PoE pi

That's hardly surprising, car batteries spend nearly all their life fully charged and only discharge a little (admittedly at very high current for a short time) when starting the engine. Their chemistry and mechanical design is optimised for this load pattern.

Try comparing them with deep cycle lead/acid batteries with a similar usage pattern, then remember that there are a *lot* of crap Li-Ion batteries on the market so make sure you are comparing similar quality products designed for similar use.

I was talking about batteries not solar panels.

Erm try 100kWHr/day system - which assuming shortest days of 6 hours is more like a 17Kw system. If I were going for fully off-grid I'd probably make it a 20Kw peak system.

See above for the corrected calculation.

More to the point I was suggesting that a system as described in the bit you quoted with a few kilowatts peak power generation and ten to twenty kilowatt hours of storage will reduce grid usage and grid dependency substantially.

Let's take a look - based on 4kW peak generation, 20 kWHr storage and daily use.

In summer with 12-18 hours of daylight it will be a rare day that doesn't see a more energy collected (48-72 kWHr per day peak) than used so there will be almost no energy drawn from the grid and quite a lot dumped, or given or sold to the grid.

In the depths of winter the system will produce less energy per day than is used and something like 3/4 of the energy must come from the grid. But that 20kWHr of storage means that it can all come from off-peak which helps. All in all with such a setup you could expect to use less than 1/8th of the grid energy that you would use without probably more like 1/10th.

In summer or winter with the batteries fully charged a full day of power cut won't cause a problem - the last time I saw a power cut of more than a few hours was more than 20 years ago - that one was three days or so around Christmas which was something of a nightmare.

It then boils down to a simple money question - does 87.5-90% of the electricity bill over the lifetime of the system exceed the cost of the system. Also how much do you value immunity from power cuts, surges, brownouts etc - which of course partly depends on how common they are.

If you can sell that summer energy and buy off-peak in winter the cost savings are even greater than the energy savings - it may even be profitable.

It's the old story - the last 10% of the job takes 90% of the cost/effort. In this case being on the right side of the 90/10 law is pretty good.