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Solar panels and batteries - How clever are systems that top up batteries at overnight rates?

  • 5 May 2022
  • 7 replies
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I am looking at the solar panel/battery combination, and wondered about using Economy 7 overnight to top up the batteries. 

What concerns me is that if the battery is filled 100% at night, there will be no space left to store any free power when the sun comes up. What I was hoping was that today’s systems would work out how much spare capacity to leave in the battery overnight to prevent that from happening.  Obviously that calculation would vary by season.

Has anyone any information about this please? I have yet to find anything online.

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Best answer by ArundaleP 6 May 2022, 14:47

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Userlevel 7

Wow such a great question this one, @warspite and not something that’s been asked here before.

 

As we’ve got plenty of community members who’ve already made the Solar and Battery investment I’m sure they’re much more in the know on the night charging aspect than me! 

 

@jenthomson, @Sean T, @JoeC, @f0rtune, @Adam Vetere, @ArundaleP - How are you finding your battery capacity? Do you make use of a cheaper night tariff to store some energy and how does this work with your solar generated power?

 

Really interested to hear your take! 🌞

Userlevel 3
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As far as I know there’s no easy way to do this, happy to be proved wrong!

Our current tariff is fixed for another year so we haven’t got economy 7 but definitely interested in moving to this when this ends. 
 

My plan would be - Nov-Feb fully charge battery overnight. Rest of the year, charge battery from solar. 
 

Our 5KWh battery has dropped our electricity usage by about 60% between march and October, a pretty incredible impact especially with rising prices.

Userlevel 6
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Hi, this can be done by seasonal power plans with SolarEdge systems but I am not sure how good the next day forecasting actually works.

To be honest with a good solar system the PV generation is very predictable and you can make battery energy plans to suit.

I have just ordered (but am still waiting) for their new Energy Bank 10kW DC battery to add to my existing solar system. The software looks very flexible and can prioritise battery charge and discharge to maximise solar self consumption and top up at cheaper rates. When I have it (sometime in June hopefully) I will do a review / case study of my own system with lots of geeky data for people to look at. 

The biggest problem with solar PV and battery storage is the integration of the two. I would recommend for someone that doesn’t yet have either, to get the system designed from scratch as a complete solar and storage system. I am a huge fan of Solaredge because of their optimisers and dc power control but Solax and GivEnergy do some very good hybrid inverters as well but I can’t comment about their software control.

I would recommend getting a quote done by a local installer with a few different options / price points. I am happy to check the details of the quote and designs once you have them. 
 

Tesla Powerwall has some very clever software features to make the best use of solar and time of use tariffs but is a standalone product from the solar system. It does offer grid back up though which most other systems do not.

 

Because everybody’s house and budgets are different I would get some quotes / options narrowed down first. 
 

 

Userlevel 3
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Hello, I have a 12kW solar PV with 2 Tesla Batteries but have only had the system since the end of April. I currently do not have a functional smart meter but hopefully this will be resolved soon and my average daily usage is 50kWh/day (mainly ASHP and AGA style electric cooker which is on all the time). At present I’m finding (on most days) I can go a full 24 hours without taking anything from the grid. Over the past 4 weeks my 350kWh/week has been largely self produced. My costs have gone from approx £98:00/week to approx £14:00/week, so good savings. I am hoping to have my smart meter up and running by winter in order to use a TOU tariff. I am likely to fully charge the batteries overnight costing approx £3/day and then take a chance on the weather. On some days I will have excess solar and it will appear I’ve over egged the grid consumption and on some days I will rely on the batteries much more but if I can survive the winter months with an electricity bill of £90/month, I’ll be happy with that. Even if I work on the basis that I pay for 6 months electricity usage/year and get the remaining 6 months self powered, I will have an annual electricity bill of £540 rather than 18,250kWh/year at £0.28p (currently) totaling £5,110. Clearly the solar and batteries are a no brainer but also create a number of options in terms of how to manage my consumption. This has also opened the door for me to get rid of my Diesel car and buy an EV with free home charging in the summer months with my excess solar production. My system was commissioned on 8/4/21 and for the remaining 21 days of April I consumed 1128.5kWh and produced 1,214.3kWh, 433.3 of which went to the batteries which I would have lost to the grid otherwise and would then have had to buy it back at £0.28p. I did lose some to the grid when the batteries were full but on the whole my savings approximate to £80/week. I will certainly charge the batteries fully overnight when on a TOU tariff rather than chance falling short of solar the following day. Therefore for me at least, once my solar production drops below 50kWh/day, I’ll charge the batteries overnight. If you get really clever you can gauge your solar production the following day from the Met Office weather forecast and fine tune the overnight grid consumption! 

Thanks everyone. Definitely food for thought. I would certainly be happy to control the overnight charging myself manually based on the following day’s forecast.

 

But I had a fitter around today, and asked him this question. He put me through to an engineer at his company, who assured me that their current systems (and presumably others too) are now clever enough to do all this for me. According to him, the programmed integration with the batteries sorts out how much to charge overnight based on my usage pattern. He also said that, in 20 years in the industry, no-one has asked him that before. Surely this feature would be a major USP? 

Userlevel 2
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Home Assistant has a solar insolation predictor for the next day.
So all the pieces are in place but at the moment there’s no common API interface across the manufacturers so each would have to be integrated.
I suspect we’ll see something like this in Home Assistant as adoption increases.


I’ll certainly be seriously looking into once OVO have finished their trail which is trying to do precisely this.  Well they’re definitely doing TOU but I assume they’ll take into account the solar PV as well.

That said, it may be OVO are more focused on peak time grid balancing than maximising self-consumption.

Userlevel 2
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I have been running solar PV and battery storage for 6 months now and have to say that there is limited prediction of solar for future days. This can so depend on cloud cover that although the basics are there, I would find it very difficult to match and leave sufficient storage capacity for the next day. 
The other consideration is power use. Most systems I have seen in a domestic sense cannot supply peak demands and so you will get draw from the battery during early light anyway if you have just a few electrical needs early morning (shower, washing machine, hair dryer, kettle, toaster, oven). So depending on your combination use (unless you’re very coordinated), you draw before you store. 
I’m sure things will progress in future. I would love to have an off peak charging slot or two, overnight or during the afternoon lull to supplement my solar, especially during the winter months. 
 

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