A: The economics of batteries is highly dependent on the local subsidies. In MA with Eversource as your electricity supply company the subsidies are very generous. I have just ordered a Sonnen battery to go with an existing array of SunPower panels. I have also just ordered a Generac battery to go with a new array of Solaria panels. Generac is not good as a retrofit to an existing array because you need to use a Generac inverter with a Generac battery. I have not bought a Tesla battery, despite it being the cheapest per kWh of storage, because Tesla is not currently able to deliver batteries.