Radically different sized RV solar. Plus solar projects.
Subtitle: Solar marginal utility and break points in solar sizing for RVing
The purpose, capabilities, and additional benefit of more watts change significantly as we move through the range of solar sizing possibilities. A lot of the legitimate debate I see seems to go back to the (mostly unstated) purpose of someone's solar system. Not surprising since we are talking 2 orders of magnitude from the smallest to the largest. A lot of assumptions go out the windows when you make them across such a large range of applications.
If one person views its main goal as being a battery charger, but another wants solar to be able to power a microwave in daylight without any draw on the battery bank, the marginal utility of adding enough watts to a solar install thatbis just a little too low to run the microwave, increasing to just high enough to do it is radically different for these two people. If you are the battery charging guy, that extra wattage in MPPT yield or a small panel addition may be waste. If you are the guy camping in abundant sunshine who could now microwave lunch from sunlight alone as desired, the value is very different from they guy just wanting to charge batteries well.
I took a shot at listing some classes of RV solar systems, their capabilities, and the marginal utility of each.
----------
System size of few watts of solar panel (smaller than needed to counter battery bank self discharge)
- Can slow down self discharge on RV battery. Handy to charge small portable devices or small portable batteries.
-Very expensive per watt
-Very good utility and marginal utility. Every watt counts and adding a few watts adds a lot of benefit.
-Benefits from tilting. In times of high production, tilting may cause a system like this to totally counter self discharge or even slightly charge batteries.
Enough watts to balance battery self discharge or trickle charge (a few watts per battery up to say 15w per physical battery, or per 100 Ah of battery when dealing with really large units)
- Keeps batteries happy and healthy in storage, generally with or without a controller
-Cost is high per watt unless you have a very large battery bank
- Very good marginal utility - when boondock camping, almost every watt you paid for contributes almost all day. The savings in less frequently replacing batteries allowed to discharge alone can make good financial sense. The convenience for RVs stored without power to have a charged battery every visit is very high. This is some of the highest bang for the buck solar you can get, especially if you find a good sale or free shipping.
- Benefits from tilting toward sun when camping, as every watt produced is hungrily consumed.
Small solar system, but large enough to require a controller.
-Often selected by people who have enough battery to do what they want, but want to extend their off grid time, or be able to conserve power a bit less.
- Cost for the solar panels just breaks into the great cost per watt price point if you shop well
-Can be hard on batteries depending on how long they are used in this mode and how well they are overcharged upon returning to a powered location
- Incredibly high marginal utility. Every watt contributes all day long. Great value for the expenditure.
- Benefits highly from heroic or technology efforts to maximize watts from small system like tilting, or like MPPT IFF the technology is efficient at the power level of the system.
Medium small
-Enough watts to be totally solar while camping with rigorous conservation and sunny days
-Still need to end trip or get supplemental charging if things are not perfect
-Incredibly high marginal utility - generally almost every watt goes into the battery all day every day.
-Hard on batteries if used for a long time in this mode, or if proper charging of the bank is not done after trip of after being totally solar.
Medium
-Enough watts to be genset free in many situations other than high loads
-Gets battery bank up to close to full pretty often unless bank is huge relative to solar watts
-Enough watts to equalize batteries (what is required varies a lot based on type and Battery OEM instructions for flooded or not) on solar alone without breaking up battery bank.
-Marginal utility or value of more watts starts to drop quickly unless special procedures or technologies are used. More often at this size of solar capacity not all the watts are used on sunny days without special capabilities like dump loads, or special procedures such as resetting some controllers to continue charging in bulk after a time out, or even panel direct bypass to battery with a timer. Each additional watt purchased (through MPPT, more solar or other means) provides less and less incremental value if battery charging is your primary objective. Not true if you do other productive things with watts not used for battery charging, such as pumping water into a tank, heating water, charging a secondary bank, running a grid tie inverter and being paid for excess electricity...etc.
Medium large
- Generally lots of watts are generated beyond what is needed for battery charging.
- Typically makes significant watts on poor solar output days
- Marginal utility as a battery charger for each additional watt harvested continues to go down. Generating more marginal benefit from more watts depends on objectives like being genset free or other design points past battery charging like resistance heating, water heating, and large inverter loads. The biggest motivation at this stage may be to be completely genset free.
Large - generally maxed out on roof space, and possibly even adding additional locations for solar panels beyond normal roof locations.
-Much more likely to be able to be genset free all the time, even with a smaller battery bank than the medium large solar size.
-Makes enough power to charge battery bank under moderate or even poor solar conditions.
-System's primary objective is probably not as a battery charger.
-Huge savings from running large loads when the sun is up stretches amp hours in battery bank due to reduced Peukert losses. Batteries really only need to be sized for power usage in darkness or as additional amps on poor solar days for large loads.
-Loads that naturally are heavier or more frequent during daylight can often be fully accommodated with solar production (coffee maker, microwave, Smaller air conditioning units, fully electric water heating)
-Conservation becomes more about time shifting loads to daylight hours than reducing usage
-Marginal utility from tilting, more efficiency in battery charging or upgrading to more efficient panels is generally low. It may be more around being able to run the next biggest load purely from solar, being able to reduce battery bank size, or be break even in solar production vs power usage under worse and worse solar conditions.
Jim
Edit - adjusted title to accommodate evolving discussion on solar projects the original thread spawned.
The purpose, capabilities, and additional benefit of more watts change significantly as we move through the range of solar sizing possibilities. A lot of the legitimate debate I see seems to go back to the (mostly unstated) purpose of someone's solar system. Not surprising since we are talking 2 orders of magnitude from the smallest to the largest. A lot of assumptions go out the windows when you make them across such a large range of applications.
If one person views its main goal as being a battery charger, but another wants solar to be able to power a microwave in daylight without any draw on the battery bank, the marginal utility of adding enough watts to a solar install thatbis just a little too low to run the microwave, increasing to just high enough to do it is radically different for these two people. If you are the battery charging guy, that extra wattage in MPPT yield or a small panel addition may be waste. If you are the guy camping in abundant sunshine who could now microwave lunch from sunlight alone as desired, the value is very different from they guy just wanting to charge batteries well.
I took a shot at listing some classes of RV solar systems, their capabilities, and the marginal utility of each.
----------
System size of few watts of solar panel (smaller than needed to counter battery bank self discharge)
- Can slow down self discharge on RV battery. Handy to charge small portable devices or small portable batteries.
-Very expensive per watt
-Very good utility and marginal utility. Every watt counts and adding a few watts adds a lot of benefit.
-Benefits from tilting. In times of high production, tilting may cause a system like this to totally counter self discharge or even slightly charge batteries.
Enough watts to balance battery self discharge or trickle charge (a few watts per battery up to say 15w per physical battery, or per 100 Ah of battery when dealing with really large units)
- Keeps batteries happy and healthy in storage, generally with or without a controller
-Cost is high per watt unless you have a very large battery bank
- Very good marginal utility - when boondock camping, almost every watt you paid for contributes almost all day. The savings in less frequently replacing batteries allowed to discharge alone can make good financial sense. The convenience for RVs stored without power to have a charged battery every visit is very high. This is some of the highest bang for the buck solar you can get, especially if you find a good sale or free shipping.
- Benefits from tilting toward sun when camping, as every watt produced is hungrily consumed.
Small solar system, but large enough to require a controller.
-Often selected by people who have enough battery to do what they want, but want to extend their off grid time, or be able to conserve power a bit less.
- Cost for the solar panels just breaks into the great cost per watt price point if you shop well
-Can be hard on batteries depending on how long they are used in this mode and how well they are overcharged upon returning to a powered location
- Incredibly high marginal utility. Every watt contributes all day long. Great value for the expenditure.
- Benefits highly from heroic or technology efforts to maximize watts from small system like tilting, or like MPPT IFF the technology is efficient at the power level of the system.
Medium small
-Enough watts to be totally solar while camping with rigorous conservation and sunny days
-Still need to end trip or get supplemental charging if things are not perfect
-Incredibly high marginal utility - generally almost every watt goes into the battery all day every day.
-Hard on batteries if used for a long time in this mode, or if proper charging of the bank is not done after trip of after being totally solar.
Medium
-Enough watts to be genset free in many situations other than high loads
-Gets battery bank up to close to full pretty often unless bank is huge relative to solar watts
-Enough watts to equalize batteries (what is required varies a lot based on type and Battery OEM instructions for flooded or not) on solar alone without breaking up battery bank.
-Marginal utility or value of more watts starts to drop quickly unless special procedures or technologies are used. More often at this size of solar capacity not all the watts are used on sunny days without special capabilities like dump loads, or special procedures such as resetting some controllers to continue charging in bulk after a time out, or even panel direct bypass to battery with a timer. Each additional watt purchased (through MPPT, more solar or other means) provides less and less incremental value if battery charging is your primary objective. Not true if you do other productive things with watts not used for battery charging, such as pumping water into a tank, heating water, charging a secondary bank, running a grid tie inverter and being paid for excess electricity...etc.
Medium large
- Generally lots of watts are generated beyond what is needed for battery charging.
- Typically makes significant watts on poor solar output days
- Marginal utility as a battery charger for each additional watt harvested continues to go down. Generating more marginal benefit from more watts depends on objectives like being genset free or other design points past battery charging like resistance heating, water heating, and large inverter loads. The biggest motivation at this stage may be to be completely genset free.
Large - generally maxed out on roof space, and possibly even adding additional locations for solar panels beyond normal roof locations.
-Much more likely to be able to be genset free all the time, even with a smaller battery bank than the medium large solar size.
-Makes enough power to charge battery bank under moderate or even poor solar conditions.
-System's primary objective is probably not as a battery charger.
-Huge savings from running large loads when the sun is up stretches amp hours in battery bank due to reduced Peukert losses. Batteries really only need to be sized for power usage in darkness or as additional amps on poor solar days for large loads.
-Loads that naturally are heavier or more frequent during daylight can often be fully accommodated with solar production (coffee maker, microwave, Smaller air conditioning units, fully electric water heating)
-Conservation becomes more about time shifting loads to daylight hours than reducing usage
-Marginal utility from tilting, more efficiency in battery charging or upgrading to more efficient panels is generally low. It may be more around being able to run the next biggest load purely from solar, being able to reduce battery bank size, or be break even in solar production vs power usage under worse and worse solar conditions.
Jim
Edit - adjusted title to accommodate evolving discussion on solar projects the original thread spawned.
