Hi,
As a thought experiment I like to consider what would be necessary for us to never import from the grid from time to time. In sunny suburban London this is wishful thinking and we have reliable supplies, though we are about zero-carbon and seemingly one of the most efficient UK households for energy.
Which grid? Just electricity for now. Note that this isn't an effort to save money: the grid is *cheap* compared to anything we could do.
We generate about twice what we use year-round, circa 3.6MWh/y vs 1.8MWh/y.
For all but 4 months of the year we generate more than we consume each day on average.
So a small amount of storage, eg a day or two or three, would mean that we could not import anything from March to October inclusive. That might cost us at £500--£1000/kWh in 90%+ efficient low-self-discharge Li batteries maybe £10k.
How to ride out Nov--Feb? At worst our PV should produce about half what we use each day on average, so extra storage for the balance of maybe 180kWh might cost ~£100k and then we need never import again. (All to save about £20-worth of imported energy!)
Alternatively we could in principle put up about another 2.5kWp of south-facing 60-degree-tilted panels if we turned over our small garden entirely to the cause to boost mid-winter generation by the short-fall. That would be £10k.
So maybe we could come off-grid (except to export for income) for £10k of storage and £10k of extra panels.
But what if we wanted to come off the gas grid too, for heating (water and space). Gas consumption last year was a little over 6MWh. Assuming for the moment that we could get a CoP on a heat-pump of 3 year-round, 2 in winter, we'd need 2MWh, amounting to an extra 20kWh/d of electricity mid-winter for heating, implying £1m in storage of £100k of south-facing panels (though only by taking over about 10 neighbouring gardens), or a heck of a lot more insulation!
So, coming off the gas grid in this house looks implausible so far due to the laws of physics and my bank account...
Rgds
Damon