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Off-Grid House

Progress Updates


WOW,! Looking back at the last post.. so much has changed. We now regularly install off-grid houses and power supplies using the Victron power supply along with solar, batteries and normally a backup generator. you can find more on this here: We are now in a new unit/warehouse workshop. We have 5000sqft of space with a 24.5kw solar array on the roof utilising Solar edge. We also now use Victron energy Inverter chargers that are hugely capable and programmable. In addition, the old house is running on traditional lead-acid batteries in HUB 4 format, this is a grid parallel system which is now legal in the UK. I will go into this further in a bit. The warehouse has a smaller system but we are testing a new saltwater battery.. yes that is what I said, a saltwater battery. It is practically inert, safe and has a cycle capacity better than the market acclaimed Lithium. The downside is they are huge. This building now has the solar and the “small” 6kwh battery bank with a maximum output and charge rate of 1.5kw. We do have a lot of solar for our usage in the summer but the proof will be in the winter. The old house project now has around 8kw of solar and 10kwh of battery storage. Pretty impressive figures. We do have a lot of solar for our usage in the summer but the proof will be in the winter. The old house project now has around 8kw of solar and 10kwh of battery storage.. here is what the latest graphs for the August show. On average during the summer we are only buying around 1 unit per day, due to battery storage.  If we didn’t have the storage this would be more like 4 units per day. This is mainly due to the ability to use solar power during the day, it is fairly difficult. Batteries make this easier to be efficient.


So you have Solar PV and you have monitored it for a while and you realise that in the summer you generate loads more than you use during the day, but have to buy electricity back after dusk. So you have a think and wonder if there is a device that you can store a little bit of that power into batteries and use it yourself at night? The answer is YES there is such a device and it stores power in batteries. But can we use it in the UK? NO! This is because of the rules with connecting power to the grid and how it should be isolated. Your standard solar inverter, if there is a power cut, shuts off. This is to protect the grid and any engineers that may be working on it (perhaps at the substation down the road). If you have power stored and your house generates mains power, you could electrocute an engineer working on the line in the event of a power cut, also the other local houses will be using your stored battery power and this will overload your system. To solve this an automatic grid isolation is required. In the UK we use a G59 contactor. This is a mechanical isolator controlled by an electrical current. In most of these battery storage devices, there’s an electrical circuit which diverts the power and cuts off the mains supply. This is the issue as in the UK we can only use a mechanical device. I am sure the manufacturers are working on it, in fact, I know this for sure and soon, hopefully, there will be a plug and play option. In the meantime,  we need data to know what size battery and how to get around any little faults and issues that may arise. This is where we come in. In stages, I will keep you up to date on the installation of the system along with my choices of design and why I have chosen this route etc. So firstly we have a south-facing roof with a 55-degree pitch. Ideal for winter generation as it has a higher angle than most roofs to collect more energy from the lower angle winter sun. 3.95kw of S-Energy modules are installed into a SMA Sunnyboy 4000TL inverter, chosen for the advanced features, high efficiency and inbuilt relays.


Choosing batteries: At the moment I am looking into battery choice. Making the correct decision on performance versus price is essential to make this system viable. The current price for electricity is around 15p per unit (KW). Other things to consider are how much energy needs to be stored for night use and poor daylight days.  This system will use a 48v battery bank. How this is made up is up to the designer.  You could use 4 x 12v car batteries or at the top end 24 x 2v cells. These 2v cells are usually much higher in power storing from 500ah – 4000ah in power. They are also longer lasting according to the manufacturers and therefore cost a lot more. Using car batteries is an option, but car or leisure batteries at 12v will result in a smaller storage capacity. For example 4 x 110ah batteries = 48v and 110ah which equated to 5280wh or 5.2kwh. As I’m sure you have realised, 5.2kw is not much power especially when your 4kw solar system generates an average of 15kw per day in the summer. This means that once again you are wasting energy. There is also a technicality with batteries that means you should only use 50% of the advertised capacity to increase its life. I will now go into the technical side of battery design a little more:
  • Batteries are normally rated at C20. This is a common test bed.
  • A battery with a C20 rating of 110ah means that this battery can supply 110ah of power over a discharge period of 20 hours.
  • A C100 rating for the same battery may be 140ah. This is because the battery is less stressed and therefore can supply more power at a slower rate for longer.
  • Conversely, the same battery may only have a C10 rating of 70. Depending on where and how the battery is to be used can determine choice in battery type.


ALL CHANGE!  Inverter change from SMA Sunny Island 6.0H which is the replacement to the ever popular 5048, to the smaller more eco SMA 2224 unit. Also now we have a battery bank, and it doesn’t comply with anything I have said previously. So here is the story… Firstly the inverter choice Initially, I chose the 6kw system to run the whole house. The idea was to run completely off-grid. The problem with this is that there may be a time, not perhaps now but later, with other owners perhaps, or even in future applications, where the user is not aware of power use. Trying to explain you can’t use the kettle and the vacuum at the same time as the oven is not always easy! So we have accommodated this, and at the same time cut costs considerably. The 2224 inverter is less than half of the 5048 plus the battery bank is now a 24v string with half the required capacity. So what is the plan? Well, we plan to segment the house from the kitchens main appliances:
  • Lights, most plug sockets, utility room, office etc will all be on the inverter.
  • One hall plug socket plus one kitchen socket and the oven will be on the mains as per normal. This is due to excessive load requirements from the oven plus the few other sockets have the capability of working if all else fails; backup sockets, if you may.
So we have a 24v to 230v 2.9kw inverter. Then batteries This was an easy choice. The initial plan, as sort of mentioned in above reasoning, was to use a battery bank consisting of 3 autonomy days. That’s the total use of the property as a daily KW rating times by 3. This has 2a few functions of reasoning. 3 times in size so that the bank has a large capacity and large loads are not stressful on the bank. 3 times so that the solar can have a few overcast days and still not require a generator input. 3 times to allow the battery to discharge to the optimal 50-60% DOD (depth of discharge). We use approximately 10kw. So 10,000w per day times by 3 = 30,000w to 50% DOD (to simplify it) = 60,000w rated capacity and at 24v = 2500AH required! At cost, that relates to around £10,000 + VAT. We started this to prove you can set up the system to work on around 750ah capacity costing closer to £3000. Well, we are now down to 380ah. 15% of the original capacity and industry standard calculations. Why, and am I mad.. ? Perhaps but this is more exciting. We have a top of the range telecoms, military and banking service support battery to try. The manufacturer Enersys have a very special AGM style battery that is used in these immensely important scenarios.  The SBS Eon is a high performing battery that typically is avoided by off-grid solar installers. AGM as a standard has a good cycle rate, works well in fluctuating temperatures, has better power output, is very efficient at accepting a charge and has a good shelf life. This means that it can be charged and discharged a lot and still perform well over time. It can remain charged without maintenance for a long period of time. It can do this better than a flooded or gel battery. So it’s perfect for solar? Not really. Aside from the cost AGM’s weakness is its charging cycles. They like high power charging for short periods and they perform best when fully charged between uses. Solar power systems cannot guarantee this. This is why Flooded or Gel are used as they can work well in this situation. Enersys are slightly different in their thinking. They believe that their modified SBS Eon AGM blocks can perform well in all situations. Each application has slightly modified manufacturing values to accommodate the particular application. After visiting their factory we believed that we can supply some good raw information and work with Enersys to develop the ultimate battery for use in the UK off-grid solar market. The inverter is now set up and awaiting the arrival of the battery bank for rugged testing. We will be installing a portal so that you can view our statistics on how the house is running, how much we are generating, using and how the batteries are performing.


So the batteries arrived last week – Enersys SBS EON 190 Big and red and weighing a small tonne. Well, 60kg actually but they definitely took some work to get them off the van and into position. Supplied with the batteries are some neat copper flat bar connectors and screws. The 4 x 12v blocks very quickly became 2 x 24v. I wired these into the fuse board making sure all of the connections are torqued up and then started the inverter. The inverter has a few simple settings to engage upon first startup or a new set of batteries. These are easy to enter: battery size, battery type and the use of the inverter (off-grid or as a backup system). Actually, there are hundreds, in fact thousands, of parameters that can be set. I won’t bore you so in short, this is how the system has been designed.
  • It will engage a backup power at 30% of battery capacity. This means when the batteries only have 30% left the inverter will activate the mains power supply and charge the battery to 40%
  • If the power demand is over 1.7kw (inverter has 2.2kw capacity non-stop and 3kw for 30 minutes) the inverter will activate the mains whilst required and for 1 minute afterwards to charge and stabilize the batteries a little.
  • After every 14 days there will be a full charge cycle and if needed the mains power will engage for this. If the sun shines lots and charges the batteries fully during the 14-day countdown, the counter will be re-set.
  • Upon connection the system showed only 46% capacity in the batteries, this is a default value and after some time the system will learn the bank and make more accurate estimations.
I am hoping that with around 1 hour of sun each day the system will not require the mains power. In the summer this is easy. Winter time, perhaps not so. Fingers crossed and I will be posting the findings each week as to how it is performing in sunny Worthing. If you believe that you have something interesting for Sunstore please contact us as we are happy to help with any project, large or small.
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