Battery, part 3

Slightly misleading as battery is charging!
I have just been going over the final handover and testing with Green Park for the Tesla Battery.

Part of it was making a power cut. This was interesting.

So a few facts that are useful to know.

  • A real power cut means a couple of seconds of no power. It is not seamless.
  • A deliberate "off grid" (done on the app) does not, so if it is a planned power cut then you can seamlessly go off grid if needed. Good.
  • Going back on grid is seamless, a few seconds while it sorts phase match and back on. Good.
  • It will take solar during a power cut, and charge the battery if needed. Good.
  • If the solar exceeds its 5kW charging capacity it will shut down the solar (done using frequency change, apparently).

The last point is a pain as the solar can do 10kW. More importantly the solar can control what it makes by de-optimising the panels, so it if knew it was a problem it could reduce power. There seems no way to link these intelligently, sadly. But once I have a second battery we will be fine at it will match the battery, at least until the battery is full. What I don't know is if there is any way it tries to turn the solar back on in such cases, maybe trying it for a few seconds every now and then? Must find out.

And as for the first point, a power cut causing a blip - I need a very small UPS for my comms rack to last a few seconds, that is the easy part.

Second battery

One battery can handle 5kW in/out, and charge to 14kWh. This is fine for most cases for balancing the solar and usage and reducing export. It is also perfect for getting on the Octopus tariff for this where they manage the battery and I pay for net usage.

However, one battery does limit my options. It does not have capability to run the house for 24 hours, meaning that I cannot make use of "cheap charge at night and run of battery for the day". If I could, I would save even more, but without that I could be using more expensive electricity later in the day, especially in winter when less sun.

One battery is also likely to have the same issue if I went on an agile tariff, in that trying to match charge at low price and usage when high would be hard if I do not have a whole day's capacity.

So for these various reasons, a second battery is on order. For now, I am trying to sort the Octopus tariff, which is fun.

Off backup

The other work still planned is an additional small consumer unit that has a few circuits which will not be on backup. The Tesla can only do 5kW, so if I have the tumble drier on, or the car charger, it won't work. There are about 4 circuits to move to that. All good fun.


  1. Do you have a hot water tank to absorb any excess you can't use otherwise?

    1. I have a hot tank, and even an immersion heater, but not set up to sink excess power. It was a consideration to avoid export, but an interesting idea for handling excess battery charge from solar. That could work if I can get the real time data and switch it.

    2. You also have a hot tub. That'll sink some excess power in an off-grid scenario.

    3. I do not (yet) have an easy way to control that, and I don't think it would be as instant as needed. But, yes, longer term, scheduling when the hot tub is on is a factor.

  2. Ah interesting about the cutover time on power cuts, I assumed they would have been able to make it almost seamless but still comply with the regs. I just bought an ME3000SP which has an emergency power supply (EPS) output, I thought that was a permanent supply output but it looks like that output is only active when the incoming supply fails, and you need to fit an external changeover relay if you want something permanently powered (ie. UPS-style).

    1. I have had switchovers that we haven’t noticed - as reported by the App. The Gateway will also disconnect the home from the Grid if there is an under/over voltage or frequency issue. It monitors the Grid output for 5 minutes before reverting back to the Grid. Tesla Netherlands has a record of the voltage that the Tesla is recording. This, along with smart meter voltage data, was enough to get my DNO to tap my local transformer.

  3. I believe from a spec point of view in an outage the gateway is supposed to open the grid contactor within about 30ms, and the powerwall begin off-grid operation within a further 25ms, so should all happen in well under 100ms, which most modern devices won't notice.

    Certainly with mine it seems to be immediate, at least when we tested at commissioning with a kettle boiling to use some power, and then using the grid isolator to kill the incoming supply, other than a noise from the gateway (presumably the contactor) there was no change, all my kit kept running fine etc...

    1. The test was done in the face of a rather heavy load, not certain why this would make such a difference as it usually is a very swift change over. If the battery was in a high charge condition and had to swap to a heavy discharge, then, due to battery protection, this could extend the timings.

  4. I'm not surprised about the cutout time on a real power cut although nice to have it confirmed.

    Not sure that seamless transition is possible on a Lithium-Ion battery system which is even vaguely affordable for a domestic install.

    If you think about it in terms of UPS systems (which are primarily lead-acid cells) you'd have to run the supply through the Tesla unit continuously to ensure supply continuity. That's a big no-no for lithium cells unless you have deep enough pockets to replace them every 5 years.

    I'd also be concerned about running inductive loads such as your dryer off the battery as those are generally "problematic" loads for battery/invertor systems.

    You should definitely be dumping excess solar power into hot water tanks as its very efficient assuming a modern hot water tank. As an aside thats why building regs from 2012 onwards specify a hot water tank for new builds rather than a combi boiler - it can subsequently be used for heat pumps and/or PV solar systems. If your hot water tank is more than 10 years old then I'd put that on your list of things to do as it'll pay for itself very rapidly given you already have an excess of solar PV.

    1. My house was built in 2018 and has a combi boiler with no hot cylinder, as do all the other houses on the estate. Handy for storage space but not so much for heat storage…


Comments are moderated purely to filter out obvious spam, but it means they may not show immediately.

NOTSCO (Not TOTSCO) One Touch Switching test platform (now launched)

I posted about how inept TOTSCO seem to be, and the call today with them was no improvement. It seems they have test stages... A "simul...