How many solar panels do you need on your house to power your heater in winter?

I would not use photovoltaic solar panels at all for heating. A solar heating system is much less expensive. I have an experiment running in Manitoba (latitude 50 deg 7 min N) that uses passive solar heating in a building 60' long by 36' wide. The long wall that faces south to catch the sun is clad with 2 layers of infrared absorbing poly. The building has 3000 sq ft of roof. The poly cost $1500. The north wall is insulated R20. The east and west walls are insulated with straw bales to R50. The east and west walls are painted dark red, which warms the external surface by approximately 5 C in sunshine. At night an insulating curtain is drawn over the south side to minimize heat loss. The internal temperature depends upon the weather. On an overcast day in February, the building is 6 C warmer than the outside. In a February heat wave the outside temperature hit +2 C and the internal temperature spiked to 36.5 C, somewhat incongruous with 4 ft of snow on the walls of the building. More typically, the internal temperature is 23 C warmer than the external temperature during daylight hours (+6 C inside - 17 C outside). In October, internal temperatures exceeded 50 C, so be prepared to vent excess heat on a warm day. For a residential building, you need to buffer the temperature by adding a heat exchanger (cooling during the day, heating at night), insulated water tanks to store the heat and a pump to circulate the water.

Edit @ Peter

6 hp = 4.2 kW. Lets take a pessimistic view and say the solar cell is 10% efficient (The best lab result is 27.4%). Assume that you store energy in a battery to operate the pump at night. In December the solar insolation in Winnipeg, Manitoba is .99 kW-hr/day/m^2.

http://solarelectricityhandbook.com/solar-irradian...

To run your pump 24 hr you need 1028 m^2 = 0.254 acres. In July the solar insolation is 5.92 kW-hr/day/m^2 and you need 0.042 acres to run the pump for 24 hours. Further south in the US, the numbers are more favorable.

Lets assume the panels are 100 Watt panels. You would need 150 panels to just provide power for the heating maybe 2-4 hours a day on nice sunny days. Get the picture? Remember the rating on a panel is peak power when the sun is at just the right position.

Solar panels are not normally a good system for heat or air conditioning. They simply get much to large and expensive, and are not reliable because the sun does not always shine. If you are in an area where the electric company will purchase any power you do not use and give you a credit, then you can build a system much larger than you need in the spring and fall and might be able to produce enough during these periods and get a credit. You then use the credits in the winter for heat and in summer for cooling.

This requires a detailed study of your uses all year, then designing a system that produces enough to cover your yearly KWH load. It will probably be very expensive, but many are using taxpayer dollars subsidies and electric company rebate programs to make these system more cost-effective. You would need to check your local area utility company.

Your house, and probably your yard wouldn't be big enough to hold them.

I did this exercise once (not doing it again) for someone who wanted to replace his 6hp outboard motor with a solar powered one.

The calculations ended up that he would need something like 1.5 acres of solar panels to get that much energy.

Not happening.

That said, an passive solar home, can get a significant amount of warmth from the sun directly. But it doesn't work very well at night, when it's coldest. (wonder why that is... probably a climate scientist could help us with that? -- oh, wait. They say warming doesn't have anything to do with the sun.)

Edit.

I didn't use theoretical stuff to come up with my numbers, I used the claims of actual off-the-shelf solar panels sold by West Marine, calculated the power, and did the math based on existing panels.

You can get over 5 times the efficiency at lower cost by directly absorbing the heat in a solar water-heating collector. Use the electrical panels for making electricity for stuff that needs it.

As a solar installer, I'd love to give you a simple answer but I can't.

Most solar contractors offer a free solar evaluation where they will tell you how much energy they can generate given your site conditions, what it would cost, and how long the payback would be. Rather than a bunch of inexperienced people giving you a bunch of hypotheticals, I think this would be the best option.

Need a lot more information than that...

What climate zone are you in?

http://resourcecenter.pnl.gov/cocoon/morf/Resource...

What kind of exposure (sun/shade)?

How much energy is used currently to heat the house (if it's an existing structure)?

If it's in the planning stages, what's the square footage?

What kind of insulation is being installed?

What kind of windows are being installed?

What type of roof and what roofing materials are being installed?

etc...

Edit:

The zone 6, forget about it... Go with solar hot water (little problem with snow accumulation**) in conjunction with a gas boiler for a radiant or radiant floor system.

For Zone 2, I'd think that the solar panels would be better used for AC than heat and used in conjunction with a ground-source heat pump could cut your energy bills for HVAC to next to nothing.

As for how much it would cost, I couldn't give you a remotely accurate quote...but I'd be surprised if a good solution wouldn't pay for itself in less than 10 years.

Edit 2:

The average home in the US consumes an average of 50 kWh per day. That's just a bit more than running a 6HP motor for 10 hours, so an acre and a half solar panels would be overkill...to power the whole house plus several more houses.

If you look at panels similar to these (14.4% efficiency BTW, Peter, d/dx):

http://www.infinigi.com/sharp-nuu235f1-235-watt-so...

And assuming about 4.5 hours of peak sunlight/day (the average in Helena Montana, which is Zone 6) you should be able to average 50 kWh/day with a total of about 30 panels which would cost about $19,000 according to the prices given in the link. The area they cover would be less than 600 sq. ft. Now, the 50kWh/day figure I think omitted the heating and cooling needs of the average house, so you have to take that into consideration...

[disclaimer - This isn't even a guestimate for the particular homes in question. It's merely for information only.]

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Aircons use a lot of power, and many types draw a large surge of electricity when first turned on, which can be quite destructive of batteries, inverters, etc.

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