Thursday, April 11, 2013

Busting our electricity bills: Part 4 - Sizing Our Solar System

Previous posts in this series:        Part 1       Part 2       Part 3

If economics didn't have a say, I wouldn't have put shiny new iron sheets on my leaking roof only to cover them up with solar panels. I would have just made a roof out of solar panels. But that didn't make sense when it came to considering our dollars. To be economically viable we needed to install only the number of panels that we needed for our electricity consumption. So we did some maths. Thinking of going solar? You can use these sums to help you choose the best system for you too.

How much electricity do you use?

This is easy, add up your total usage found on your electricity bills for spring and summer in one coloumn, and winter and autumn in a second coloumn. Divide these figures by the number of days in each period. Now you have a figure for your daily average electricity usage for both the sunnier (peak solar gain) and gloomier (minimum solar gain) periods of the year.

This is how ours came out:

Total Spring and Summer kw/hrs 2190 / 182 days    = average daily usage 12kWh
Total Autumn and Winter kw/hrs 5200 / 182 days    = average daily usage 28.5kWh
Total Annual kw/hrs 7390 /365 days                         = average daily usage 20.2kWh

Back to the grid solar systems only provide electricity during the day. By taking into consideration what appliances we run and when, we estimated that we consume approximtately half our power during daylight hours. This reflected our household habits of one person working from home and using appliances during the day. If your household has no one home during the day you might expect to use only a third of your power during daylight hours to run a fridge, dishwasher etc. By halving the above average daily usage figures, we arrived at an estimate for our average daytime electricity use.

Spring and Summer average daytime usage   12kw/hrs per day / 2       = 6kWh
Autumn and Winter average daytime usage   28.5kw/hrs per day / 2    = 14kWh
Annual average daytime usage                       20.2kw/hrs/ 2                 = 10.1kWh


Solar production estimates

Your solar supplier should be able to provide you with data for how much electricity each kw of solar panels produces in your locality. Solar gain varies from place to place, increasing as you near the equator. Other factors that effect solar gain are the placement of your panels, whether they face east, west or north, and whether your roof receives shading from nearby trees, tv aerials etc. 

Solar energy production estimates for Newcastle, Australia for a 1 kw panel:

Annual average on a north facing roof:   4.6 kWh/day 
Annual average on a west facing roof:    4.0 kWh/day

Maximum (January)                               5.16 kWh/day
Minimum (June):                                   2.4 kWh/day

What sized solar system do you need?

Now to crunch the numbers. Divide your annual average daily electricity usage by the annual average solar energy production for a 1kw system, choosing the one that is applicable to your roof orientation. 
Annual average daytime usage 10.1kW/hrs / Annual solar energy average on a west facing roof: 4.0 kWh/day         = 2.5kW solar system

This sized system based on our average usage (10kWh/day) will meet our needs most of the time. In practice this means that we will be producing about 45% more energy than we require during the day in summer, both our minimum use and the peak solar gain period, and we will experience an approximate short fall of 42% during the day in winter, both our peak use and the minimum solar gain period. 

Making a decision on the best sized solar system for you is dependent on what your goals are. Do you want to produce enough energy from your solar system to meet all your daytime energy needs all year round? Or do you want to produce enough energy to meet your average needs? The size of your roof or budget may place restrictions on your choices of what sized system you can install.

How much of my overall power consumption will come from my solar system?

The below figures show how much energy will come from both the grid and from the solar panels to meet our energy requirements. You can apply these sums to work out what percentage of electricity your solar system will provide.

10kWh/day x 365 days    = 3650kWh/per year from a 2.5kW solar system
Annual kw/hrs 7390 consumed - 3650kWh of solar energy produced         =3740kWh/per year from the grid

These figures show that by choosing a solar system that produces enough power to meet our annual daytime average consumption we will produce approximately half our overall electricity needs from the panels. However, during summer we will be producing more energy from our solar panels than we require, feeding the excess energy back to the grid. This means that we won't actually be using all the energy that we produce. To make an allowance for this we approximated that about one third of our electricity consumed will be provided by a 2.5kW solar system over the course of a year.

How did your sums come out. What sized system did you decide on?

Read the next post in this series... Part 5: Will Solar Pay For Itself?

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