# PV Generation: Lessons I Have Learnt Part 2

Being a bit of a geek, I thought I’d ‘do the math’ on my PV system’s generation, total household usage and the amount of my own electricity I’ve consumed over the last 7 months.  I chose 7 months, rather than the total 10 months of PV generation, as it was 7 months ago that my electricity meter was swapped for a new backstop meter; before then, because of the backwards running meter issue, it was impossible to tell how much of my own power I was actually using.

So, what are the scores on the doors?  Since the end of September 2011, I have clocked up approximately 1000 kWh on my new consumption meter.  This being the amount I have needed to pull from the grid.  However, because of my nifty Wattson meter, I have a record – month by month and day by day – of my actual total electricity consumption over that time.  A brief calculation on my usage data from October until the end of April shows I have actually consumed a total of 1250 kWh.

Hence by the powers of subtraction I can say that, from the period from October to end April, I have used 250 kWh of my own generated power.  That doesn’t seem a huge amount really! Then I looked at my total generation over the same period, and it totaled a rather meagre 350 kWh.  So, this shows that I have used just over 70% of my total generated power from October to the end of April, higher than the average, assumed, 50%.  And I can also say that my system has provided me with around 20% of my total electricity need over the same scale.

I will perform the same calculation again at the end of the summer, it’s my theory that my percentage of used PV power will drop considerably as I’m expecting, if we do actually see some sun this year, that my generated power will start to really pick up in the next 3 months, but I will not be able to make optimum use of it as there is only so many laundry washes you can put on when the sun is shining!  The unfortunate truth is your peak demand for electrcity is in the evening, when you need light and are cooking, hence you still need to top up on grid electricity, even if you have generated more than enough power to meet your daily needs during the day.  I’m more than ever convinced of the urgent need for a small – in both size and capacity (only needs to be a few kilowatts), cheap, battery storage solution for domestic PV systems.  Only then will we really begin to make headway in reducing peak load issues.

So, what else have I learnt lately?  One thing is to check your system after a power cut!  A while ago, when the solar flares were threatening to hit earth and ‘destroy human life as we know it’ but then didn’t, we in south London suffered a power cut in the middle of the night.  It was all back to normal by morning, but, a couple of days later when I was absentmindedly checking my generation on a sunny day, I realised that I wasn’t generating anything?!  Cue mild panic and worry that maybe the ‘deadly’ solar flares had killed my system, and worried calls to my PV installer.  It turns out that it is not necessarily a power cut that can turn your system off, but if there is a power surge before the black out then that can certainly trip the system and it doesn’t automatically reset itself when the power is restored.  So the lesson to learn is regularly check your system, especially if you haven’t got a display unit that easily shows you what you are generating at any given time.  It could be easy to lose days of good generating potential without even realising it.

Related blog posts:

Lessons I have learnt Part 1

Electricity meter going backwards after PV installation?

Unintended consequences of PV systems and meters running backwards

• Andrew Wong May 11, 2012, 2:18 pm

Hi Paula. Interesting information on your blog since we 'spoke' nearly 3 months ago when I got my PV panels fitted. Since then they have generated 330 kWh from the 1.44 kW system.  Not bad for Scotland.  I have a question about the inverter which has a spec stating a maximum output of 1.2 kW.  This is confirmed by the little red LED on the generation meter which flashes for each watt-hour produced (i.e. 1000 flashes per kWh).  At best (when the sun is high and bright) it flashes once every 3 seconds and no faster.  This equates to the 1200 W stated (3600 secs per hour / 3 secs per watt).
I know there will be losses in the system but why isn't the inverter bigger so as to cope with up to 1.44 kW, e.g. the Schuco Sunny Boy 1700 instead of the fitted 1200 model?  The supplier says it is the right inverter for the 6 panel installation but are unable to explain why.  Any ideas?

• Paula May 11, 2012, 5:58 pm

Hi Andrew

Good to hear from you again. Interesting question, and I know there is a good reason for why they size the inverters to be close to or slightly less than the peak load on the system – but I can’t remember exactly what it is now. Let me get back to you on it. I know for my system (1.68) i have the 1.7 sunny boy inverter.

I’ll find out and get back to you

Paula

• Andrew Wong June 9, 2012, 2:26 pm

Hi Paula, have you been able to obtain any information of the inverter sizing question?  Many thanks. Andrew

• Paula June 11, 2012, 2:16 pm

Hi Andrew

Sorry for delay in this. This is what Jamie Beevor, Tech director at Paula Owen Consulting, has replied to your question:

My understanding from a conversation I’ve had with a PV installer friend in the past is that it is customary to undersize inverters because you rarely get peak output from your array. The greater the disparity between typical flow and inverter capacity, the greater the inefficiencies. So by undersizing the inverter you’re operating closer to optimum efficiency for more of the time. I believe that on days of high irradiance the power will be clipped to the inverter’s max capacity so you’ll lose a little bit on very sunny days but that’s offset by the improved efficiency during normal conditions.

They don’t have much detail on this in the installation guidelines but it is mentioned:

http://www.bre.co.uk/filelibrary/pdf/rpts/Guide_to_the_installation_of_PV_systems_2nd_Edition.pdf

• Andrew Wong June 26, 2012, 6:30 pm

Thanks for the very helpful advice on inverter undersizing.  This is what I suspected but the installer couldn't explain it.  It is none-the-less a bit disappointing on a clear sunny day to see the inverter limiting generation to 1.2 kW when the PV panels are going 'full tilt' probably generating the nominal 1.44 kW.  Ce la vie. Cheers.

• Gavin June 28, 2012, 4:31 pm

The size of the inverter is usually determined by the software that the installer uses to calculate the solar PV analysis. I believe that along with system size and panel type, the number of strings,  orientation, inclination and latitude are all used to determine inverter size,  location is also a factor.
I opted for an inverter that was larger than my system called for, but  has a low start up voltage with a wide range, 120vdc-450vdc.  This is currently producing about 40% more than forecast. I have yet to check this figure and it should be treated as an estimate only, although you are welcome to look and do the math yourself!
http://www.renewable-energy-resource.net/performance.html