Get the work sheet
The attached work sheet is a generalized guide to average current draw for most of the electrical appliances found on recreational vessels. It is meant as a guide only, for determining daily usage. The most accurate method is to actually measure the draw of each device with an accurate digital amp meter, but this is a time consuming exercise. The spread sheet can be further refined by checking the documentation for each device provided either in the owner's manual or from the specification plate affixed to the device itself, however often manufacturers are some what optimistic in their ratings.
The spread sheet is reasonably self explanatory, identify all of the instruments and appliances installed or planned for your boat, enter the current draw in amps for each device and multiply by the maximum hours of anticipated use .Some estimation will be required. For example, if your radar draws 3 amps and you intend to operate it for 10 hours you will have consumed 30 amp hours. My fresh water pump draws 8 amps and runs for an average of 20 seconds to recharge the pressure tank. Convert to amp hours as follows, 20 seconds is .33 minutes (20/60=.33) 1 minute is .0167 hours (1/60=.0167) (.0167x.33=.0055 hr) .0055hr x 8 amps =.044 amp hours per cycle. If the pump runs 10 times per day total consumption in amp hours is .44 or a little less than 1/2 amp hour. Perform this calculation for each device for one 24 hr day and the total will be daily usage. You will probably be surprised at how much you actually use. Additionally, there are many "ghost loads" such as clocks, inverter stand by, and electronic controls. Remember to add a little for these.
When you have your daily total, multiply by 3 and you will have a fairly accurate indicator of required battery bank size. If you use lOO amp hours per day you will need a battery bank of 300 amp hours at the 20 hr .rate. This size to consumption ratio will discharge your batteries to about 66% of capacity. Although you can discharge the bank to 50%, your battery longevity will increase dramatically if you design for the lower discharge target and the extra money spent at the outset will more than return in battery life. These figures are based on quality deep cycle batteries. Starting batteries and cheap "deep-cycle/marine" batteries will require a much larger bank. If you have an ongoing charging source such as solar panels or a wind generator you can factor in their contribution and subtract that number from your daily usage. I recommend, however, that you size the bank as described above and the charge source contribution will decrease depth of discharge and further lengthen your battery life. Remember that for each amp hour used, 1.15 amp hours must be returned to compensate for losses in the system. If you intend to return the balance of the deficit with an engine alternator, remember further that when the alternator is hot and fully saturated it will probably average about 80 % of it's rated output so you have to add in additional charge time for that too.
The above will give you one 24 hr day with no recharge, for each additional day, multiply the total figure by the number of days, for example if you want to go 3 days with out recharge in our hypothetical 100 amp hour system, you should have a bank of approximately 900 amp hours at 66% level of discharge or 600 amp hours if you plan to discharge to 50%. A down-loadable copy of the spread sheet can be obtained by e-mailing email@example.com and typing "battery sizing spreadsheet" in the subject box.