Sizing a battery bank and inverter/charger

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hammer asked May 25, '19

Sizing a battery bank and inverter/charger

Hi, apologies in advance for the all the scatter gun questions!

Hopefully someone here might be able to help me to size and choose the right equipment for a bill of materials.

I know what they say about assumptions but, let us assume that I need to operate consuming approximately 27.5kWh.

Our energy supply costs for the year are on track to come in at £30K+ per annum, at least whilst we continue to consume nearly all of this power direct from the grid. With that in mind we need to design and build an energy storage system that would help to reduce this particular operating cost significantly.

Based on my research to date I seem to be on the right track, but given the likely sum of our intended investment I would like to make sure that I am not undersizing or oversizing (or going wildly over budget!)

The intention is to scale up to generate and provide up to a 15kW input from renewable energy (wind/solar/biomass). But would this be able to provide enough charge assuming I can consistently supply the 15kW, whilst consuming 27.5kWh?

I read somewhere that there is usually a need to respect a 40/60 ratio in charge vs consumption, but if this is incorrect then someone please correct me! :) Also do Li-ion batteries allow for a much better performance in this respect?

Anyway, I am looking at the Victron Quattro 48/15000-200-100/100 15KW off-grid solar to run alongside a suitably sized battery bank (vendor/size suggestions welcome).

So, is this a wise product choice, and would we be able to run on continuous mains power via the Victron Quattro + battery bank storage for our power needs, until we have all the other RE components in place?

Many thanks!

battery charging victron products battery capacity design

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ejrossouw commented · May 26, 2019 at 05:20 AM

Just some food for thought. I am not exactly sure why you require a battery bank? Won't a grid-tie solution will be a more efficient use of the energy produced? Also, should you not in conjunction rather look at the consumers and make those more efficient? Given your consumption is so much higher that what you plan to generate, at a glance it is obvious you will only ever play catchup ... and not to mention the weather dependencies. Another thing to consider, is ROI just don't be fooled by the apparent savings as I suspect you may have three phases and in case of e.g. solar (30-40 panels required plus charge controllers etc. etc.) and wind need serious hardware just to ensure you can deliver close to the ambitious 15KW you mention ... plus it has to be maintained. For this one, I'd definitely get a professional involved. To conclude, you will more than likely not save much given the large investment required, but rather do it to reduce a dependency on fossil fuel and ripoff energy providers.

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hammer ejrossouw commented · May 26, 2019 at 05:45 PM

Thanks for the detailed reply.

Reading back what I have written, it is not very clear in relation to how we are attempting to achieve the numbers mentioned. This will be an incremental deployment though and will likely need to be a grid-tie solution, you are right.

The consumers themselves are being looked at in parallel but this is an equally expensive undertaking, and so the priority would initially be to bring in the Victron Quattro, with a battery bank large enough to at least reduce power costs.

If we can have a design whereby the power is taken from the stored battery power, and flip over to mains when the charge gets below a certain threshold, I can see immediate cost savings there - this is of course if we are using grid power during off-peak times.

Incentives to move off fossil fuels are high so we are motivated to beef up what we already have, but also take your point about getting professional advice.

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Answer 1 · 2019-05-25T22:42:05Z

Pat Davitt answered · May 25, 2019 at 10:42 PM

I assume that the 27.5 Kwh consumption is daily. So your yearly consumption is about 10,100 Kwh per year. That same usage in the US would cost about $1,274 USD per year based on the 2019 average cost of .127 USD per Kwh. I am having a hard time believing that the same consumption in the UK cost 30,000 Quid per year. Could you please clarify.

Thanks,

Pat

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hammer commented · May 26, 2019 at 02:26 AM

Hi Pat, that's 27.5 kWh hourly consumption... hence the £30K! We do have a modest solar and wind presence, otherwise we would probably be upwards of £35K per year.

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Answer 2 · 2019-05-26T02:39:44Z

Paul B answered · May 26, 2019 at 02:39 AM

Hi Hammer,

You would be best to go to you local Victron Dealer/ Distributor and they will help you configure/spec out the system

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hammer commented · May 26, 2019 at 07:39 PM

Agreed. Thanks!

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Answer 3 · 2019-07-29T14:30:50Z

neo answered · Jul 29, 2019 at 02:30 PM

Hi,

I bumped into your post by mistake but all I can say is that sizing a renewable installation is not so easy because of all parameters involved : battery capacity, c-rate, technology used, PV capacity, geographic PV seasonality, consumption profile and tariffs... so it's quite hard to find the most economical solution (for the short and long term RoI)! But I can surely help on that matter. Just contact me directly.

Regards

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hammer commented · Jul 30, 2019 at 12:28 PM

Thanks! I've no idea how to contact people directly here, but appreciate any assistance you're able to provide on this subject matter.

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neo hammer commented · Jul 30, 2019 at 09:38 PM

Me neither I have to be honest... first time I log in !

My personal email is neolec.ch (a) gmail.com

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