Living An Environmentally Friendly Lifestyle

Tuesday, April 30, 2013

How Much Solar Do I Need to Run My House?

solar panels on roof
The biggest question I hear when someone is considering adding a solar power system to their home is "How many watts do I need?"  And there's really no definitive answer.  But I'll try to explain in both detail and layman's terms of how you can figure your system needs. 







*The following assumes you have a grid-tied system with net metering.

Your daily electrical usage varies widely from day to day.  Some days you vacuum and some days you do laundry.  On hot days you run more fans or air conditioning, on colder days you turn up the heat.  Every day is different. 

With solar power, your power production will vary widely on a day-to-day basis as well.  A cloudless long sunny summer day will produce much more power than an overcast short winter day.  And there's every conceivable variation of weather in between.

With all these variations of energy production and usage, it's impossible to determine the exact size of the solar power system that you need.  But there are steps you can take to average it out...

Most (if not all) utility companies monitor your daily, monthly, and annual usage, and figure the average use for your household.  My utility company has the information readily available on their website when I log into my account.  My household averages about 8KWh per day in the colder months and about 13 KWh in the warmer months. 

So technically, if I want to cover 100% of the usage in my house for the summer, I'd need a system that can produce at least 13KWh during the summer.  However, on the days I'm not using 13KWh, my system would be producing more than I need.  And since solar panels and equipment are a fairly pricey investment, you probably want the smallest system possible to cover your usage.

My utility company offers net metering (they buy my excess electricity), so I could simply send the excess back out on the grid and get paid for it. But initially, buying a 13KWh system would be a
 fairly excessive system to meet my needs and I don't see the justification in paying extra for it.

I don't necessarily feel the need to produce 100% of my power, just the majority of it. If I average out the entire year my annual average power usage is about 10KWh daily. 

So what would it take to produce exactly 10KWh of solar energy each day?  Well...a miracle.  Remember, day-to-day your systems total energy production will vary based on weather conditions.  But using a few calculations, I can figure out the size of the system I'd need to be in the general area of my energy needs.

First, you need to know the average 'Peak Sunlight Hours' for the area your solar panels will be installed.  There are many websites out there that offer assistance in finding this number if you take the time to google them.

*A helpful equation to figuring out a systems' daily production is to multiply the total wattage by the peak hours per day.  This gives you the Watt Hours produced per day.

So lets say that at my location, I receive an average of 5 peak sunlight hours per day.  Now I know that I have roughly 5 hours a day to make 10 KWh worth of electricity.  This comes to about 2 KWh each hour or 2KW worth of panels.  Because a 2000 watt array working at 1 hour would be 2000 watt hours or 2KWh.

Also, it's common knowledge that a panel rated at 100 watts will rarely ever produce exactly 100 watts. So, I'm going to average it down a little,  lets say 80%.  So with all of the 'averaging' and 'estimating', if I bought a 2500 watt or 2.5 KW system, which worked at an average of 80% production, I should average about 2000 watts per hour, times 5 hours i could produce 10 KWh per day.  More in sunny summer conditions, less in cloudy winter conditions, but it should average out.

I would feel comfortable with a 2.5KW system to meet the needs of my household.  On days when I require more but produce less, the needs would be filled by the grid power.  On the days when I produced more than my needs, the electricity would be fed back onto the grid and the utility company would buy the electricity I feed at roughly the same cost as they sell the electricity to me. 

With this method, each month my electric bill would fluctuate some, but it would average out in the end.  For example, lets say that throughout the year, I used a total 100 KW hours from the grid due to bad weather or higher daily usage than my system was producing.  Lets also say that my system sent about 100 KW hours back to the grid on days i used less than i produced.  Throughout the entire year, my system averaged out to cover my needs.


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