“I want 100% solar!”

“I want 100% solar!”

Update 12-31-14

I frequently get asked if a person can get 100% of their heating and hot water from solar. I always answer with a story.

Let’s say you need 500,000Btu (500kBtu) on Sunday for heating and hot water in the dead of the winter.  So, we size a solar system to produce 500kBtu on a clear day in February.

This means on Sunday we collect 500kBtu of energy and everything is fine.  However, if Monday is rainy and cold, we don’t get any energy from the solar system.  That means we have to go back to Sunday and double the size of the system to collect enough energy to cover Sunday and Monday.  Well, what about Tuesday?  It could be rainy and cold on Tuesday too.  Back to Sunday we go and triple the size of the system to cover Sunday, Monday, and Tuesday.

By the time your great uncle tells you about the blizzard of 1935 which lasted for two weeks, you have now increased the size of the solar system by 14 times and it costs more than your house.

Obviously, this is not the right approach. We need a different perspective.  We need to think of solar energy as an irregular source of energy.  We collect it.  We store it, and we send it out to heating and hot water.  When it is gone, we wait for some more and repeat the cycle.  We can’t turn solar energy on in the middle of the night like a furnace.

“Solaring” is a lot like “farming”.   A farmer knows how much sun and rain he will get in a typical year.  He knows how many bushels per acre he can expect.  This data comes from averages over many years.

But don’t expect him to know how tall his corn will be on June 15.  He has no idea.  If this year has low rainfall, or has been cold and cloudy, it may be half as high as normal.  Solar is the same way.  We have 30 year average weather data, but we don’t know anything about what will happen on a given day, or month.  We have to plan on the average.

With a properly designed system, we can collect and use a significant amount of solar energy over the course of a year.  That energy directly reduces the amount of energy we have to buy from the electric, gas, or oil company to heat our homes and water.  The utility bill at the end of the month tells the story.  We are harvesting a free resource that reduces our energy expenses.

There are several key things this story teaches us about solar energy, some of which have been mentioned before.  We also have to plan for no solar energy on the worst day of the year.  That means there must be a full power conventional heating system in place.

Solar reduces the energy used by the other appliances, thereby reducing total energy costs.

A little considered fact is that if the furnace runs only 60% of the time because of solar energy, it will last 40% longer.  Not only does the solar system provide free energy, it extends the life of heating equipment by having them run less.  This rule may not apply to water heaters whose lifetime is based on rusting out, not whether it is running or not.  But the water heater electric elements will last longer.

Second, the solar system is always put on a south facing roof, shading the house from the hot summer sun.  In my home state of North Carolina, 100 ft2 of shading on a south roof can equal about ½ ton of air conditioning.  So, the summer cooling bill goes down.

Finally, the shingles under the collectors will last almost forever.

– Dr. Ben