Monday, November 5, 2012:  Updated November 11, 2012

Why Be Efficient?

Wouldn't it be great to be able to satisfy all your energy needs from sunlight collected on your own property?   No fossil fuels, no pollution or CO2 emissions!  Certainly this is possible considering that over 1.4 Gigawatt-hours/ year of solar energy falls on a typical 1/5th acre lot in North America.  Solar energy is very plentiful. The only difficulty comes in how to collect all this energy, both efficiently and cost effectively. 

If you were to take the average, 2-car driving, American family household and sum up all the energy they consume in their home for heating, cooling, lighting and daily transportation, it would be on the order of 73 Megawatt-hours of equivalent energy usage per year.  

The energy unit of "watt-hour" was chosen so I can compare all forms of fuel on the same energy measuring stick.  

Let’s take a look at how that typical American family uses 73MW-hr of energy each year.  

  • 12MW-hr in electricity (1000kW-hr per month)
  • 830 therms of natural gas (or 24.3MW-hr in equivalent energy usage).  
  • 1008 gallons of gasoline in two cars annually (or about 36.7MW-hr in equivalent energy usage).   

To generate enough electricity from photovoltaic panels alone to satisfy this huge energy demand would require a 40kW – 50kW solar array and would cost about $150,000.  It would also be 10-15 times larger than the available collecting area on a typical house roof.  

For 73MW-hr of conventional fuels, at a combined energy cost of $6100 annually, it would take about 25 years to offset the cost of the solar array. This is not cost effective.  

A Need for Higher Efficiency:  

The typical American (internal combustion engine) vehicle gets about 20 mpg and is only 15-20% efficient at converting its fuel into movement of the vehicle and occupants from point A to point B.  

Electric cars by contrast are 80-90% efficient at converting fuel into movement.  By simply switching over to electric vehicles, a reduction of 31.7MW-hr of energy can be saved annually.  

Modern homes are not designed for efficiency.  In ancient Roman times, fuel was scarce.  They built their homes oriented towards the sun so as to collect as much sunlight as possible.  Sunlight was stored as heat in the thermal mass of the structure and would keep the occupants warm while it radiated the heat back throughout the night. The thermal mass also kept the structure cooler in the heat of the day.  All this was done without electricity or fossil fuels.  

Just because we have cheap and plentiful fuel doesn't mean we should continue to build our homes stupidly inefficient.  If we can pattern our homes after this ancient Roman idea, our homes could take advantage of more of the sun’s free energy.  Today we also have the advantage of modern building materials with superior air sealing and insulating properties.  

A highly insulated and air sealed home, with proper window placement and roof overhangs, (designed to block out direct sunlight in the summer and let in direct sunlight during the low-sun-angle winter months), will require 5-7 times less heating and cooling energy than a typical, modern house today.  Combine this with energy efficient electric appliances, electric vehicles, efficient habits and you have a powerfully synergetic concoction. 

While the energy from natural gas costs 2-4 times less than electricity from the power grid, gas appliances are less efficient than their electric counterparts.  

  • Gas furnaces are about 80% efficient.  Electric ones are 100% efficient.  
  • Gas water heaters are only 55% efficient.  Electric ones are nearly 100% efficient.  
  • Electric heat-pumps (because they are able to rob heat from the environment) are 250% - 300% efficient.  

Once a home is designed with uber-efficiency in mind, it might be tempting to stick with conventional, inexpensive fossil fuels.  While it won't affect your immediate bottom line, you are still left with a large waster of energy.  

Energy generation and transmission.  Natural gas loses 1.4-10.8% of its energy to the air through transmission.  Electricity loses about 5.7% of its capacity through transmission.  Modern clean coal powered electric plants are at best only 37.6% efficient at converting the energy from coal into electricity. The production of gasoline from oil is only an 85% efficient process.  Extraction and distribution of oil results in further losses. 

Having a local solar plant on the home eliminates all fossil fuel usage and its associated generation and transmission losses.  

In this ideal home, fossil fuel (oil, natural gas and gasoline) usage would be zero and total electrical usage would be about 10MW-hr of energy per year.  5MW-hr per year would be consumed in the household energy needs (HVAC, appliances, gadgets) and 5MW-hr would go toward charging and driving the two electric vehicles a combined 18,000 miles per year.  

A 6kW grid-tied solar array satisfies the demand for 10MW-hr per year of energy and is small enough to fit aesthetically on the home’s roof.  It is also reasonably affordable.  Upfront cost for an array this size is about $22k (in 2010).  Not paying for any energy bills, including gasoline, (using the above example of $6100 annually), the brake-even point for this investment would happen within 4 years.  Current government subsidies and utility rebates shorten the pay-off to under 18 months.  After that, it’s all free energy.