To charge my 24volt battery on the boat, do I need a 24volt or higher solar panel?
I see mppt can work for both 12v and 24v batteries but do they step-up if solar panel voltage is too low ? Or do I need solar panels with higher voltage output ?
For 24V, you need 2x12V panels that run around the 21V mark open circuit in series to get around 42V open circuit. You need +5V above the battery for the MPPT to start. You can use one panel, but its voltage needs to be above 35V to be safe. Above 40V is best for 24V. That’s what I have found.
My 24V is fully charged. I have 2x 12V panels. You see, I get just above the 5V needed to start the MPPT when they are fully charged.
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No, the MPPTs from Victron are Step-Down only.
Since I do not know, with how many Amps you want to charge or how much space you have for the panels, it is better to go with the Solarpanels near the maximum Voltage the MPPT support (do not forget the Voc and the Temperature coefficient)
E.g. a BlueSolar MPPT 75/15 support 440W @24V, hence you can use 3 solar panels 200Wp with 18V (Voc 22V) in series which give you even energy in dim light