We currently have an LPG fridge on our boat, which we would like to change to an electric. I would appreciate any wizardly comments about my calculations and design. (Consider that we use the boat only during summer months, and we spend 2-3 days “in the wild” where we don’t have access to shore power.)
Any help would be great. I know this is simplistic, but I have to start somewhere.
- There is a website that lists the average KWH/year for small AC electric fridges ranging from 110-150. My simple-minded approach is to divide that by 365, giving a range of 0.3 - 0.5 KWH/day (let’s use 0.5/day ).
- So 0.5 KWH/day = 42 AH/day at 12 volts (0.5/12*1000). We also need about 13 AH/day for lights, pumps and misc. That means total 55 AH/day.
- We have two 110 AH 12 V lead acid batteries that when fully charged should theoretically be able to power 55 AH/day for up to 4 days (110*2/55=4). (I know about Peukert’s law, but don’t know how to factor it in.)
- We have a 900 watt inverter (but may need to get a bigger one to deal with the fridge’s compressor start-up amp draw). Assuming 85% efficiency of the inverter in the above calculation we would get more like 3.4 days power for the fridge -- call it 2-3 days.
- We have a 100 watt solar panel, so assuming +/- 5 hours of sun/day, that would put +/- 42 amps into the batteries per day (100/12*5= 42).
- Running the motor while cruising every other day or so for 2-3 hours would recharge the batteries (but not sure how much…how can I find out the charging capability of the motor’s alternator?). The only reliable way to recharge in the wild would be to idle the engine for a while if we are not moving (we don’t have a separate generator).
- We would be connected to shore power every few days, which our battery charger would use to fully recharge the batteries.
Any help would be great. I know this is simplistic, but I have to start somewhere.
