Questions from newbies! PV capture 'capping' ? Different PV voltages?

70sbudgie

kW=amps x voltage

In a house the voltage is usually fixed (+/- a few %).

However, I have noticed that my pv panels voltages vary. So if your batteries charge from the panel's DC, it may allow you to set a charge rate that somewhat reflects your panels performance? 

Can you select a different discharge to charge rate? Because you might not want to mess with your discharge rate too much.

nitsuanomis

Thanks for everyone's input so far - really useful and interesting; we do appreciate you taking time out of your days to respond so comprehensively.

If we understand it correctly so far, the key takeaways seem to be:

1. The inverter could be more powerful - we're 'wasting' power that we could otherwise harvest on a good day (essentially, we can't generate more than the 3.68kW we can export, even if we could consume some of it ourselves)

2. Delaying the charging of the batteries will not help with maximising usage in our case (as we can both charge the batteries via DC and export up to the capped 3.68kW maximum at the same time)

3. Switching to a larger inverter that could export more would require G99 certification from the DNO (which can be time consuming and expensive, but if desired should be done sooner rather than later due to possible future reductions on export limits)

3b. That said, a larger inverter that limited the grid export is a possible alternative (is 'limited via software / configuration' valid here, or does it need to be 'hard wired' somehow?)

4. An iBoost (which we do have) will allow us to capture some of that 'would have been exported' energy into our own 'heat store', but reduces the exported power by the same amount (same reasoning as in 1. above)

4b. Note that this was turned off in the previously posted graphs as it always seemed to be constantly on; we have since increased the 'trigger' wattage and it is behaving much more as we expected it to

5. A third battery may well help to remove the grid demands during peak consumption (and is something we're considering anyway to help cover our 'hours of darkness' usage)

Firstly, have we understood correctly? We're still very much learning, so more than happy to be lead back onto the correct path if we've strayed anywhere!

Secondly, do these observations ring true:

i. Although the PV array is 'oversized' for a sunny day, it will certainly help keep up PV production in the dingier seasons?

ii. An additional battery - even though the ROI will likely take several years - would be a useful investment to both help handle the current peaks and save more of that exported power for our own use?

iii. Like many others, we are planning to transition to an electric vehicle sometime in the next few years - this will help us capture some of that additional power (although we'd still be limited by the 3.68kWh maximum generation); could we somehow as the charge point for this is installed leverage additional PV generation capabilities?

Many thanks in advance for your continued assistance!

Mikeyboy01443

Sorry if i have hijacked your thread a bit 😁
Would you speak to your installer about sizing up? They might do a swap at smaller cost... i cant even get a response from mine these days doesnt fill me with confidence if there was an issue....

70sbudgie said:

kW=amps x voltage

In a house the voltage is usually fixed (+/- a few %).

However, I have noticed that my pv panels voltages vary. So if your batteries charge from the panel's DC, it may allow you to set a charge rate that somewhat reflects your panels performance? 

Can you select a different discharge to charge rate? Because you might not want to mess with your discharge rate too much.

Yes i can set a different charge and discharge rate, both currently set to the maximum 30a, i think this may go up when (if) my second battery ever materialises... this is solax kit btw

Reed_Richards

I have grid export limited by software, I don't know how it is done but I'm pretty sure it is not "hard-wired".

I have a 6.5 kWh battery and an all electric house.  At the moment my battery sees me through to the next day, typically, so I have no need of a bigger one (except for peaks in power demand which are usually quite short).  In winter my battery never gets the chance to charge significantly so a bigger one would not help.  Thus I would actually only benefit from a bigger battery for a while in spring and autumn.  The battery size that is optimum depends very much on your pattern of use.

nitsuanomis

Interesting idea, @Mikeyboy01443 - I might talk to the installer about options here, although I too am waiting for an update on an additional battery... Guess supply < demand at the moment

Thanks for the confirmation, @Reed_Richards - I guess the export limit is not necessarily a safety thing then, as software control could relatively easily be overridden.

Heedtheadvice

Hmm......network safety thing it is as the DNO needs to ensure that local network capacity is not exceeded. If it is safety of supply can be affected.

You might think  that green generation is a great thing (which it is) but in some areas where there is already a lot of renewable generation or simply there is not the safe capacity on an old  network, the capacity is simply not there at present. Maybe at some times but the generation and supply needs to be balanced and meet spec at all times.

As writ In a post by @Solarchaser previous South of Scotland area if difficult to get approval, as an example. The network is being expanded to provided improved links for all the Renewables generated in the area but still we get high mains voltages intermittently. Uncontrolled generation exacerbates the problem. Part of the  control mechanism is to require approval for higher generation connections. Network development is not really keeping up fast enough, IMHO!

Mikeyboy01443

Magnitio said:

The following graph shows an example of battery charging as a way of using the output from solar panels when it exceeds the AC limit of the inverter.

In this case, the AC limit on the inverter is 5.5kW. By setting the battery to charge at 1.25kW, output from the panels up to 6.75kW can be handled.

And here is an example on the same system where the battery was fully charged too early in the day:

As you can see, the output was limited to 5.5kW for several hours.

If you have Agile Outgoing Octopus and the ability to control the battery, then you can export some of the energy at high rates in the early evening or early morning and then let the battery fill up during peak solar hours. This does mean checking the forecast and electricity rates and fiddling with the setup, but it is quite interesting.

I think i saw further evidence of this today, my batteries were full but it was sunny, so my inverter hit the cap of 5.5kw, i had my car plugged in (2kw) and it was exporting at about 2.something kw.
But if my batteries were charging then that would have in theory been able to pull the extra load from my panels.
So the question being if i want to limit my charge rate, as a test, what could i Set it to (in amps) to achieve a lower charge rate? Is currently set to 30a max, it is currently charging at 946w and the info says the current is 7.8a... so I'm guessing setting to 10a would give me a charge rate like yours of about 1.2kw?

Edited to add i have set my charge rate now to 10a, i can tune this as i need to but I'm interested to see if it works as i think it will 🙂

70sbudgie

Heedtheadvice said:

Hmm......network safety thing it is as the DNO needs to ensure that local network capacity is not exceeded. If it is safety of supply can be affected.

You might think  that green generation is a great thing (which it is) but in some areas where there is already a lot of renewable generation or simply there is not the safe capacity on an old  network, the capacity is simply not there at present. Maybe at some times but the generation and supply needs to be balanced and meet spec at all times.

As writ In a post by @Solarchaser previous South of Scotland area if difficult to get approval, as an example. The network is being expanded to provided improved links for all the Renewables generated in the area but still we get high mains voltages intermittently. Uncontrolled generation exacerbates the problem. Part of the  control mechanism is to require approval for higher generation connections. Network development is not really keeping up fast enough, IMHO!

This is the main difference between G98 notification and G99. The limit on G98 correlates with standard equipment for domestic properties 16A), so the DNO can assume that the equipment ratings are very unlikely to be exceeded. It is also unlikely to have so many houses with solar panels in an area that there is a net generation feeding back upstream - your generation will offset your neighbours' demands, but are unlikely to exceed it. However, as domestic generation becomes more prevalent, the DNOs are starting to crack down on registrations and the SEG will help them do this.

For G99 level generation, you have to apply in advance and I think (but am not totally certain) that the DNO is allowed to charge you if they have to upgrade any equipment as a result of your application. They are not allowed to say no you can't connect, but they don't have to let you connect for free and of course you can always choose not to pay the costs and therefore not connect.

Edit: If you want to get an idea of the potential scope of you connection, have a look at the cut out - they come in difderent sizes.