FAIRBANKS, Alaska — Karl Kassel fell in love with the site while ptarmigan hunting in 1975. At the top of Murphy Dome, his octagonal wood house overlooks Denali and Murphy Dome Valley. It was 2 below zero outside and 70 degrees inside when he woke up on a recent Wednesday. He hadn't heated the home for three days.

Energy from his windows, solar photovoltaic panels, wind turbine and solar thermal panels was being stashed for next winter.

"If it stays sunny for the next several days, we'll probably not have to light the fire again until October," he said.

Kassel, his wife Billie and their 15-year-old son moved into the off-the-grid house last January.

"It's crazy the amount of energy that's available here in Fairbanks, that we waste," said Kassel, who had been dreaming up the house since the 1970s. "I finally said, 'I'm going to do this. It makes sense, the energy's available, the efficient designs are here. We know how to do this. Why aren't we?'"

So Kassel hired Thorsten Chlupp, general contractor and owner of Reina LLC, to build a highly efficient home that made the most of the area's resources. They designed a house that combines passive solar design, renewable technologies and energy storage to make the variable sunlight in Fairbanks last almost the whole year. They are studying its performance to make the design more efficient, less expensive and less dependent on fossil fuels.

The southern side of Kassel's 1,800-square-foot house is mostly windows, letting in every drop of available sunlight.

The octagon is 20 percent more efficient than a square design because it has a lower surface-to-volume ratio and softer corners, which is where you lose the most heat, Chlupp said.

It was just as warm in the shadowed arctic entryway as by the sunny window.

"You don't have any loss, you don't have any drafts," Chlupp said. "You can feel very comfortable in a house like this at 64, 66 degrees."

The interior has an outdoors theme with tree sculptures, a free-form island in the kitchen and locally made birch cabinets with hand-picked rock handles. The house contains 16 tons of rock collected from river trips and a rock party. A blue river flows through sandy banks on the stained concrete floor and the living room is trimmed with plant beds containing two tons of dirt.

"We like being outside and floating rivers," Kassel said. "So we designed the house to be like camping on a gravel bar."

All of these items, plus nearly 13,000 gallons of hot water, are designed to absorb and save the maximum amount of heat.

The heating system focuses on saving energy in the water tank for later use. Heat travels, via glycol, from solar thermal panels into loops in the floor and a 120-gallon domestic hot water tank in a closet. When Kassel starts a fire, coils in the masonry heater carry heat into the water tank. When the indoor tank hits 140 degrees, it diverts extra heat through a heat exchanger to a 12,000-gallon tank buried outside.

The electricity generated by a 1.6-kilowatt solar array and 3-kilowatt wind turbine first powers the refrigerator, water pumps and other utility needs. The system was producing 400 watts Wednesday afternoon and the house, with lights on, was only using 290. The extra power charged a battery bank sitting in the closet, which stores about three days worth of power, Kassel said.

Once the battery is full, wind and solar are channeled into an electric heating element to heat the water tank.

"The PV panels are awesome," Kassel said. "They operate more efficiently in cold weather, and you get double sunlight because of the reflectivity off the snow."

The wind is also a surprisingly steady resource, he said.

"There are many days I can hardly feel a breeze on the ground but my wind turbine is 80 feet up and it's just humming."

If there's no sun, no wind and not enough heat in the water tank, it's time for a fire. Kassel used two cords of wood in the last year to satisfy two-thirds of his heating demand. The rest came from a propane hot water heater.

He has a backup diesel generator, which only kicked on three times in the last year, to recharge the batteries.

He went through 170 gallons of propane — counting appliances — and expects to consume 20-30 gallons of diesel this year.

Though the home already screams efficiency with 18-inch walls, triple-pane windows and efficient lighting, both Chlupp and Kassel are learning how to shave energy use — and costs — even more.

With bigger coils in the masonry heater, he wouldn't need the propane. A thicker foundation, with a sandbed under the floor, would also cut heat demand, Kassel said.

"That's a big lesson we learned here. We need more mass," he said.

Chlupp wants to eliminate any unneeded part from the system.

"Simplification of the mechanical system is what I'm after — less pumps, less electricity, less things that can go wrong," he said.

Kassel spent about $100,000 on the renewable energy systems, he said.

He received a $7,500 rebate from the state and is eligible for more than $20,000 in renewable energy tax credits through federal programs that give credits for 30 percent of renewable energy systems.

The rest should pay for itself within seven to 16 years, depending on the system, the conditions, the price of oil and many other factors, he said.

But he doesn't focus just on payback.

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"It's the kind of investment that is hard to put a value on. ... When you paint your house, what's the payback?" he asked.

You can't quantify the creature comforts, small carbon footprint and stable energy costs, he said. Or knowing you have one of the most efficient homes around.

"Seven months in Fairbanks without heating your house. ... How many people in this town aren't heating their house right now?"

Information from: Fairbanks Daily News-Miner, http://www.newsminer.com