Just curious how would you determine the size battery back up bank needed for a 240 volt single phase 40 amp resistive load? Pardon my lack of understanding on this subject.
Assuming lead-acid batteries, roughly half of the Ah capacity is usually usable before the voltage really begins to drop. DC-DC converters will extend that quite a bit, as they output a steady voltage while their input voltage begins to drop. It's not a linear relationship - discharge rate VS usable capacity - thanks to the dark magic that is battery chemistry. Also remember that deeply discharging lead-acid batteries will drastically shorten their lifespan, "deep cycle" or not.Wouldn't the capacity need to be higher than that to make sure the voltage doesn't sag near the end of the desired run time?