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Summer/Winter difference

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Summer/Winter difference

Posted 06 Dec '16 12:09 AM

We use a lot of electricity with a pool pump operating 6+ hours during the summer and underfloor heating in winter via heat pump and water pipes in concrete floor. A PHEV hybrid electric SUV.

We recently took the plunge and have a 2.9kw solar system installed as of last week with no batteries at this stage. We could only fit 10x 290 watt panels on the roof facing north due to a thermal solar hot water system taking a lot of room.

With almost cloudless days (early December) in Tauranga, here are some stats for a 24 hour period...

Peak generated around 1pm: 2.3kW
House Load: 32kWh
Imported: 19kWh

So we saved importing 13kWh.
5kWh was uploaded to the grid. Trust power pays 7c.

I expected a higher peak power generation with only 15 days to peak sun radiation on 21 December. Will it get to 2.9kW?

What would be expected on a sunny day at noon in mid-Winter?

We paid $13,225. I was expecting the return on investment to be around 8 years but I'm not so sure now.

Cheers,
Ross.

RE: Summer/Winter difference

Posted 06 Dec '16 01:51 PM

Hi Ross,

I could help you figure this out.
What is the angle of your roof? Do you have micro inverters or a string (central) inverter? Do you know what size your inverter is rated? Often you can have solar panels which will have a different total kW size from your inverter.

We have had some great sun in Tauranga recently, so it should be hitting close to it's peak.

Cheers
Kristy

RE: Summer/Winter difference

Posted 13 Dec '16 12:04 AM

I replied to Kristy a few days ago but it doesn't appear.

I think the roof is 20 degrees, perhaps 18.
Enphase M250 microinverters which I now realize is what is limiting the output. The graphing of the solar output is very flat between 12 noon and 3pm. I'm questioning why S280 were not used at only $15 USD more each.

I'm still interested in what generation in winter will be possible.

Thanks,
Ross.

RE: Summer/Winter difference

Posted 13 Dec '16 10:11 AM

Hi Ross,

A peak of 2.3kW sounds about right for the peak power output of your microinverters which is 250W. This means for your system of 10 panels, the maximum amount of power your system will generate at any given time is 2.5kW of solar power. If the conditions are right you may get slightly closer to reaching 2.5kW output, but there are quite a few factors to consider that will decrease your solar generation. You will lose solar power generation due to inverter's efficiency which is 96.5%, a small amount of power lost in the wiring, any dirt or dust (pollen) on the panels will decrease the output and if the temperature is above 25 degrees then the panels performance will also drop slightly, by about 10%.

The optimal angle for solar panels in Tauranga is closer to 25 degrees, but you are only losing about 0.1% with your solar panels being a 20 degree angle.

The limitation of the micro-inverters doesn't actually effect much of the overall power generated during the day. If you look at how much power the system generated over the day, to some degree the power generation on a line graph will look like a bell shaped curve. Limitations of the micro-inverter output size essentially shaves a small about of the bell shape curve at the very top.

This diagram shows the inverter undersized effect well: http://www.yourenergysolutions.com/wp-content/uploads/2016/02/Enphase-Clipping-vs-Not-Clipping.png

This clipping effect will only occur on the sunnier days of summer, it won't effect the systems performance in winter.

I think the 8 year return on investment estimate is about right, particularly as it sounds like you'd be using a lot of solar power directly rather than exporting it to the grid. The important numbers to look for in a solar power system design is how much power will a system generate over the length of year rather than any given second of the day to make sure you get a good return on investment.

Do you have your pool pump and heat pump on timers or controllers to make sure they use as much of the solar power as possible?

Regards
Kristy


RE: Summer/Winter difference

Posted 13 Dec '16 10:14 AM

Hi Ross,

I work for Enphase. As a general rule we recommend oversizing the PV for the inverter. It tends to maximise the yield. You can read some more background here; https://enphase.com/en-us/blog/over-sizing-v-right-sizing-new-ideas

The S280 isn't available in the NZ market at this stage. The M250 is a good choice for a 290W module. The 290W rating is based on standard test conditions, which is 1,000W/sqm at 20 degrees Celsius. You are likely to get more than 1,000W/sqm in the BoP, but that will usually be when it is very hot. Temperature has an impact on PV module performance. Roughly half a percent drop in efficiency per degree celsius.

Hope this is useful.

Cheers,

Chris

RE: Summer/Winter difference

Posted 14 Dec '16 02:04 AM

Thanks for the replies. Sounds like M250's may be adequate in that case. Could be useful now during summer but not after a few years.

Maximum generation I've seen so far is 2.26kW and very steady on that between 12pm and 3pm which I think suggests the panels are producing much more than 250W each and the limitation is in the inverters. Using the 96.5% advertised efficiency, I would expect 2.41kW, so there is some additional loss occurring there somewhere. Perhaps in the cables?

But going back to my original question, what should I expect the system to be producing on June 21 at midday if it is a sunny day?

Cheers,
Ross.

RE: Summer/Winter difference

Posted 14 Dec '16 09:55 AM

Hi Ross,

Sorry I don't know the answer to that question. All I know is that you'll generated approximately half the amount of solar power during the month of June as you would for December, so that's half the expect kWh for the month. But that doesn't mean that any given second that your system is expected to generate half the amount of output (i.e. kW), because in winter you have to account for shorter days. Your solar installation company should be able to help you with that question.

Regards
Kristy

RE: Summer/Winter difference

Posted 15 Dec '16 02:06 AM

That's answered it, thanks.
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