Your Questions About How Much Solar Power Needed To Run A House

Henry Dover answers:

I have just finished building a solar room addition. When I put plants in it I will call it a greenhouse.

A greenhouse of any style is an attempt to modify the environment. Glass or plastic walls and roof are often used because they allow sunlight to enter. Inside it becomes heat. There are a number of ways that heat is transfered. One way (radiant heat) is blocked by the glazing. This causes a greenhouse to heat up and even without plants an attached solar room can help heat the house it is attached to.

The things needed to “run” a greenhouse are those things that are first necessary to maintain the environment and secondly are convenient for gardening. My uncle had a commercial greenhouse business. It had a huge boiler that used coal to create heat and send it to the greenhouses. They let most of the heat escape into the air. There was no air conditioning.

A solar greenhouse will sometimes have double glazing to keep heat inside. As an alternative it may have insulation that is put in place at night to help keep the heat inside during cold months. The other important thing a solar greenhouse will have is a lot of “thermal mass.” This is often brick or stone walls inside of the insulated outer walls or lots of water in containers. The thermal mass soaks up extra heat and when the greenhouse is cool it releases heat to the inside. The effect is to keep the greenhouse from getting too hot or too cold.

There are also greenhouse designs that use compost that is mixed in a way to heat up. This adds not only heat but can change the air mixture to something that plants will like even more than normal air.

Most greenhouses will not have air conditioning to cool the air. Plants can usually tolerate a bigger range of temperatures than people can. Often there is a way to ventilate the greenhouse in the summer that may be sufficient to keep it cool enough. This is especially true if there is enough thermal mass.

Plants and the ground around them give up a lot of water. If it gets too humid the structure may suffer. Again ventilation may be good enough. Additionally, there are ways to make the heat of the sun create a draft in a kind of solar chimney that will pull air like a fan through the rooms. Ventilation can be aided with vents that open automaticly without the use of electricity.

Greenhouses are not all the same. If they are commercial they may be designed for a particular crop. Also the environment where they are built will make a difference. In a cold environment the growing season will be shorter and heating concerns will be greater. In a warm environment ventilation will be more important. In a dry environment keeping water vapor inside may also be a consideration. In a dark enviornment some consideration might be made about lighting.

Once the inside environment is stable and sustainable some attention may be made to planting concerns: growing beds, soil, water supply and irrigation.

In my greenhouse I just finished I have put a couple of potted plants but it is still freezing outside some nights. My next concern is to get some accurate idea of how the room is heating up compared to the outside temperatures. I have a thermostat for inside and outside but I have ordered one that I can hook up to the computer to log the temperature changes.

Henry Dover answers:

Square foot has little to do with it.

Electric heat, lots of AC, electric stove etc are a lot more important. What you need to do is look at old electric bills. Look at summer and winter and get the number of kW-hours for each month to get an average number, plus highs and lows.

Also important is location. Deep south with lots of sun versus northern maine with little sun. Do you have a roof that faces south with no shade at any time of the day?

First decision you need to make is grid tie or lots of batteries. Grid tie lets you sell excess power to the power company when you have a lot of sun, and buy it from then when there is no sun. The alternate is a huge bank of expensive batteries to store energy. And it can come to hundreds of batteries at hundreds of dollars each.

If you go with batteries, you have to pick a number of hours that you want to get power from the batteries when the sun is out. You can have many days with no sun, and that means a lot of batteries. If you decide to supply power for 5 sunless days as opposed to 1, then you need 5x the number of batteries.

Get a number of kW of power you need, and multiply that by a factor, perhaps 3, so you have excess to charge the batteries or sell to the power company. Then you have a start on sizing the panels themselves.

Get all those numbers together, then I can go over the numbers with you. Be warned, even with no batteries, this is more than US$30,000. Perhaps a lot more.

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