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Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 9:43 am
by mbbphoenix
Morning all
Firstly, thanks Will for setting up this forum, and also for the helpful YouTube videos about HomeAssistant! (I've managed to get the RS485 interface running using your instructions)
This is a great place to discuss ideas, and it's not FaceBook!
I have recently had a Hybrid installation; 19 off 380W panels, an H1-5.0-E inverter, and 2 HV2600 batteries.
I like the kit, but didn't have a great experience with the installers and hence why I'm posting my question to the forum rather than consult the original installers.
I have 2 questions:
- Is the 5kW inverter sufficient for a solar setup with (in theory) a peak generation of 7.22kW? (or is it standard practice to "de-rate" the inverter assuming that the panels are not 100% efficient)?
- The installers claimed that the panels can generate power faster than the batteries can be charged (which seemed reasonable), but also claimed that adding more batteries wouldn't make any difference, ie more batteries would still charge at the same rate (but I assume would charge for longer) and hence there would still be same peak excess power. Is this correct?
Basically I'd like to retain as much generated power in the batteries rather than sending it back to the grid, and am trying to work out if I should get more batteries and if so, how many.
Many Thanks
Martin
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 10:54 am
by Will
Hello,
On the inverter sizing, yes it's best practice and normal to underrate the inverter slightly vs the full possible panel generation potential value. The panels will normally only generate their rated wattage in lab conditions so in the real world they'll perform slightly worst. Inverters have an odd characteristic that means it's better to undersize than oversize your inverter. This is because the panels and inverter work best near their maximum ranges for efficiency reasons. It's better to have more hours at peak generation even if slight clipping is happening. Installing a 6kW inverter that is hardly ever producing 6kW will perform worst than a 5kW operating close to the limit more of the time.
On battery charging, if you have a 5kW inverter, it can only "invert" 5kW maximum which is a number that is shared across the solar generation, house load and battery charge/discharge. So regardlesss of how many batteries you have you can never charge them at more than 5kW but this would be that nothing else can be using any power share on the other inputs/outputs.
For example, on a perfectly sunny day your solar generation is 6kW solid (unlikely but bare with me), if you house load was 1kW then only 4kW could go to charging the batteries, the spare 1kW would simply be lost to clipping. if your house load was 4kW your batteries can't charge faster than 1kW. If your house load was 5kW, regardless of solar generation your batteries wouldnt be charging until the house load reduces freeing up inverter capacity to do the battery charging.
Your inverter will never output more than 5kW regardless of source. Should your solar system be producing more than 5kW the inverter will simply derate the solar output to keep it around 5kW max. This process is called clipping.
The more batteries you add the more capacity you have but the charge rate will always be 5kW so the charge time just increases as you add more batteries.
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 2:07 pm
by mbbphoenix
Hi Will,
Thanks for your detailed response.
I'm intrigued by "clipping". I see that the inverter has a max power rating (in my case 5kW) which it can split between its "outputs", ie house load and/or batteries.
In the (albeit unlikely) event that the solar panels are generating more than the inverter rating, where does this "clipped" energy go? You mentioned that in this scenario the solar output is "de-rated". Does the inverter somehow control the panel efficiency, or does it just dump this excess energy somewhere? For example, does it feed it back to the grid (somehow bypassing the inverter)?
Regards more batteries, am I correct in thinking that I need to measure the following condition:
the smaller value of "solar panel output" and "inverter max rating - house load", when the batteries are fully charged.
Converting this into kWh will give me an indication of how much more battery storage I could have achieved.
Is that correct (or have I missed something)?
Once again, many thanks!
Martin
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 3:21 pm
by Will
FoxESS like many inverters have MPPT solar features which is Maximum Power Point Tracking which can alter panel efficiency to match demand and capacity of the inverter.
I believe the unused solar is converted into heat by the panels and inverter.
Regarding what size battery you would benefit from. I would look at it another way, it's all about usage and ROI on the battery purchase.
Most people in the uk use between 12-15kWh worth of power a day. You can find yours by checking an energy bill from before you had solar.
To cover the average home you would need 5 - 7 HV2600 batteries.
I would focus on how much power you're using from the grid and slowly add batteries to get closer to covering your daily usage even during poor solar days.
The aim is not to always cover 100% of your usage but enough that you gain a good ROI on your system.
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 5:00 pm
by mbbphoenix
Thanks for the explanation of "clipping" and MPPT - very interesting.
Regards the number of batteries, I agree that the ROI has to be weighed up in all of this. However my aim was to use the actual inverter data (hence the reason for hooking up the RS485 interface) to better predict a "sweet spot" of cost vs performance rather than gradual incremental steps. I also thought I'd seen in another post that that new batteries couldn't be added to a setup more than a year after the first battery was commissioned. Do you know if there is that limitation?
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 5:03 pm
by Will
Good call on the data driven decision, home assistant will clearly show you how much battery/solar and grid you're using.
On the battery side, there is no formal requirement that you install new batteries within a year, it's simply best practice but you could technically install more batteries in 2 years' time and it will work.
Re: Sizing a Hybrid Installation
Posted: Tue Oct 18, 2022 5:12 pm
by mbbphoenix
Thanks for your comments - as always, very helpful.
As an aside I know that HomeAssistant is popular and very powerful (and has been very useful in debugging a few RS485 interface problems I had). A work colleague has done a lot with it, and raves about it.
However I personally don't find it that intuitive; I think I may have to brush up on my Python and write something simpler and targeted at exactly what I want
Re: Sizing a Hybrid Installation
Posted: Mon Jan 02, 2023 2:35 pm
by RedBaron
I suspect the issue with fitting further batteries later will be the impact on return on investment. The whole battery system will likely get scrapped at the same time, so if some batteries are fitted later they get less use, and thus negatively impact ROI compared to fitting them at the start.
In principle, this would suggest considering adding new battery stacks (assuming system can handle this), rather than batteries to an existing stack, if you want to add a significant amount of capacity.
Re: Sizing a Hybrid Installation
Posted: Mon Feb 27, 2023 11:45 am
by spmaid
Hello
Just wanted to pickup on a small item of detail on this, regarding inverter size, batteries and house load. I have a Fox Hybrid inverter and I can still get the AC rating out of the inverter and charge the batteries if the PV generation is greater than the inverter AC rating. i.e. I have a 7.8kWp array, 6kW hybrid inverter and when at full solar production, I can use 6kW in the house and still charge the batteries at up to about 1.8kW, as they are on the DC side. Once the batteries are full though, then my production is clipped to 6kW as that is all the inverter can output to AC. I therefore try and maximise use in the house during high generation to lengthen the time the batteries take to fully charge so as to not lose up to 1.8kW of production. But this has to be balanced with making sure I have enough leftover during the day to fully charge the batteries of course.
Right now, I am generating 7.106kW, using 4.351kW in the house and the batteries are taking 2.645kW. I have a screen shot but can't seemingly paste in to my post here.
Will wrote: ↑Tue Oct 18, 2022 10:54 am
Hello,
On the inverter sizing, yes it's best practice and normal to underrate the inverter slightly vs the full possible panel generation potential value. The panels will normally only generate their rated wattage in lab conditions so in the real world they'll perform slightly worst. Inverters have an odd characteristic that means it's better to undersize than oversize your inverter. This is because the panels and inverter work best near their maximum ranges for efficiency reasons. It's better to have more hours at peak generation even if slight clipping is happening. Installing a 6kW inverter that is hardly ever producing 6kW will perform worst than a 5kW operating close to the limit more of the time.
On battery charging, if you have a 5kW inverter, it can only "invert" 5kW maximum which is a number that is shared across the solar generation, house load and battery charge/discharge. So regardlesss of how many batteries you have you can never charge them at more than 5kW but this would be that nothing else can be using any power share on the other inputs/outputs.
For example, on a perfectly sunny day your solar generation is 6kW solid (unlikely but bare with me), if you house load was 1kW then only 4kW could go to charging the batteries, the spare 1kW would simply be lost to clipping. if your house load was 4kW your batteries can't charge faster than 1kW. If your house load was 5kW, regardless of solar generation your batteries wouldnt be charging until the house load reduces freeing up inverter capacity to do the battery charging.
Your inverter will never output more than 5kW regardless of source. Should your solar system be producing more than 5kW the inverter will simply derate the solar output to keep it around 5kW max. This process is called clipping.
The more batteries you add the more capacity you have but the charge rate will always be 5kW so the charge time just increases as you add more batteries.