Energy and Power

Probably most people reading this have a good grasp of this subject.

HOWEVER read the section on ‘Nameplate’ and ‘Actual’ capacity.

The SI unit of energy is the Joule J with prefix Kilo, Mega, Tera, Peta etc

Power is the rate of energy ‘expression’ measured in Watts W.

1 Joule per second is 1 Watt

(As an important digression, energy is not created or destroyed, just converted into a different form)

(‘Energy Loss’ implies the effects of entropy and means that energy went to a non useful form)

To confuse things, Energy is often described as Power for a certain amount of time.

Thus the normal domestic electrical power bill is described in Kilo Watt Hours.

The old fashioned household electric bar heater was 1KW per bar, thus 1 bar on for 1 hour was 1KWhr.

A KWhr is 3.6MJ there are many online coverters

An electric car uses approx 15-20KWhr per 100km

(so for 20,000km per year you need 4000KWhr, 4MWhr, 14.4GJ. So at least 4KW of solar panel and 40KWhr of storage just for charging your car)

(to multiply confusion, battery capacity is often described in amp hours which is meaningless without reference to the voltage)

As there are 3600 seconds in an hour 1KWhr is 3,600,000Joules or 3.6MJ

Domestic power use is usually measured in KWhrs (kilo, thousands)

Industrial power use in MWhrs (mega, millions)

National power use in TWhrs (terra, billions). However PetaJoules are a preferred standard.,-work,-and-heat/petajoules-to-terawatt-hours

This inconsistency of units for power and energy requires mental agility when comparing ‘apples and oranges’

Stored power, energy (simplistically) in coal is approx 22GJ per tonne, useful energy will depend on the moisture content etc. Coal can commonly be 30-40% water. Diesel moisture content would be 0%.

(Moisture contents are another source of confusion, as they can be measures as a ratio of water to dry product, or as a proportion of water in the wet product, thus raw pine is 50% water but sometimes will be described as 100% moisture content ie there is contained the same weight of water as dry wood in the wet sample.)

Energy content of some fuels (indicative only)

  • 1 kilogram of dry wood 5.3 kwh 19.0 mJ>
  • 1 kilogram of coal 8.1 kwh 29.3 mJ
  • 1 cubic metre of natural gas 8.8 kwh 31.7 mJ
  • 1 litre of petrol 9.1 kwh 32.6 mJ
  • 1 litre of diesel-oil 10.0 kwh 35.9 mJ
  • 1 kilogram of hydrogen 33.6 kwh 120.8 mJ
  • 1 kilogram of Uranium 235 22.2 million kwh 80.0 million mJ

Power Generation

Nameplate capacity/baseload/wind and solar/EROI

Certain electrical generating plants are described as ‘baseload’ which 24/7/365 can, and usually do, produce power at their ‘Nameplate’ capacity. So for example a 1GWhr coal plant might be a baseload plant and runs at that capacity day and night all through the year. Likewise geothermal and ‘run of the river’ hydro-electric usually are baseload.

Solar panels operate on a totally different paradigm. The solar panel’s nameplate capacity is usually the amount of power it produces when exposed to 1KW sunlight per square meter. Solar panels have a power efficiency of between 10%-20% (light energy into electricity). Thus a 1 square meter panel exposed to 1KW of sunlight may produce 100Watts when full sunlight at 90 degree incidence shines on the panel.


  • The sun by definition does not shine at night
  • Day length varies with seasons
  • The sun’s position moves thus a static panel does not intercept sunlight at 90 degrees
  • Non direct sunlight and cloudy days produce minimal power

As a rule of thumb, the annual output of a domestic solar panel is its nameplate power in watts as KWhr. Thus a 100W panel has an annual power output of 100KWhr. If the panel is 1 square meter and produces 100KWhr (360MJ) per year its average power output 24/7/365 is 10Watts per square meter. Often a square meter of panel is further down rated to 10 square meters of land, due to the casting of shadows. So then you end up with 1 Watt of electricity per square meter of land average generation through a year. Interestingly biomass production and wind harvesting often equates to similar power density per land area.

    Manapouri Hydroelectric power station has a nameplate capacity of 840MW (thus in a year can produce that power multiplied by the number of hours in a year) There are 8760 hours in a year (so theoretical annual output is 7,358,400MWhr) actual output was 5,100GWhr

    How many hectares solar farm would match the annual output of Manapouri?

    Manapouri’s actual output was 5,100GWhr equates to average 582MW power output.

    At 1Watt per square meter, a hectare of land is 10KW thus 100Hectares for 1MW.

    THUS Manapouri could be replaced with a solar farm of 58,200 hectares!!!

    “Refining New Zealand is considering spending up to $39 million to build the country's largest solar farm next to its oil refinery in Northland. The 26 megawatt farm, would be built on 31 hectares it owns next to its Marsden Point plant.”

    In reality this solar farm will be closer to a 0.3MW baseload rating compared to the publicised nameplate capacity of 26MW.