Weather and overnight charging

[oleboyredw]

I have an issue I cannot seem to get around. Over the three months or so of having our installation (5.7Kw of panels and Powerwall 2) we’ve seen quite a few instances of a couple of sunny days getting over 20kW of generation followed by a days of bad weather which is quite understandable for this time of year in the UK.

Everything works great on the sunny days and we have little or no charging of the Powerwall from the grid. However, for a few days after that the intelligence in the Powerwall/gateway assumes same, does not top up the battery and we end up using expensive daytime electricity from the grid. When our prime time (day) electricity tarrif is treble our cheap (night) tariff it makes quite a difference.

Does anyone have a solution to this?

I can see that a simple manual override to top up the battery in the app would ve good enough (for me) or sonething more soohisticated that looks just a day or two out on weather forecast.

 

[sashton]

In a very similar position here. Whatever Tesla uses to calculate the off-peak charging really doesn't work well in the UK. One bright day and the algorithm overreacts and the subsequent day you can guarantee insufficient overnight charging. The long-promised backup gateway may offer more options via reservation setting but it isn't the right way to go.
On various fora I have asked for the ability to set a target SoC for the end to the off-peak period, including submitting an improvement suggestion via their support channel but have never had a response.

 

[oleboyredw]

I found emails get responded to quite slowly, hoever my most recent ine asking for what the app was next likely to be updated got a phone call response. I said that Ihad a problem with battery/gateway and it’s reaction ti weather and the oerson gave me an 0800 number to ring. Planning on calling them Monday. Just working on best wording to present this to Tesla. Having read the comnents here and orevious responses I l’m not expecting much in way of a response. But if you dont ask ....

 

[JCUK]

We have observed that after a sunny day the powerwall will retain enough charge to last to the time that solar energy generation normally starts. It will do this for the following three days even if we generate very little due to bad weather. It will then charge to a significant level approximately 90% to 100%.

This has happened this week 3 days of using peak energy, then charged to 100%, this morning; today is a sunny day, and have generated 28kWh and been sending to the grid. I expect the powerwall will use this valuable energy overnight in off peak, and tomorrow when heavy cloud and rain is expected, we will be using peak electricity.

I think that John Ratseys suggestion of setting a % target balance for off peak is a simple idea which would be easy to program as it appears to be similar to the backup system.

 

[JohnRatsey]

Unless Tesla integrate solar generation forecasting into the off-peak charging algorithm it will never work very well with the variable UK weather (and once the Australians move towards winter they'll start complaining again). I have sent numerous emails expressing exasperation to the Tesla EU support and finally someone is showing a little interest in my complaint which is basically that sometimes off-peak charging is followed by a sunny day with power then going back into the grid which results in an overall financial loss (I've less frequently encountered the situation of insufficient off-peak charging although the battery hit empty when there was snow on the panels at the start of February).

I've also done some mental exercises in trying to estimate how much I would charge the battery based on the weather forecast and discovered that it's not an easy exercise based on the relative simplistic weather forecasts. Low cloud is much worse for solar generation than high cloud but they both look similar on the forecasts while the sunny/cloudy forecast gives no indication of the relative split between sun and cloud. As the average daily solar generation is increasing the need for off-peak charging is reducing as is the number of occasions of the battery getting it wrong. Tesla might better understand our problems if they had a time-battery % charge graph similar to that generated by PVOutput.

 

[SimonT]

We are thinking of having a powerwall fitted and have a engineer coming round Wednesday.

Am I correct in that you plan do charge the battery using off peak electric? Is this in option or is it programmed into the software to always do this?

I would have thought the most effecient way was to charge the battery using solar during the day for free and not charge it at all from the grid?

We have a 5.5kw and would have thought that apart from really horrible days we would get a reasonable if not full charge into the battery.

Over the last 15 months we have averaged 12kw generated per day.

 

[JohnRatsey]

Am I correct in that you plan do charge the battery using off peak electric? Is this in option or is it programmed into the software to always do this?

I would have thought the most effecient way was to charge the battery using solar during the day for free and not charge it at all from the grid?

The battery has several different operating modes which are not well explained by Tesla here. The backup mode needs the backup gateway which is not currently available in UK. The other modes can be selected using the Tesla app. The cost-saving mode is intended to use off-peak charging when appropriate to supplement solar generation in order to avoid the use of more expensive peak electricity. However, Telsa's algorithm is currently based on past generation and usage and fails to use any forecast of solar generation. Hence this discussion about providing users with simple controls for off-peak charging. You will also need an Economy 7 or similar tariff to get cheaper off-peak electricity. This should be easy if you have a smart meter.

 

[SimonT]

The battery has several different operating modes which are not well explained by Tesla here. The backup mode needs the backup gateway which is not currently available in UK. The other modes can be selected using the Tesla app. The cost-saving mode is intended to use off-peak charging when appropriate to supplement solar generation in order to avoid the use of more expensive peak electricity. However, Telsa's algorithm is currently based on past generation and usage and fails to use any forecast of solar generation. Hence this discussion about providing users with simple controls for off-peak charging. You will also need an Economy 7 or similar tariff to get cheaper off-peak electricity. This should be easy if you have a smart meter.

Thanks - do you mind sharing which supplier/tariff you are on?

Do you use the off peak rate to charge the battery or something else such as a car as well?

From my limited research it looks like the solar powers the battery during the day then discharges at night when the PV stops working. The biggest difference I can see between a cheap rate economy 7 and a standard rate is about 7 or 8p so depending on how much is used in excess of the battery you are talking less than £100 per year on 3 to 4 kw per night.

Does the cost saving mode try and charge the battery as well? If so can this option be turned off and just use solar?

 

[JohnRatsey]

I'm with Octopus.

No car, just battery using off-peak charging and that's only in the winter half of the year. I still get some peak power usage when domestic consumption exceeds the output of the battery and/or solar panels (if generating). Both of these are capped at 16A as I haven't tried to get dispensation to potentially export more than that into the grid.

You can see what's happening with my panels and battery at PVOutput. You will need to click on the right-most blob below the date in order to see the battery data. Go to the middle of winter and the erratic off-peak charging becomes evident.

 

[Shygar]

Just curious, do you not have the equivalent of net metering for your power company? For PG&E, we are given credits for those sunny days where we send back power to then use when we have those cloudy days to use those credits. So for me it really doesn't matter as long as my usage over 12 months nets out to be around the same.

 

[SimonT]

Just curious, do you not have the equivalent of net metering for your power company? For PG&E, we are given credits for those sunny days where we send back power to then use when we have those cloudy days to use those credits. So for me it really doesn't matter as long as my usage over 12 months nets out to be around the same.

No - currently we get paid a "Feed In Tariff" the rate of which was determined at the time your solar was commissioned.

The rate has been dropping steadily and I believe is being scrapped next year.....

 

[sashton]

FiT ends sooner than you think: End of the month for new installations.

 

[oleboyredw]

We are thinking of having a powerwall fitted and have a engineer coming round Wednesday.

Am I correct in that you plan do charge the battery using off peak electric? Is this in option or is it programmed into the software to always do this?

I would have thought the most effecient way was to charge the battery using solar during the day for free and not charge it at all from the grid?

We have a 5.5kw and would have thought that apart from really horrible days we would get a reasonable if not full charge into the battery.

Over the last 15 months we have averaged 12kw generated per day.

I have mine set up to charge battery during day from solar and also to top up overnight during cheap period. Currently on Robin Hood Energy Evergreen V7. (Daytime 19.3p, Night 9.6p, sc 21.48 and yearly discount of £22.50 for online management). I've recently tried to find cheaper for my usage profile and uswitch failed to better it. However, as I've only had solar/battery since end of November I need to check again later, think I'm stuck this year.

 

[mikemillar]

My Powerwall 2 should be installed in the next few weeks. I have s Tesla model S charging at night, sometimes, so my daily consumption is rather variable. The ability to forecast tomorrow's solar radiation and charge at night accordingly is critical to my financial model. I have been lead to believe that the Gateway software accesses forecast solar radiation data and optimises charging on that basis. There is quite an industry in forecasting solar radiation. I use the paid for Topmeteo site for aviation forecasting and it provides a fairly good hourly forecast of solar radiation up to 5 days ahead. If the Powerwall only bases it's night charge on the last few days solar it's not going to work very well in the UK! Looking at my neighbour, SJOG's PVOutput data (look for LC at Hassocks north of Brighton) it normally works well (zero grid inout) but was pants over the last few days of rain and cloud! (until it sorted itself out after a few days).

If it doesn't use real forecast data there needs to be a manual over-ride. Would setting the backup reserve to a high value when there is a change from a sunny to overcast/rainy day achieve the desired result? I can see I'm going to have some fun optimising this!

 

[JohnRatsey]

I have been lead to believe that the Gateway software accesses forecast solar radiation data and optimises charging on that basis. There is quite an industry in forecasting solar radiation. I use the paid for Topmeteo site for aviation forecasting and it provides a fairly good hourly forecast of solar radiation up to 5 days ahead. If the Powerwall only bases it's night charge on the last few days solar it's not going to work very well in the UK!

My experience last winter was that, at time, Tesla's off-peak charging algorithm used data from the previous few days to decide what to do next. Eventually Tesla support confirmed my suspicion that no solar radiation forecast was used (it's very annoying to see the battery charging overnight when a sunny day is forecast) and agreed to pass a request to the software development team for provision of user control over off-peak charging. I wait to see if this appears over the next few months. There is a manual fudge to limit the mount of off-peak charging by reducing the length of off-peak period but I haven't figured out how to get the battery to charge overnight if gloomy weather (or snow on the panels) is expected (there is a fix but it costs money - get a backup gateway!).

 

[mikemillar]

That's rather disappointing. I will be getting the Gateway 2 backup gateway, it's the only one they supply now, at an extra £1000 over the original gateway, so I'd be interested to know how the backup gateway differs from the original one in respect of controlling charging. Can you enlighten me, John?

 

[mikemillar]

From my limited research it looks like the solar powers the battery during the day then discharges at night when the PV stops working. The biggest difference I can see between a cheap rate economy 7 and a standard rate is about 7 or 8p so depending on how much is used in excess of the battery you are talking less than £100 per year on 3 to 4 kw per night.

Rather late replying, SimonT, but some EDF rates have a massive difference between peak and night rates - about 24p peak and 5.4p night rate. For a lot of people that would be disadvantageous, but as I use 3 times as much night rate compared to peak, it still works out the best for me even without a Powerwall. (electric car and immersion heaters on timers). With the addition of a powerwall, my goal is to use zero peak rate. If that's close to achievable it doesn't matter what the peak rate is, getting the lowest night rate is the important thing. I assume that the wide split with EDF is because they operate nuclear power stations, which generate fairly constant uncontrollable output, so they need to dump a lot of night generation.

 

[mikemillar]

That's rather disappointing. I will be getting the Gateway 2 backup gateway, it's the only one they supply now, at an extra £1000 over the original gateway, so I'd be interested to know how the backup gateway differs from the original one in respect of controlling charging. Can you enlighten me, John?

PS I presume adjusting the backup reserve with the backup Gateway is the way to over-ride the charging algorithm when low solar is forecast?

 

[JohnRatsey]

That's rather disappointing. I will be getting the Gateway 2 backup gateway, it's the only one they supply now, at an extra £1000 over the original gateway, so I'd be interested to know how the backup gateway differs from the original one in respect of controlling charging. Can you enlighten me, John?

The backup gateway appears to provide a route to more user control over off-peak charging. The Australians seem to have gone furthest down this road as they've been getting backup gateways as standard and some also suffer variable winter weather a bit similar to ours. There's some discussion somewhere in this forum but probably the most comprehensive discussion is at Whirlpool (link to what is currently the last page of a long thread).

Here's an extract with an example of the process:

If you wanted it in it simplest form the powershell module I have will just tell it what reserve to set.

eg
pwlevel.ps1 50 #sets to 'self consumption mode' with reserve to 50%
pwlevel.ps1 -20 #sets to 'self consumption mode' with reserve to 'current SOC' less 20%
pwlevel.ps1 0 #sets to 'self consumption mode' with reserve to "current SOC'
pwlevel.ps1 b #sets Powerwall to 'backup mode'
and makes no changes if the powerwall is in 'autonomous TBC mode'

To avoid any further scripting this could just be fired by windows task scheduler at 10pm to tell the battery to keep reserve at the current SOC (or perhaps charge to 100% depending on your offpeak tarrifs etc) and another at 7am weekdays to drop the reserve back to start using it again.

 

[mikemillar]

Thanks John. Useful link, I'll look into it when my PW2 with back Gateway arrives!