Basics on Designing a Solar System

How much Roof Space do you need?

With advances in silicon design and module efficiency, 72 square feet of solar photovoltaic panels can now produce approximately 1 kilowatt of power. Under ideal conditions this means that you could get a measured 1000 watts out of this 72 square feet of panels. To achieve this, however, the weather would have to be really cold and the sun would have to be perpendicular to the panels. This 1 kilowatt would produce usually between 4.5 to 5 kilowatts of electricity every sunny day. You can determine what number to multiply by with the use of a solar insolation chart. This would tell you how many hours of sun your site would get on an average for a year. The numbers on the solar insolation charts are quite conservative and if the panels are facing south and set at about the angle of your latitude (for instance, 38 degrees for the San Francisco Bay Area) this amount of electricity should be attainable.

Sometimes it is a better idea to have the panels at the best angle to use summer sun, or even better, have the angles of the panels adjustable for seasonal sun. (See our notes under PV Modules’ Orientation. )

If your electric company puts your solar system on a yearly contract, you can use the excess generation of the longer hours of the summer months and you can take it off your winter usage.

String Sizing: Series and Parallel Series

Series = + to –

Parallel = + to + and – to –

With a solar PV system it is important to size the strings of panels for the correct voltage of the inverter. When you connect panels in series, you add the voltages for each of the panels together but the current on the string stays the same as one panel, no matter how many panels you use. This is the way to make a very efficient system. It would give you high voltages and low current.

If you were connecting up the panels in parallel you would have the same voltages but the current would be added for each of the panels. You would end up with a system with low voltages and extremely high currents. This would require very heavy gauge wire and this method would not give you as efficient a system.

This is why solar designers try and get the highest voltage possible, with the least amount of current.

Distance is part of the equation so if the solar array is far from the inverter  you can use smaller gauge wire by using a high voltage string of panels.

Temperature and Panel Output

Another thing to be aware of is that cold panels put out more power than hot panels. When a panel gets hot the semiconductor junctions have more resistance so the panel has a lower output. On the spec sheet of a panel there will be number for the computation of output on a temperature basis.

How Much Electricity Do You Use?

When designing a solar system it is important to know how much electricity or gas you are using and the simplest way is to look on your electric bill. It is easiest to figure out the numbers in kilowatt hours per day or therms per day.

If you are going to be off the grid, you can calculate all the different appliances and lights you would use and multiply this by the hours in use. This can give you a fairly accurate number for the amount of electricity you would need to generate in kilowatts per day. (See our System Loads worksheet to calculate the loads on your system.)

If you are looking at how much gas you are using, one therm equals 100,000 btus.

Can you reduce electrical needs?

Before you come to a conclusion as to how much electricity you will need, you should first think about how you could reduce your electricity usage without changing your lifestyle. The easiest way to do this is usually with more efficient air conditioning and refrigeration and more efficient lighting.

LED Lighting

LED lighting has really improved over the last year. You can now get a 3-watt LED light that would give you the equivalent light of a 20 watt halogen light. Be careful when you buy LED lights not to get the kind that have multitudes of regular LEDs wired together, as these don’t give off enough lumens and the small LEDs have a lens that is preset for a very close distance. The best LED lights, with the most lumens, are ones that are surface mounted with the Cree or SMD LEDs. These LEDs can sometimes be focused with a lens and they have a very long life expectancy (11 years). They also produce no RF or heat and don’t contain mercury.

LEDs come in two basic colors, not including the more decorative colored LEDs. For white light, you can get 5600 Kelvin which would equal what is considered to be natural light. The light would have a blue hue to it. 3600 Kelvin would be equivalent to tungsten. Some people seem to prefer the 5600K while others show a distinct preference for the 3600K. You get more lumens out of the 5600K lights as the tungsten color is produced with yellow/orange filters. These lights can be with the Edison base to screw into a regular socket or MR16, like halogen lights, which is used in many track lighting systems.

Solar Hot Water

Solar hot water systems are very efficient, low cost and perhaps give the best return for your energy efficiency spending. We used to sell flat plate collectors, but when the evacuated tubes became available we quickly became converts.

The evacuated tubes produce about 40% more hot water than a similar sized flat plate panel. The tubes also produce equally as well in very cold weather. This is because of the vacuum that exists between the inner and outer tube. A flat plate panel could lose a lot of its heat to the ambient air through the front glass. Not so with an Evacuated Tube System. With the evacuated tubes a 30-tube system produces about 40,000 btus every sunny day and about 1/2 that amount on a cloudy day. 40,000 btus can heat 80 gallons of water up 60 degrees, so depending on the temperature of your incoming water, a 30-tube system can heat up 80 gallons very hot.

We think the evacuated tube systems will eventually become very popular for air conditioning. Right now absorption air conditioning systems are available but cost about $7,000 per ton of air conditioning. (The ton number is an air conditioning number that indicates the amount of AC it would take to freeze a ton of water.) This price should be coming way down when these systems become more popular and you can just buy one off the shelf. So when designing a solar hot water system it may be a good idea to design for the future installation of a solar air conditioning system.

Another idea is to design the system with a heat dump. A pool or hot water heater is perfect for a heat dump and sometimes this is necessary as the evacuated tubes produce so much hot water. One of our favorite uses of the evacuated tubes is to run a kick-space heater. These heaters usually have a 1/2 inch input and output and hot water circulates through the system. When the water gets hot enough, a small fan pushes air through a small radiator, such as the types used for under the counter installations with hydronic heating systems where baseboard heaters do not fit. The kick-space heater can also provide a dump load if the air could be redirected outdoors when the weather is hot. In the winter months the air could be used for space heating. Kick-space heaters can be small and produce 2000 to 3000 btus but they can also be big enough to produce 7000 to 8000 btus and one or more kick-space heaters can be combined with the evacuated tubes. We usually don’t recommend putting more than 90 evacuated tubes in series as the water would get too hot. However, you can parallel plumb systems if you do need more hot water. The tubes are ideal to put into radiant floor heating systems and if they are installed in the concrete floor, the concrete can act as a heat sink to release the heat into the house throughout the night.