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Otautahi Christchurch
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Otautahi Christchurch
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richms:
fastbike:
... richms:
The issue I was told with excessive overpanneling is with the MPTT when there is a sudden change in the sun, and how fast that it can track to not blow itself up. I guess some are just built with more headroom than others.
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That's why you follow the datasheet ... if the panels are within the allowable limit there will be nothing "blowing up"
Yes, this is why the theory of if you stick to below its max V input isn't all there is to it. I guess its a similar issue to when you load dump on a charging device but happening on the other side of it.
Sorry - your final statement doesn't make sense. If in doubt, consult an expert.
Nevertheless, if you stick within the limit on your datasheet, then you are fine. There is no problem from a 'sudden change in the sun".
You look at the maximum your inverter can take compared to the maximum voltage from your panels.
Maximum voltage from your panels is simply Voc multiplied by the number of panels in the string, with some temperature compensation (this is a relatively small correction, but important nevertheless if you are getting close to your inverter limit or operating in cold conditions).
The temperature compensation is easily calculated using the Temperature Coefficient of your panel. For example, let's say your inverter had max 550V rating, and you live in a climate where you want to allow for operation down to 0C, i.e. a drop of 25 degrees Kelvin.
From the Hyunsai Solar panel datasheet extract below note that nominal Voc is measured at 25C.
Then the temperature compensation percentage is (-25 K ) * (-0.22%/K ) = 5.5%
So, the temperature compensatd Voc for the 450W panel is 36.72 * 1.055 = 38.74V
So the maximum number of panels you could use would be ⌊550V/38.74V⌋ = 14 panels.
You may like to build in a little safety buffer, noting the +/-3% test uncertainty, and also use a lower temperature than what you'd ever expect (e.g. calcualate to -5C rather than 0C in case of freekish weather - though bear in mind that daily min temps typically happen in teh dark hours of early morning)
If in doubt, consult an expert.
[note - corrected from original post, as i had accidently used the value for the temp coeff on Pmax rather than on Voc in the example)
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Otautahi Christchurch
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Otautahi Christchurch
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Otautahi Christchurch
LightbulbNeil:
I feel like I have been living under a rock since Oct 2023. Was talking to people about power prices, and was rather stunned at the $2.85 a day to $3.25 a day line charges , and then the actual unit prices on top of that.
Does anyone have any insight as to the current predictions of supply charges and the increases in unit costs might be over the next 3 years?
It seems that now it is looking like we will be needing a battery etc which I am looking at and doing the math for it with the amounts we use over night and the ability to charge during the day from the roof. Some days in Winter the battery will not charge from the roof alone.
When then , becomes the best power plan with a 16kw/h or 32kw/h battery that may need charging? Do you charge during the day , or do you charge during the night time.?
I am looking forward to after 2028 when our plan changes, but wanting to have a plan in place in case something does change dramatically and we are not on the power plan we have currently.
Currently we are on a fixed plan that does not change until October 2028 with Meridian, and have been very happy with the current arrangement.
Any information will be appreciated.
Our system is 3 phase 15kw inverter with 15 kw of panels. In winter on a good day produces 38kw/h and on a bad day is down to 3kw/h in Hamilton.
It depends ... and trying to predict electricity prices changes is easy - they always increase. And applications to increase charges to support infrastructure upgrades are normally approved by the regulator.
I think the line charges depend on your location e.g. here in Christchurch (Orion) they seem to be about half of what people are paying 20km away in Kaiapoi (Mainpower).
We have a 10kW 3 phase inverter.We are on a fixed Meridian plan until Oct 2029: 104c per day, 17c export, 18.4c offpeak (9pm-7am) import, 31c peak (7am-9pm) import.
We have a 99% electric house (gas hob) and use heat pumps for winter warmth - the outside unit can pull 4kW. I installed a 34kWh battery in March 2025 and doubled it in Dec 2025 so we now have 68kWh gross. However I have set the depth of discharge to be between 15% and 80% so it is effectively 44kWh scaled capacity. This means the battery lifetime will most likely outlast my use.
Here's May,June,July, Aug 2025 data. You can see the consumption and import figures. If the forecast showed a rubbish solar day for tomorrow I would charge the battery to close to scaled 100% (within the DoD as above). This year with the two batteries I currently have a rule that runs from midnight to 6am to charge to 40%. If the forecast is bad I then charge between 6am and 7am to 60%.
HTH
Otautahi Christchurch
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