people, plants, and winds in weather systems. It puts things in motion. It can take a lot of forms, like electricity, chemical energy, or heat, and is measured in joules. Energy is never destroyed, only transformed. A large fraction of the chemical energy content of combusted fuels in industry, transportation and electric systems is wasted, usually dissipating into the air as heat.

Power is a rate of energy use or production. Power is how much energy is provided over a certain amount of time. Appliances use electrical power measured in watts, with 1 watt equal to 1 joule per second.

How much energy does it take to power a person for a day? Food has energy in its chemical bonds, typically measured in Calories. The dietary recommendation for an adult is between 2000 and 3000 Calories a day, equivalent to around 10 million joules per day.

When we’re talking about a continuous flow of energy, the more appropriate quantity is power, the energy provided per unit time. An average person’s food supply is equivalent to around 100 watts.

A standard, energy-inefficient incandescent lightbulb uses about 40 W. Incandescent bulbs are inefficient because they release a lot of heat when they’re on. An LED bulb can produce the same amount of light with only 5 watts of electrical power, by releasing less waste heat energy.

On a clear summer day when the sun is directly overhead, the Sun produces around 1000 watts in every square meter. Energy from the Sun comes in the form of electromagnetic radiation, or radiation. This is light, visible or invisible, including infrared and ultraviolet.

On your electric bill, energy is probably recorded as kilowatt-hours (kWh). An “hour of power” is a measure of energy. Over a year, one watt of power adds up to (24 hours/day) · (365 days/year) = 8.76 kilowatt-hours of energy.

Representative power values for various sources and uses are below.

Sources of power on Earth

The Sun is the ultimate source of nearly all the Earth’s power. Other lesser (but still tremendous) sources of power include geothermal, which comes from heat diffusing upward from the core of the Earth, created from the initial formation of the planet and from the radioactive decay of certain elements, and tidal power, created from gravitational pull of the Moon and Sun. The steady rotation of the Earth gives two high tides per day in most locations on Earth, roughly when the Moon is closest and farthest from the water.

Because different parts of the planet receive different amounts of sunlight, circulations develop in the atmosphere and ocean in order to distribute energy more equally, converting thermal and gravitational energy into kinetic energy in the process. In the atmosphere, these motions take the form of air currents like monsoons and the high and low pressure systems that characterize day-to-day weather systems over much of the planet. Flows in the atmosphere and ocean are strongly influenced by the spin of the Earth, which might seem slow in rotational frequency (23 hours 56 minutes per rotation), but is rapid in speed (1670 km/hour at the equator). 

Water power can be harnessed in rivers and dams because these same atmospheric circulations carry evaporated water upwards into the sky, where the vapor eventually condenses and precipitation occurs. Rainfall that happens over the land probably puddles at an elevation higher than sea level before flowing downward into rivers and eventually out to the oceans. Gravitational potential energy is continually resupplied by the water cycle.

The world nameplate capacity, summed over all countries and all types of electricity generation, is over 8 TW (terawatts, or trillion watts). Solar and wind now each make up over 10% of the world nameplate capacity, and are soaring, up from less than a percent two decades ago. There is more than 700 times the generating capacity of solar power as in the year 2000.

The notion of a capacity factor is important when considering power generation, especially from renewables. The capacity factor is the total energy produced by a source divided by its maximum possible power, i.e., its nameplate capacity. Capacity factors that are less than 100% occur both from physical constraints (solar panels will never generate at night, for instance), and management (plants go down for maintenance or are shut off when power is not needed). For solar and wind, capacity factor is an averaged-out depiction of the intermittency of generation. Rooftop solar capacity factors typically vary between 10% in high latitudes to 25% in subtropical deserts. Offshore wind turbines can exceed 50% capacity factor.