Thanks @toodles  

Writing the text isn't the issue,
it's creating the graphics which takes time...

... especially for an introductory topic!

This post is here as a 'Marker'

It'll be replaced with the intro and graphics when I have a moment!

I thought I'd share my story/journey and experiences here (not sure if it's the best location, but much is relevant to this topic).

I have an existing solar array (3.6kWp) but no battery storage. We also have an ASHP. Our usage in summer is around 10-15kWh/day which is around half our peak generation of ~25kWh/day. In the depths of winter we can consume 40-45kWh per day depending on how cold it is (I do not expect our consumption to exceed 50kWh/day)

We wanted to add battery storage to (a) make ourselves self sufficient in summer, (b) maximise ToU tariffs / cheaper unit rates in winter, and (c) provide whole home backup in the event of a power cut (thankfully rare here, but a feature still very much worth having). One of the most critical aspects for the home backup is in relation to keeping the ASHP running in cold weather in the event of a power cut to prevent it freezing up.

We have no suitable internal locations for a battery, so any solution will need to be sited outside. My research indicated that the Tesla PW3 is industry leading in terms of it's thermal management which is key when the battery will be operating outside of it's optimal 15-25C operating range when sited outside and exposed to the elements, so this seemed like the ideal candidate for us. The battery comes with it's own built in 11kW inverter which can be set in 1kW steps anywhere from 3.68kW, 5kW up to 11kW, together with three MPPT strings for connection of up to 20kWp of solar panels. The 13.5kW capacity battery can sustain a continuous load of 11kWh and is sufficient to ensure we can run completely from solar for 6 months of the year with zero import, and in winter we can use the Octopus Cosy tariff which has 3 cheap rate slots spread throughout the day to charge the battery and see us through to the next cheap rate slot meaning we can maximise the ToU aspect to run completely from cheap rate electricity. An overnight cheap rate tariff (e.g, EV tariffs) would not work for us as the cost of the required battery capacity would be prohibitively  expensive, requiring at least two PW3 batteries, maybe three. Without a heat pump (and in summer), a cheap overnight tariff is certainly a possibility if we can fully charge overnight at cheap rate, run off battery constantly and export everything we generate, although that would be dependant on (a) having sufficient export limit (more on that in a moment) and (b) being able to configure the battery not to charge during the day from solar.

So we submitted an application to our DNO (National Grid) to add a PW3 and an additional 3.6kW solar array giving us a combined total 7.2kWp of south facing solar and 13.5kW of battery storage. The increased solar would result in generation of around 4 times our daily usage in summer resulting in considerable daily export, and the additional generation would be particularly welcome in winter and should cover or significantly contribute towards the ASHP usage, even on a cloudy day. However, all did not go to plan as the DNO originally came back with an offer that we could install the Tesla PW3 with an inverter setting of 8kW to cover the combined 7.2kWp array, but that export was to be limited to just 1kW. The low export limit would cripple us in summer as we would be unable to use, store or export most of what we generated and that export was critical in getting the financial case to make sense. @transparent had advised me previously that my application may be difficult due to my location and the saturated nature of my local grid.

To cut a long storey short, we worked with the NG who advised us of a '5kW+5kW' scheme where they would permit two inverters, each of up to 5kW and a total of 5kW export, which works for us. So we will retain our existing solar array and inverter, and add an additional 3.6kWp solar array connected to the Tesla PW3, with a limit of 5kW export from the property. In summer we should see around 6.6kW peak generation, can export 5kW so providing we can use or store the remaining 1.6kW of generation, we are good to go.

Posted by: @toodles

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Individuals are I think, the only sector at the moment who have (possibly, like me), invested in any energy storage. I won’t pretend that my aims are purely altruistic, I want security against power cuts, I wish to buy my energy at the cheapest and greenest rates of my chosen tariff; I like the idea of being able to choose what I do with the solar energy my panels produce for me.

There is commercial battery storage (these usually take the form of storage containers filled with racks of batteries of either 1h or 2h duration). There are two (were 3, one was recently bought out) investment funds listed on the FTSE (GRID and GSF) that invest in and operate battery storage in the UK. Their investment model is to construct and operate commercial size battery storage facilities and to trade / make available to the grid that storage capacity for balancing . These listed investment funds look to pay a dividend to their investors based on their generated income. I invested in both, but it has not been plain sailing. For reasons I do not fully understand, the grid has not been fully utilising these facilities recently (@transparent can probably tell us why), and as a result their earnings and hence share price has plummeted. The grid is working to improve the way this works, but currently I'm sitting on a large capital loss due to these operating inefficiencies.

Interestingly, one of these funds (GRID) has recently signed an agreement with Octopus Energy for about half of their 1,072MW operating capacity (568MW/920MWh), enough to power a city the size of Birmingham.

Octopus press announcement

GRID press announcement of Octopus deal

One assumes Octopus will use this storage (and other storage available to them through tariffs like Intelligent Octopus Flux where they take control of customers batteries) to minimise the amount of electricity they need to purchase at peak rate for their customers, much as a home user does in maximising a ToU tariff.

The other sector that has invested in commercial grade battery storage is solar farms, where some are starting to co-locate on site battery storage. I assume the business model for this is to store some of the solar generation during the day and be able to sell that generated energy at a more favourable time/rate.

I'm not sure about EV charging stations, but one would think these are also ideal locations to co-locate battery storage, so the operators can purchase and store cheap rate electricity to supply the EV charging points which they can then sell at a premium unit rate to EV drivers. So if you see a number of storage containers on site, these are almost certainly packed with battery storage.