Just a few points to help the discussion.
PVWatts is a US Government built site that also covers Ireland. Pop in your location and proposed system stats and it will use historical weather data from your nearest weather station to estimate annual solar PV production:
https://pvwatts.nrel.gov/pvwatts.php
I'd advise anyone considering solar and/or a home battery to install a home energy meter with online stats like an OWL (
http://www.theowl.com/index.php/energy-monitors/remote-monitoring/intuition-e/ ) or Smappee ( [broken link removed] ) first.
Get at least a month of data. See what the impact of switching to a nightsaver tariff may be, look at your consumption during peak daylight hours and compare to the hourly average production numbers you get from PVWatts.
Some definitions:
kWp - kilowatt (peak) - This is usually a stated spec on PV panels/arrays and is the peak power production number under ideal lab conditions.
kWh - kilowatt-hour - this is a unit of energy equivalent to the energy a constant 1kW load would use in an hour. This is what your units on your electricity bill represent and the capacity of a battery is measured in.
kW - kilowatt - this is a unit of instantaneous electrical power. As an example a standard household ESB connection is rated for around 12kW, if you powered on twenty 1kW appliances at the same time you'd blow your main ESB fuse.
A home battery system will have three primary metrics.
Capacity - which is measured in kWh - there is also sometimes a difference between the nameplate capacity and the usable capacity, though normal practice is to only state the usable capacity.
Power output (sustained) - measured in kW - this is the highest sustained load the battery can support. This is a spec of the inverter, which in some installations is built-in to the battery and in others is your solar inverter (which may have a different rated output for power from the battery to power from solar)
Power output (peak) - measured in kW - this is the highest peak load the battery can support, usually for a period of time measured in seconds (often important to allow for loads like refrigerators which have spikes in load when first powered up)
If you turn on appliances exceeding the kW power output the battery can supply the remainder needs to be supplied from the grid or any remaining solar power.
Some examples:
Tesla Powerwall 2. It's usable capacity is 13.5kWh (enough to support a 1kW load for 13.5 hours). It has a built-in inverter with a sustained power output of 5kW and a peak of 7kW (for up to 10 seconds).
SonnenBatterie Eco. It's usable capacity is 5kWh (enough to support a 1kW load for 5 hours). It has a built-in inverter with a sustained power output of 2kW and a peak of 3.3kW (for up to 10 seconds).
LG Resu 10H. It's nameplate capacity is 9.8kWh, but usable is 9.3kWh (enough to support a 1kW load for 9.3 hours). It has no built-in inverter. A typical inverter used with this system would be a 5kW rated model. However while the inverter may support 5kW of power from solar... it may only support only 3kW of power from the battery.
Lithium is not a rare material... it's also non-toxic, common (we even have quite a lot of lithium carbonate in Ireland), relatively clean to extract vs other materials and is usually only 2-3% of the battery by weight. It's a Lithium battery because lithium is the active component...not because it's a large portion of the material used to make up the cell. For most home storage batteries aluminium is actually the largest constituent material by weight followed by graphite.