Should You ALWAYS Charge Your Home Battery Overnight?

* NOTES SINCE PUBLICATION * 1. One benefit I failed to mention was that having a near-full battery most of the time really helps in the event of a power outage, which could have any time. 2. If you have an oversized array with a DC-coupled battery and your inverter supports charging of your battery with solar generation above the AC limit, it may be more financially beneficial NOT to charge your battery overnight when it's going to be sunny - instead charging it using a strategy that maximises that feature.

Another great video which really chimes with our situation. We fitted 9kW of solar (6kW peak E/W arrays), EV charger, solar diverter and two powerwalls (27kWh) in March 2020. Back then the aim was to self consume as much solar as possible as the tariffs at the time did not reward export. We exported less than 10% of our solar then but everything has changed with Octopus intelligent Go. We now charge the powerwalls, heat our water, charge cars etc. overnight and export ‘everything’ we generate while running the house on the imported overnight energy in the powerwalls. As of today we have exported over 70% of our generation ytd and expect to be net zero electricity cost for the year including two plug in cars and an ASHP. I don’t expect this tariff to remain unchanged (too good to be true?) but it highlights the advantage of having a flexible system which can adapt to a volatile and rapidly evolving tariff environment.

Good to know I got it right using much simpler modelling spreadsheet, but also good to know your about your model too Gary. I think it’s fabulous that Octopus give a high enough export tariff that it pays you to charge overnight and export in the day. They are actively helping to balance the grid in this way.

Another fantastic informative video. I regularly forward them on to friends and family who are either considering PV and battery systems or really should.

For me a fully charged battery means more export and since Octopus upped their export payment to 15p is incredible. Paying 7.5p overnight is a win win. Also it is nice to know we have some cover if there are any power cuts.

Very interesting. I will admit i've always set to solar only historically, but this might make me change tact

Another great video, if you have export at double than import it's a no-brainer. Now into spring I'm exporting by 8am. Best move I made was oversizing the array to the inverter.

Thanks Gary, good video. I'm in Canada and just finished installing my solar system and that's my exact plan. One other benefit with a full battery at night is that you will virtually always be ready with a full, or close to full charge if the grid goes down.

Good video. I've got a very complex node red flow controlling my ESS which does a similar thing to what your software does. On intelligent Octopus Go. I look at the next day solar irradiance, battery SOC, and predicted consumption. Then at 23:30 I adjust the charge current for the 6 hours and it works really well. Keeping the charge current as low as possible allow the system to.run at maximum efficiency. It's rare that I have off grid days and find it always charges at night. I'm either running of cheap off peak or free energy from solar. 😊

I have to manually control charge and discharge as, now with decent irradiation I try to deplete my 2 SolarEdge energy banks to allow them to take some charge from ordinarily clipped power. My 10 kWp array can generate 9kW and with a 6kW inverter 3kW would be wasted. It’s tricky but satisfying when you get it right. Very informative video await the next in series

I love your vidoes Gary, learning so much before my install takes place. I had been considering a second battery to my system to benefit from a very good 12 month SEG agreement of 40p. What I am now calculating is not purchasing a second 9.5kwh battery but instead using the 12:00 to 07:00 off peak period to fully charge, discharge and then recharge to 100% at an inport rate of 6.9p and an export rate of 40p. My 7.4kwp solar system to do it's thing and export throughout the day.

Thanks Gary. I always charge the battery overnight. I’m currently on Agile with a small PV/battery system. My last few bills have an average import rate of 12p, so Agile is working well for me. The only thing I might manually do is to inhibit battery discharge during the late morning/afternoon until the 16:00 import peak starts. I do this particularly when the daytime and overnight import rates are similar (like in the last week). However I think I’ll do some calculations to see if the other tariffs are better in the summer. Keep up the great work!

Definitely my strategy in winter with Intelligent Octopus Go, but in summer I want to limit my DC charge due to an oversized system

Great video. I wonder if the next stage is for the app to calculate the best possible tariff given location, array & battery size etc

We are currently on Flux and about to switch to Intelligent Go next week when our EV arrives. I had been trying to keep some space in the battery and adjusting charging rate to avoid clipping during the day as we have a 5.5kW inverter and 6.5kW of PV, but with the UK weather being what it is, it’s quite different to judge. I think I’m going to revert to the fully charging overnight strategy.

Another clear video, thank you! What I don't see in your modelling is the losses which mean that when you put 10kWh into a battery, you'll get less coming back out. For our installation, the installer estimated the total round-trip loss occasioned by the battery and the two trips through the inverter to charge and discharge as about 10% in total. I've no idea whether I'm seeing that sort of loss or not, but any loss means that paying to put charge into the battery and then selling that electricity back to the grid as soon as the sun comes up is not quite as profitable as just looking at the import and export rates suggest... I'm not suggesting that charging overnight is a bad idea, I'm just noting that whether you charge your battery to 100% on the grid, or on the solar, you'll lose quite a bit of electricity on the round-trip through the battery. Then again, I'm coming to the conclusion that absolutely nothing in this game is quite what it first seems! All the best, Gary...

Great video. I have just had my export tariff set up so can get payment for export at last. My problem with overnight charging of my battery is that my array peaks at 5kW when the sun shines but my DNO restricts export to 3.6kW. I guess I would only charge the battery overnight when the forecast is cloudy. Maybe even restrict the battery charge rate during the day to lengthen the window when some of the generation was going to battery (when it’s sunny)

Hi Gary while I commented earlier this is my current strategy I’ve just discovered why you maybe wouldn’t when guaranteed sun. My inverter will export its maximum kw ( ac ) and charge the battery at the same time if you have solar PV over the rating of your inverter ( not all inverters may do so ) but my 5kw inverter can export 5kw and charge at 5kw simultaneously. I have 7.5kwp so there is a case for charging in the day so I wouldn’t be impacting/reducing any export 👍👍👍👍

Very good Gary. Something I considered a while ago and realised whatever you put into your battery in excess of what you need gets exported and you get paid back at twice the rate on Octopus Intelligent Go for example. However, have you considered what happens when the battery reaches 100%, say two hours into the six hour charge period and then you run for four hours supplying the household load from grid which is possibly significant such as a heat pump? That four hour period is a grid draw and expense that isn't recovered by export. Arguably as soon as the battery reaches 100% it would be better to stop charging and start to consume battery charge. Even better would be to delay the charging so the battery reaches 100% as late as possible making export happen as early as possible. I must admit I've not modeled this yet as I've only just considered it. It's the long six hour off peak on Octopus Intelligent Go as opposed to standard Go, where my battery struggled to reach 100% over most of winter, that's highlighted the issue. I was wondering if it's better to stay on Intelligent Go or switch to Flux over summer as I had presumed was the best thing to do. Flux has a better export rate between 4pm and 7pm but doesn't have as cheap an overnight import and for only half the time. However if you can't charge the battery with the extra cheap hours because it's already full that dead time won't turn into earlier export so perhaps Flux regains the advantage. For the moment I've set my Intelligent Go charge time to three hours whilst I think about it.

Another good one Gary. I recently switched to Octopus Intelligent Go I have now changed my charging routine. As you suggested, I now charge my batteries overnight between 11.30pm & 5.30am without exception. I also charge my car & run any appliances overnight that have a delay timer all on the 7.5p/kWh night rate tariff. In addition, between 11.30 & 5.30am I prevent / pause my battery from discharging (GE Portal setting). This prevents the car, or anything else, from draining the battery overnight & ensures I start the next day with the highest possible battery SOC. If the sun is out next day then I'll be exporting to the grid & earning 15p/kWh as early as 7am & for most of the day (ESE array). If there's no sun then I run my house from the 19kWh of stored battery energy plus whatever solar is available throughout the day. I'm already seeing a dramatic reduction in our monthly energy bill by way of generation credits & best of all I don't have to watch the weather any more like a Met Office geek!! 🌧🌞