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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.

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:

  • Furnace/boiler
  • Refrigerator
  • 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.

Solar panels
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.

Inverter
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.

Batteries
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.

Generator
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.

System cost
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!

Comments

Excellant work!

Your story is enough to sell a system to almost anyone!

Is all your heating from the fireplace?

Wow, I've been intrigued for some time regarding radiant heating and solar power. My husband and I just moved to Puyallup and are considering building a house. It's encouraging to hear your story and I'm sold.

I strongly commend your efforts because I have alot of interest in alternative energies.

Now tell me something, can solar power drive a single plate cooker? If so what size of panel and battery is sufficient to drive this load?

The powerhouse looks very nice, great job! If you ditch the 220v well pump in favor of an adequate solar pump, I bet you will never need to upgrade the generator.

Nice setup!

We live in the Spanaway area. We are taking baby steps toward disconnecting household devices from the grid. Purchased 7 15watt solar panels with battery setup. Able to run TV, computer, lights off the grid. Can't afford the cost of a complete system yet. Yours is very encouraging price wise.

Did you set it up all yourself?

Awsome work!
I hope you would built Small Solar power generation units in Sizes of UPS or small generators which can run few energy savers, Fans and handheld device chargers. If you commercialise share with us, There is a great market for it.

Sizing solar power systems seems like simple math, but it gets complicated. It becomes a question of how much battery capacity you need, and then how much PV it takes to charge the batteries.

For small systems like Paul's, you need to take into account the efficiency at your latitude, and sun days, at least. For larger domestic systems, you also need to consider line loss, temperature impact, and other factors.

The package Asim is asking for would be easy to configure... some PV modules, a few batteries, a charge controller and an inverter. Put it all in a cabinet, mount the PV on top, and run a wire to the loads. (For that last part, consider hiring an electrician.)

Hello
I want to buy small solar energy unit to produce electricity for only 4 100watt fans, 4 15watt enrgy saver bulb.i havent got enough money to buy heavy unit of solar energy.I live in Lahore(Pakistan), so please advise me about this and do inform me about the prices.
Thanks
yours sicerly
Mubarak

Hello
Could you share your knowlege to me , I need wiring diagram to install the system with battery back up .
Thanh You very much
trinhphuc@yahoo.com

Phuc Trinh

where can we get a diagram for off grid with batterie backup . we have ordered several books but have learned nothing. your system is awesome.

Maher Abuhamda
mabohamda-hotmail-com
+971-501721738

Hello;

I have an interest in a solar panels could supply a house hold electricity 220V 50~ for the mid east. Please let me know if you have a dealer in UAE or if I could buy a volume from you directly.

Regard’s
Maher Abuhamd

what's the cost of diesel? the diesel generator is so cheap comparing with the PV.

The dollar cost of diesel is only part of the equation. Certainly $4 per gallon diesel makes the PV pay for itself within our hold horizon for the home. The irony is to live in a county where PUD power is all hydro (zero carbon, zero imported fuel) and then run a diesel generator for several hours per day on average (at approx 20 lbs of CO2 per hour). PV costs less -- financially and environmentally.

Hi,
Could you maybe tell me more about the Inverter,and how it controls the solar power diferences.A diagram wold be nice,or anything technical.Or the make.
Thanks,

we have a house with about 13000 wh consumption which needs about 3300 wp panel and about 3000 Ah battery bank for one day autonomy.Ihe point is that ,the this house is vacation house and is usually used at weekends.In such a case can we use less numbers of pv,letting them charge the battery during the week?lets say 600 wp panel?i have read about a special kind of charge controller that lets the batteries be charged in sequence and one after the other,does that work?

farya, You have to consider the % sunshine in your location, the % de-rating of your array due to latitude, and % de-rating of batteries due to temp and age.

You won't want to completely drain your batteries, so you have around 1,500 Ah available -- is that enough for a cloudy weekend?

Expand your spreadsheet and standardize on a unit of measure, such as kWh AC, and this will be easier to figure out.

Hi Denis,
great article, I did my Environmental degree a few years ago after retiring from the UK North Sea offshore Oil & Gas Industry, through ill health.
I concentrated on renewable energy in the degree, and the final project was to run an old, off-grid farmhouse on renewable energy. anyway I passed, but I wish I had seen your article then. I write for Brighthub.com and I was researching wiring diagrams for PV arrays when I came across your excellent article. I am a mech eng by profession, so not as smart as you electrical wallas.
I am semi retired, well my old construction yard is making big beastie wind-turbines set on floating structures, so who knows I might get back yet!
Loved the bit where your system paid for itself!

Sound interesting
But why not led lights, solar water heater and use all DC appliances

That was nearly 7 years ago. I wonder if the cost has come down and the efficiency improved?