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Add an extra battery to solar system question
We already have solar panels with microinverters and looking for additional battery storage. We have decided to go with 2x Fox EP12 batteries as we’re a high energy usage household. Our avg daily consumption excl EV is between 20-30kWh. We would like to add a 3rd battery at some point in the future when budget allows. The plan is to charge the batteries overnight on our cheap tariff.
What size inverter should we go for? A 6kWh or 10kWh given we would like to add a 3rd battery in the future? Common sense tells me we should opt for the 10kWh inverter but I have also read about not oversizing inverters for efficiency! Any advice much appreciated. Thank you!
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19 Replies
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I would go with 10 at least. You are using a lot of KW’s is that for a ASHP or weed farm?
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Haha! It’s neither but I am wondering the same. We’ve recently moved into the property so need to do some investigation as to why we’re using so much electricity. (It is a bigger property with 2 adults at home most of the day running laptops, monitors, networks, 2 fridges, 2 freezers etc but still…)
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How do you heat your hot water? We have a similar daily consumption in a household of 4, but I know the bulk of our usage is due to the fact that our gas boiler is not plumbed into our hot water tank (thanks to previous owners of the house) and we’re reliant upon a 3kW immersion heater.
Fortunately, we are on an EV tariff (no EV and not required with Good Energy) so heat up the tank fully overnight during the cheap window and charge up the batteries for use throughout the day meaning we don’t need to import any peak rate electricity.
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you really do need to investigate I think. We’re a large property. Effectively 2 semi-detached houses as my mum and dad live next door but we all run off the same system. They’re in the 70s/80s so home all day. My husband and I are both retired. 4 freezers, 2 fridges, usual other IT stuff, 2 washing machines and dishwashers. Plus an EV. Our average daily usage is nowhere near yours.
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One factor that will affect inverter size will be what the DNO allows. If you already have an existing PV system, depending where you live the DNO may not allow an extra 10kW inverter. This is something that would be worth discussing with an installer.
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Would this still be the case if it’s AC coupled? We wouldn’t want to export from the batteries and the solar has it’s own inverter.
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or depending on the existing PV system and location they could impose an inverter limit. It will be down to the DNO to decide.
the fact it’s AC coupled is the reason it would need a DNO application and approval before installation. The DNO may put a limit on inverter size dependent on already installed capacity and your local grid infrastructure.
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That’s a fair usage number, do you have scope for further panels to be added alongside battery storage? Would give you the best of both worlds if so.
DNO are going to be the ones to determine your capabilities with import and export and you won’t know until this has been submitted and the response from them unfortunately. I’d suggest going for the 10 and then seeing what they come back with.
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Hi Cheryl, this video might be helpful to you:
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I’d first be investigating why such high energy use if you don’t already know. Your energy use is similar to mine when I also include an EV driven 80 miles daily and a heat pump. With regard to inverter sizing, there’s also two use-case considerations; max draw load likely at any one time, but more importantly the ability to fully charge your batteries in your off peak period. For example if you empty a 30kWh battery daily, you’ll need at least a 5kw inverter to charge it back up under octopus IOG.
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You can pretty much refill 3 of those batteries in 5 hours with your EV tariff on a 6kW inverter but I’d look at the cost implication of going bigger likely only a couple of hundred £?
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yes, cost wise, its not much more for a 10kWh inverter. It’s just whether there are any drawbacks in getting a larger inverter now whilst we only have 20kWh of battery.
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look at the start up values for each model. Quite often they are the same on the next size up. I’d say 8kW’s would be perfect if Fox make one that size.
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Larger inverter is helpful as will cover larger loads and charge batteries quicker, your EV usage does not need covering so 2 batteries are probably ok.
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there are two main factors that influence the inverter size
1. Household sustained peak load – if you run multiple 3kw cookers for hours a day then you’ll need a bigger inverter than if you run 1 cooker for an hour. The emphasis here being on sustained load. My main household peak load is around 3kw, although I have a 9kw electric shower in the morning. However as it only lasts 10 minutes a 9kw inverter would be overkill.
2. Size of the array on each string. Most inverters have limits in terms of the amount of solar they can take on each string input (MPPT) and in total across all arrays. It is industry practice to slightly undersize the inverter and whilst this can result in clipping, it’s a balance between an oversized inverter that may not generate well in low-light/morning/evening and an undersized one that could result in clipping. This post explains a little bit more
Freedom Forever – Solar Power for your home – Freedom Forever
Lastly whilst not really a consideration, it’s important to be prepared to change the design depending on what limits your DNO may put on your system from an export perspective. Having a huge array and a large inverter that is restricted by your DNO to 3.6 kW export might be a waste unless you have a significant household demand that can consume the majority of what is generated and therefore you’re not losing much through an export restriction
Another consideration in your case is that you already have some inverter capacity through your micro inverters. As such you will need to apply to your DNO for a G99 for the total capacity of all inverters in your household.
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We do a lot of Fox multi battery installs, and inverter sizing is the most common thing customers underestimate.
Simply consider tariffs being used and charge periods for off peak, naturally we want to have as many available for varying lengths (sizing an inverter for a 5 hr window will leave you short if they drop to a 4 hr window), we would then also need to consider household load I.e what are you using that means you need 3 batteries?
This in most cases, with a desired end result of 31kWh of usable storage means you will be best placed to size up at minimal cost now, rather than later. Generally a few hundred pounds for the KH range.
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It’s probably best to check with your local dno to establish what you may be able to install…. unfortunately we can’t just add more batteries when we can afford them as we may be over the local capacity…
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Battery storage with fox is limited to their own system and about 30kwh on a single inverter. If you want more storage you can install an extra fox inverter in parallel (has too be a gen2 inverter, the gen1 doesn’t have the ports) you can install another battery stack per inverter, up to 300kwh. Alternatively you would need to swap the inverter to another brand like Sunsynk which supports 3 party batteries like Fogstar which are the best value per kWh. Some excellent info on this channel
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My original install was 2 EP11hs, now 3. I got the KH10 inverter despite my 10kWp array being split over 3 aspects (so unlikely to require the 10kW inverter in itself) as it was a few 100 more than the 9kW inverter and has 4 MPPTs over 3 so an extra array should I want to.
The only time it ever get maxxed is when I’m getting paid to suck from the grid for a short period and want as much as possible
🙂
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