Off-Grid Solar Powered Home: The PV System
Case study of building a solar power system for an off-grid home, as told from first-hand experience by our Editor. He talks about the decision to go solar, how the system works, and what it cost. Photos.
September 16, 2004
When we started shopping for a property to build a vacation home, we knew very little about renewable energy; we thought it would be nice to have some solar power at our future mountain hide-away. But we hadn't considered living without utility power.
Then we found our dream property in the Cascade Mountains of Washington State -- and it was off the power grid.
Should we pay the $30,000 price to have the grid extended to this remote site? We could buy a state-of-the-art solar electric system for half that amount. Could we live normally in an off-grid solar home? I started researching.
|Solar panel array -- view from the back, showing pole mounted frame. To get the assembled array onto the 15 foot pole, we hired a well pump installer with a boom truck.|
Going solar was a big step for us, and the decision involved a lot of research -- we interviewed solar experts, families who had lived on solar, electricians, builders, manufacturers and even the PUD. There were many surprises.
What surprised me most was how little electricity it takes to live normally. Our grid-tied suburban home consumed an average of 45 kilowatt hours (kWh) per day. Our off-grid solar home is designed to use less than 5 kWh per day.
That's a big difference. What do we give up for a 9X increase in efficiency? Does it mean living in a straw-bale home, banking the wood stove and reading the Old Farmer's Almanac by kerosene lantern?
Not even close. As I designed our photovoltaic (PV) system, we were also talking with the architect about a conventional home design. After all, if we wanted to be without modern conveniences, we could just go camping.
How does it work?
Think of a solar powered home as a home that runs on batteries. The batteries are charged by the sun. When the batteries and sun aren't enough, a supplemental generator kicks in.
The batteries' direct current (DC) power is converted to alternating current (AC) and wired to the service panel. The electric loads in the house are all normal 110V AC lights and appliances.
|Inverter -- wall mounted (largest unit, center) with related electronics including transformer (white unit below inverter) and charge controller (small unit in upper right). Large flex conduit holds battery cables.|
How big is big enough?
Photovoltaic (PV) equipment is a big investment. It's important not to buy too much, or too little, capacity. Too much capacity is wasted money. Too little capacity would take excess generator time to make up the difference.
In designing the PV system for our house, I assumed we would employ some energy-efficient practices, designs, and appliances.
Energy efficient practices make a big difference:
- Turn off lights
- Cool with ceiling fans
- Power down electronics, like computers
- Avoid big peak loads
Design options were open to us, because we were designing for new construction:
- Compact fluorescent lighting
- Passive solar heating
- Radiant underfloor heating
- Passive cooling techniques
- High R-value insulation
- Fireplace as supplemental heat source
- Gravity-fed water supply with large storage tank
- Siphon activated septic system
In the average home, appliances use most of the electricity. The following appliances can run on propane (LP) and consume very little electricity:
- Clothes dryer
- On-demand water heaters
To size the system properly, I considered each electric appliance and its hours of use. We calculated that our daily winter consumption would be just under 5kWh, so we assumed 7kWh to be safe. It was a big Excel spreadsheet when I was done.
The largest normal simultaneous load would be around 2000W and the highest AC load appliance would be a dishwasher drawing 1250W. The well pump draws about 2000W; it normally operates only when the generator is running.
Our system has 5500W of peak load capacity, 2000W of rated solar generating capacity, 10kWh+ of battery storage, and a small 5kW backup diesel generator.
|Batteries -- Phil Glass, owner of renewable energy reseller EnviroSafe, straightens batteries weighing over 200 lbs. each. The house can run on batteries for several days without sun or generator. Batteries need maintenance twice a year and last about 10 years if well cared for.|
Most of our power will come from the sun. We purchased 16 Kyocera 120W PV panels to be installed as two arrays of eight panels. Each 1kW array produces 48V DC. They feed into a Trace (now Xantrex) charge controller, which prolongs the life of the batteries by charging them scientifically and maintaining optimum voltage levels.
Our solar panels provide DC power to a Trace SW5548 inverter. In addition to changing 48V DC to 125V AC, the inverter is a programmable brain that automates much of the tedium of using a backup generator and caring for batteries. For example, when it detects low voltage in the battery bank, it starts the generator, but not while we're sleeping. The power from this inverter is true sine wave, so it gets along with our computers. Our deep well pump uses 220V power, so we have a 120-240V transformer.
Often our power consumption exceeds what we can get from the sun. It's inevitable at night, when we use the most power. It also happens on cloudy days.
The power we generate on sunny days is stored in Surrette 530 batteries. These are deep-cycle 6V batteries strung together to provide 48V. The bank of 16 batteries holds 10kWh of backup power, enough to last a few dreary winter days. These two tons of batteries are housed in a custom-built cabinet with steel-reinforced shelves and special ventilation.
This being primarily a summer home, our generator demand is relatively small. We started out with a 5.5kW gas generator. When the smaller generator wore out, we bought a 5kW air-cooled diesel genset.
The design calls for a 7.5kW water-cooled genset with remote start capabilities, that runs at 1800 RPM on propane. When we upgrade to it, the larger generator will be quieter, more fuel efficient and longer lasting. If we anticipated more generator use, we would switch to diesel (possibly using biodiesel) for its lower maintenance and fuel costs.
|Powerhouse -- This fire-resistant shed houses the batteries, inverter equipment and generator.|
Someplace to put it
We built a 10 foot x 12 foot powerhouse for all the PV equipment. This is a forest-fire-prone region, so the powerhouse is built to withstand fire. (Sustainable/Firewise forest management is a whole 'nother story.) The roof and exterior walls are clad with metal roofing sheets and fully insulated. Vents are completely covered with wire screens. The door is a rolling overhead steel door, like you would find on a mini-storage unit. The generator is inside; its fuel is stored outside.
This complete system cost us about $16,000 plus the generator. For the PV equipment we qualified for a 15 percent State of Washington rebate through Western Sun, and it was exempt from sales tax. In all, the incentive programs subsidized about 23 percent of the PV cost.
The diesel generator was $2,000 more. The next generator upgrade will cost about $8,000. Total cost (after incentives): $22,000
Given the $30,000 cost of a grid connection, this system paid for itself the first time we switched on a light. And now we know how it feels never to receive power bills: Great!