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

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! 🌞

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.

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? 

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.

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. 
 

Hi 

I am in the process of getting quotes for solar panels and battery storage. I cannot find any info on the price OVO charge to force charge the batteries at night. Can any of the forum member help please?

Many Thanks

Cathy

Howdy @Bosscat0110 !

Given that OVO doesn’t currently have any Time of Use tariffs as such, it’ll just be the usual rates that you pay. If you’re on Single Rate, then it’ll just be whatever your flat rate tariff is. If you’re on Economy 7, then it’ll depend on exactly when the batteries begin to charge and stop charging vs your E7 times.

Thanks for your swift reply Blastoise186, a number of salesmen have promoted battery storage as having he ability to force charge batteries at 0.7p/Kwh. Is that just sales pitch? l could find info on OVO site relating to EV charging at 0.5p, however i don’t have an EV but was very interested in force charging my batteries between 00:01am-5.59am at a much cheaper rate. There doesn't seen to be an option to move suppliers, Octopus Agile do quote very low rates to force charge batteries, bit surprised OVO is not offering the same service. 

Yes @Bosscat0110 it depends on your current plan so there’s no ‘forcing’ a different rate for charging batteries. 
It would be helpful if there was though!

@Bosscat0110 without the benefit of an EV for cheaper off-peak rates your best fixed rate option may well be Economy 7, especially if you can pick a supplier with a very low night rate at the expense of a high day rate.

The agile tariff you talk about is a bit unreliable price wise lately. Generally even the overnight prices are high, often above 30p/kWh, although there have been one or two plunges recently. You'd have to work out how much you'd be importing and at what time/rates to see if a time of use or good old Economy 7 would work out better.

There may be other options coming soon so keep an eye on any announcements.

Hi all

After some more advice, I am looking at a 5.0kw Lux hybrid inverter with 20x405w Longi solar panels and 3 x 3.2 Greenlink batteries. I have been told that i will require a G99 certificate for the 5.0kw inverter but the company will install the system on a G98 certificate and cap the inverter at 3.6kws until the G99 certificate is approved (likely to by 8-12 weeks) Does that make sense to people. I have my L plates on when it comes to solar but looking to reduce my massive electricity costs.

One more question please, how long will it take to fully charge my batteries from the grid overnight, looking at either economy 7 or Octopus Agile to take advance of the cheaper rate during the winter to use during the day when the panels are are not as productive.

@Bosscat0110 the approvals are all about local grid and DNO notification. The difference between G98 and G99 is mostly about system size
This might help

https://www.deegesolar.co.uk/dno_for_solar_panels/

Hi BPLightfog

Thanks for your response. My concern is to do with capping the inverter until the G99 has been approved. I am not sure if this is possible/legal or just a sales pitch to commission the sale. What would be the impact of generating 8.1kw PV array during solar production hours with an inverter capped at 3.6kw? 

I am no expert on the approval process @Bosscat0110 but from my electrical knowledge, if power is being generated, it has to go somewhere. The DNO approval seems to be connected with their systems ability to cope with the potential feed back into the grid.

The only way that I can see a system being capped is not to connect all the string - which seems strange to me, unless the inverter is capable of being controlled to this level of output.

National Grid have their own advice for connections here https://connections.nationalgrid.co.uk/g99-connection-procedures

Some DNOs appear to be taking a more relaxed approach with export limitation. They used to require a G99 even if you were limiting to 3.68kW. Now I've heard to at least one DNO who is happy for an export limited inverter to be fitted under a G98. It's probably worth checking with your DNO.

Battery charging rates depend on your inverter and batteries. They will each have a maximum rate but you'll get the lower of the two.

Your inverter can charge at 3.6kW but the batteries charge at 2kW. I think the system might be able to charge all the batteries at once so your limit would be the 3.6kW of the inverter. That means it would take 2 hours 40 mins to charge them (in practice it's probably closer to 3 hours).

Hello everyone,

I have had solar panels and several additional updated storage heaters fitted with a Government grant. I have an Economy 7 system and previously the hot water was heated at night, as was my one old storage heater, however the charges are going up every month, much more than under the old system. Is it because the solar panels are only contributing to day time electricity use ( although with it being Winter they probably don’t do much anyway?) 

would it make sense to alter the timer for the hot water to be heated during the day? Your thoughts would be appreciated

Hi @Rubberducky , you should still be getting a cheaper off peak rate overnight for your storage heaters and hot water. The only bit to think about is if you also have to add heat to either the storage heating or hot water during the day. 
Solar PV will help but without batteries it can only give what it is producing towards whatever you’re using at that time. So at the moment, it will be on lower generation during winter but will help more during the rest of the year. 
There are devices which can use excess solar to heat your water but again this will only work when it is producing more than the home requires. 
 

Thanks for this, looks like I need to add batteries.