Matthew,
The battery chemistry that you chose has a big effect on the energy lost while sitting. Flooded cells often have self discharge ratings of about 3-5% per week, while AGM or Lithium batteries self discharge at about 2-3% per month.
If you're using flooded cells, it is reasonable to lose about twice your total battery bank capacity per year to self discharge (4%/week x 52 weeks). Batteries discharge faster in warm conditions, so the rate changes over the course of the year.
As far as your comparison to petrol, the common rough estimate amongst E-boaters is the energy delivered from 10kWh is close to 1 gallon of petrol, so you used the equivalent of about 4 gallons (15 liters) of fuel during your 8 trips. You can compare the $6.50 spent on electricity for propulsion to your local petrol prices.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Matthew Geier <matthew@...> wrote:
>
>
> With my boat well and truly 'stored' for the southern winter, and the
> Winter Solstice just passed, I totalled the power used by my boat over
> the last season.
>
> I used the following Kw/Hrs -
>
> Maintenance charges 18.90 Kw/Hr 33.57%
> Post trip charges 37.40 Kw/Hr 66.43%
>
> The boat was used only 8 times over the summer (It's on a trailer and
> I don't own a suitable tow vehicle, so each use requires weeks in
> advance planing, no 'it's a nice day, lets use the boat' spur of the
> moment thing.)
>
> So I can say for me, â…" of the power my boat consumed from the 'grid'
> was used solely to replenish what simply leaked out of the batteries.
>
> However the total cost of that electricity was in the $10 region. Each
> time the boat was used it cost around $12 in diesel fuel for the 4wd
> used to tow it from my house to the boat ramp. (Usually a Toyota Hilux).
> The last time the boat was used with it's petrol engine was 20 years
> ago, so sorry, no before and after comparisons of running costs!
>
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