A lot of ideas, advice and equipment have been thrown at Mr. Wonderful, but no one has really taken the time to begin the process of establishing an energy budget. How can he build a system to meet needs that haven't been reasonably quantified?
Since your expressed needs are simple, Here is an article that is clear and summarizes what I'm about to say. And there is much more educated and sage advice along the same vein from other trusted sources, among them Stu Jackson, who is a guru in this area, and who has many helpful articles and comments on this site. Search any of his posts to find a treasure trove of helpful information.
To Draw up an Energy Budget
First, make a list of everything that will draw power. Find out what each draws when operating. For each, determine how long that device will be drawing power in a 24-hr period (you may have to estimate, or observe closely, the actual run time for equipment that cycles on and off, like a refrigerator, to arrive at its 24-hour total appetite: for example, if you see by the tag your dorm fridge draws 60 watts, and by observation you conclude it runs 20 min. in each hour, that's 60w x 1/3(hr) x 24hrs, or ~475 watt-hours every 24hrs). Or how long each day you intend to use something, like a plotter, anchor light, VHF radio, or a guitar amplifier, or whatever else you figure you'll be doing. Add everything up, and you'll have at least an idea of a daily energy requirement in terms of amp-hours likely to be consumed. (Formula for converting watts into amps, the common value, below). Add 20% for inverter inefficiencies, then add another percentage for reserve (or in case the real world is different than your math: maybe 40–50% total, depending on how conservative/secure you want to feel). Now you have a reasonable estimate of your daily power needs.
Second, choose a battery that has a capacity that will provide for this final number, within its usable capacity. a 100 amp-hour battery will not give you 100 amp-hours of usable power: regularly discharging a battery completely can severely shorten its long-term performance, so, to avoid that degradation in performance over time, there is a practical discharge limit— for lithium figure you can go down to 10–20% depending on the manufacturer; for other battery chemistries, the minimum recommended SOC (state of charge) is typically higher. If you encounter cloudy days, which happens on Earth, you will want to anticipate that by selecting a battery (or perhaps a bank of two connected batteries at this point) that has even more storage to get you through days when there are fewer photons to collect. There is a practical top SOC that can be achieved through mechanical charging methods (alternators, etc.). Bottom line: lead-acid chemistries typically can be used between a 20–80% state of charge, because as the internal resistance in such a battery rises with charging, it becomes impractical to "top it off" to 100%— that leaves you with a usable capacity of 60 amp-hours from that 100 amp-hour battery. With solar charging you can achieve a 100% recharge on sunny days and so can dismiss this top-end consideration; meaning with lithium and solar, on sunny days you will be able to achieve 100% SOC and draw the bank down to 10% SOC (or whatever the manufacturer recommends). In this case, a 100 amp-hour battery will let you use 90 amp-hours of that 100 amp-hr. capacity.
Now you will have a fairly accurate idea of how big your battery bank (whether composed of one or more than one battery) will have to be go give you the performance you anticipate needing, while accounting for capacity limitations, inefficiencies, low-sun days, etc. Size your bank accordingly, and like anchors, a little bigger is better.
Caution: to cable two batteries together to give you more amp-hour capacity but stay at 12v, they must be connected in parallel, not in series. Read up on that before attempting if you try to do it.
So much for draw and storage: now to charging.
Solar panels are wonderful, but have limitations, as has been mentioned. Their actual consistent output is dependent on conditions (including nearby objects throwing shadows on the panel's surface) but always less than their rated (maximum) wattage, so conservatively figure your overall average output during the day will be half of whatever number it claims: less in the morning, more mid-day, and less as the sun lowers. Since you're working in amps, convert that average output wattage into amps on paper with the formula amps = watts/voltage (you have a 12-v system) to get the equivalent in amps, which is your usable number. Multiply that number by the number of hours each day you expect the panels to produce, perhaps 5–6, to get the number of daily average amp-hours that you can reasonably expect to be supplied to your batteries. (This is the same conversion formula to determine the total amp-hour draw of any devices that list their operating draw in terms of watts in the budget inventory described above, so that everything is expressed in amps).
Those are the essential steps in creating an energy budget for your small yacht.
Other Considerations
Systems: considering your small yacht and modest needs, the only really essential components are: panels, a solar charge controller to feed the juice to the batteries safely & let you monitor the panel's output, a battery bank in which to store the energy, & a battery monitor that will give you a digital display of battery bank amps-in and amps-out, and so serve as the "fuel gauge" for your battery bank. For such a simple system, going beyond that is nice, but not strictly required.
Making out an energy budget following a method like this will take you from wild guessing and blind equipment purchasing to educated estimates of your real-world needs.
Kind Regards.
Since your expressed needs are simple, Here is an article that is clear and summarizes what I'm about to say. And there is much more educated and sage advice along the same vein from other trusted sources, among them Stu Jackson, who is a guru in this area, and who has many helpful articles and comments on this site. Search any of his posts to find a treasure trove of helpful information.
To Draw up an Energy Budget
First, make a list of everything that will draw power. Find out what each draws when operating. For each, determine how long that device will be drawing power in a 24-hr period (you may have to estimate, or observe closely, the actual run time for equipment that cycles on and off, like a refrigerator, to arrive at its 24-hour total appetite: for example, if you see by the tag your dorm fridge draws 60 watts, and by observation you conclude it runs 20 min. in each hour, that's 60w x 1/3(hr) x 24hrs, or ~475 watt-hours every 24hrs). Or how long each day you intend to use something, like a plotter, anchor light, VHF radio, or a guitar amplifier, or whatever else you figure you'll be doing. Add everything up, and you'll have at least an idea of a daily energy requirement in terms of amp-hours likely to be consumed. (Formula for converting watts into amps, the common value, below). Add 20% for inverter inefficiencies, then add another percentage for reserve (or in case the real world is different than your math: maybe 40–50% total, depending on how conservative/secure you want to feel). Now you have a reasonable estimate of your daily power needs.
Second, choose a battery that has a capacity that will provide for this final number, within its usable capacity. a 100 amp-hour battery will not give you 100 amp-hours of usable power: regularly discharging a battery completely can severely shorten its long-term performance, so, to avoid that degradation in performance over time, there is a practical discharge limit— for lithium figure you can go down to 10–20% depending on the manufacturer; for other battery chemistries, the minimum recommended SOC (state of charge) is typically higher. If you encounter cloudy days, which happens on Earth, you will want to anticipate that by selecting a battery (or perhaps a bank of two connected batteries at this point) that has even more storage to get you through days when there are fewer photons to collect. There is a practical top SOC that can be achieved through mechanical charging methods (alternators, etc.). Bottom line: lead-acid chemistries typically can be used between a 20–80% state of charge, because as the internal resistance in such a battery rises with charging, it becomes impractical to "top it off" to 100%— that leaves you with a usable capacity of 60 amp-hours from that 100 amp-hour battery. With solar charging you can achieve a 100% recharge on sunny days and so can dismiss this top-end consideration; meaning with lithium and solar, on sunny days you will be able to achieve 100% SOC and draw the bank down to 10% SOC (or whatever the manufacturer recommends). In this case, a 100 amp-hour battery will let you use 90 amp-hours of that 100 amp-hr. capacity.
Now you will have a fairly accurate idea of how big your battery bank (whether composed of one or more than one battery) will have to be go give you the performance you anticipate needing, while accounting for capacity limitations, inefficiencies, low-sun days, etc. Size your bank accordingly, and like anchors, a little bigger is better.
Caution: to cable two batteries together to give you more amp-hour capacity but stay at 12v, they must be connected in parallel, not in series. Read up on that before attempting if you try to do it.
So much for draw and storage: now to charging.
Solar panels are wonderful, but have limitations, as has been mentioned. Their actual consistent output is dependent on conditions (including nearby objects throwing shadows on the panel's surface) but always less than their rated (maximum) wattage, so conservatively figure your overall average output during the day will be half of whatever number it claims: less in the morning, more mid-day, and less as the sun lowers. Since you're working in amps, convert that average output wattage into amps on paper with the formula amps = watts/voltage (you have a 12-v system) to get the equivalent in amps, which is your usable number. Multiply that number by the number of hours each day you expect the panels to produce, perhaps 5–6, to get the number of daily average amp-hours that you can reasonably expect to be supplied to your batteries. (This is the same conversion formula to determine the total amp-hour draw of any devices that list their operating draw in terms of watts in the budget inventory described above, so that everything is expressed in amps).
Those are the essential steps in creating an energy budget for your small yacht.
Other Considerations
Systems: considering your small yacht and modest needs, the only really essential components are: panels, a solar charge controller to feed the juice to the batteries safely & let you monitor the panel's output, a battery bank in which to store the energy, & a battery monitor that will give you a digital display of battery bank amps-in and amps-out, and so serve as the "fuel gauge" for your battery bank. For such a simple system, going beyond that is nice, but not strictly required.
Making out an energy budget following a method like this will take you from wild guessing and blind equipment purchasing to educated estimates of your real-world needs.
Kind Regards.
