Posted by: nativeiowan | January 21, 2014

The green discussion…

Ok, I just grabbed this online from a local supplier offering a special, while stocks last, deal. So commercially we can quickly see that off the street you can buy…

kwa copy

Average annual usage is a bit confused, I will aim to break cost down demographically but we well know that the few consume more than the many…

kw use copy

So I’ll use a high average for western world domestic private consumption of 8000 kWh pa. 8000 kWh pa = an on-call supply of 22kW per day with peak demand being night time.

Every once in a while I’m dragged into these discussions mixing apples n eggs. In most of those conversations it’s common when someone starts mixing up the units: kW vs kWh. Since they both mean something completely different, it’s important to use the correct one.

kW is a unit of power. Unfortunately they often use kWh (kilowatt hour) which is a unit of energy. Let’s apply this on an analogy that most people can relate to: cars. Power is measured in bhp or kW, energy is measured in…. gallons of fuel that can fit in the fuel tank!

So if you are one of those people who mixes up the units I hope this helps.

Now, we’re discussing the “advantage” a photovoltaic collection system offers, noting that most of the published “facts” indeed claim there to be an advantage… lets discuss what we are measuring… and it is easy…

Easy: so say we’ve need 20 kW. That’s 20,000 W, which is 20,000 J/s.
An hour has 3600 seconds (60 seconds in a minute, 60 minutes in an hour).
In an hour we will use up 20,000 J/s * 3600 s = 72,000,000 joules. A lot of zeros is annoying so let’s shorten that to 72 mega joules or 72 MJ. If we kept up our 20 kW use for a day we’d use 72MJ/hr * 24 hours = 1728 MJ or 1.728 GJ.

So I think it’s obvious that this is… less than ideal. If you use a lot of power or measure over an extended timespan you’ll end up with numbers that are less than meaningful… something economists and the World Bank love. But not ideal for the average consumer.

So let’s make it a bit easier and make up a new unit: kWh!

kWh means kiloWatt-hour. Writing it down as you’ve learned in your physics lessons it would look like: kW*h or kW times an hour. In the real world it’s the amount of energy that you would use if you used 1 kW of power for 1 hour.

Back to our 20 kW that, for 1 hour, uses 20 kWh of energy. Easy right? If we kept the use up  for 24 hours it’s 20*24=480kWh. One year later we’ve used 175,200 kWh or close to 175 MWh. Much easier to calculate, right?!

Ok, so we see the discussion is not as straight forward and that the numbers both bamboozle and confuse.

Also, an average photovoltaic system can only produce full capacity about 50% of the time, noting annual output, rain or shine, for a 1.5kW system would be less than .75kW.  But, even at full output for 10 hours per day a 1.5 system will produce no more usable electricity than a total 5500 kW per annum, and in all reality is at least 50% less for 2700 kW pa.

Thus, if you were aiming to have 100% of your 8000kWh usage covered by a collection system, in my opinion,  you need , conservatively, a full 10 such 1.5kW systems, with a serious battery bank. The system would need to be cranking hard to produce all your day-time power plus would need to be recharging the battery-bank for the night’s demands.

I know this is a flawed equation but continuing with the green theme and the inherent delusion being marketed we see that a system which would cater to a modern domestic household would need to be:

a)    Able to produce for use and storage a static 1.5kWx 10 for an on call 15kW at any given time x 10 hours per day. Note that storing the energy produced requires batteries, a lot of batteries.

b)    The foot print of this system would be huge. The panels plus the batteries would require some serious infrastructure and loads of maintenance.

c)     Buy-in cost 10 x $2399.00 for a total of $23,990.00. This does not include battery cost which could be up to double the cost of the collection system.

d)    Looking at a 10 year life span on the collection system we’d  be taking 2399.00 annual cost / 8000kW used pa for an average costing = $0.30kWh.

e)    If my experience is correct and you only produce 50% of capacity from a collection system, the cost jumps to  $0.60 kWh.

And we don’t have a battery bank factored in here.

I know there is no “definitive” answer in the “energy discussion” but I keep coming up against confused thoughts and discussions when it comes to what is GREEN. We like the warm-fuzzy feel of GREEN. But, indeed, is IT, any of IT,  green?

Noting the world prices for electricity:

electricprices copy


I still fail to find an advantage to a domestic photovoltaic collection system, unless – like the one on my roof – it is subsidised.

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