Available sunlight differs every day with factors like clouds, smog, humidity, rain and seasons. There is often excess sunlight during the summer and not enough during the winter, depending on location. You may think it best to calculate your PV panels' energy generation for the worst time of the year, but this raises the system cost unnecessarily and increases excess energy wastage during summer. For example, if you live in Miami, you will receive 5.6 hours of solar illumination per day during the worst months. Assuming your average electricity usage you want to cover is 15KWh per day. The PV array size required with this figure is:
15,000 watt-hours/day ÷ 5.6 sun hours/day = 2,680W PV panel
Don’t forget to decrease the manufacturer’s panel ratings (assume a 20% factor):
2,680W real rating + 0.8 'reality' factor = 3,250W manufacturer rating
Miami has excellent sun conditions - so you can run a sizeable load with just 13-14 panels. As you can see, two PV arrays with practically the same power offering do not always cost the same. This is the retail world - just as you shop around to purchase a car, shop around for a more affordable PV array of the same power. An energy-to-cost ratio gives a fast comparison of available module types and needs only a simple calculation: divide the module cost by the rated wattage. For the above PV modules:
Suntech Module: $0.78
Sun Solvello Module: $1.19
Remember that more modules may be needed to cover 100% of your energy requirement during the winter when there are minimal hours of sunlight: in summer, the sun hours can double. The result is clear: with maximum sun hours, you require only half the modules needed during the worst days of winter. When annual monthly averages are used to calculate energy requirements, the figures will not match daily energy generation and consumption needs.
But if your PV modules are in a holiday cottage or cabin, you can calculate each period's best sun hours - for a brief visit during winter or fall, add another panel or two. Full-time residences require more complex choices. Usually this means more panels, although a complementary renewable resource like wind power can be considered. Alternatively, add a propane, diesel or gasoline generator as a backup production booster for the worst days.
Exactly how many PV panels you need depends on several variable factors. Even with all the necessary PV modules mounted on your roof, it's a good idea to