Jan 7, 2010

Solar Power - How Many Watts, Panels & Batteries Do You Need?

If you’re thinking about setting up a residential solar power system, you’ll need to know about your power consumption to (read more about how solar cells generate power). The table below shows average electricity consumption by different household electronics such as the refrigerator, microwave over, ceiling fan, light bulb and so on. (Table credit, UK Solar Energy).
Items Description
Average Electricity Usage (per hour)
Ceiling Fan
80 Watts
Floor Fan
125 Watts
Energy Saver
24 Watts
60, 100, 200 Watts
300 Watts
80 Watts
550-800 Watts
AC 0.75 Ton Window
900 Watts
AC 1 Ton Window
1200 Watts
AC 2.50 Tons Window
3500 Watts
AC 1 Ton Split Unit
800 Watts
Air Cooler
325 Watts
Microwave Oven
1400 Watts
1400 Watts
Washing Machine/Dryer
400 Watts
Water Pump
1 HP (746 Watts)
Computer Desktop
400 Watts
Monitor 17"
120 Watts
Computer Laptop
45 Watts
Tube light (modern)
40 Watts

How many panels do you need?

Once you know your hourly power consumption (say, 200 watts), multiply it by the number of hours you get sunlight in a day (say 14 hours) to determine the total power that needs to be generated by solar panels in a day (200x14 = 2,800 watts).

Next, go shopping for the ideal solar panel. Different residential solar panels usually offer anywhere from 10 watts to 200 watts of power per hour, which depends on their size and price. 

If you pick the 200 watts product, you'll need 14 panels. Each panel costs over $1,000, so the total solar panel cost would be over $14,000. If you pick the 10 watt panel, you'll need 280 of them! These come a lot cheaper, around $40, so your total cost comes to just over $1,000, which definitely seems like a better deal (provided we ignore eqipment costs).

What battery size do you need?

At the same time, you’ll need a battery to make sure you have power at night or on cloudy days. Solar power systems rely on acid-lead batteries to store power and one unit usually stores up to 1000 watts of power, while having an efficiency of 85-90%.

So one you’ve calculated your maximum power consumption (again, 200 watts per hour), you multiply the figure with the ‘number of hours without sunlight’ (say, 10 hours) to determine the size of the battery (200x10 = 2,000 watts). During summers, and in countries near the equator, the Sun will shine for much longer compared to winters and countries near the poles. Plus, the angle of the Sun will impact the amount of power solar cells create (but this problem doesn't impact a new type of solar cell).

After that, you simply divide the figure (2,000 watts) by the typical load of one battery (1,000) to get calculate the number of batteries you need (2,000/1,000 = 2 batteries).

Of course, there are some other things to remember:

  • if you consider the efficiency rates of lead-acid batteries, you’d need slightly over 2 batteries, or you’ll need to lower you power consumption. 
  • depending on the type of solar cells, you may need fewer batteries to rely on 
  • In the end, it’s important to remember that these batteries will get damaged if they’re over-charged or under-charged so you'll need a controller too.
That’s it. You can now start setting you home solar panel system!

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