Portable Solar Hookup Question

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Re: Portable Solar Hookup Question

Postby Socal Tom » Sun Dec 03, 2017 1:42 pm

John61CT wrote:I'm amazed 50W and under 100AH is enough for a fridge.

Longterm boondocking up north I bet not when cloudy weather sets in.

The best battery value by far is Duracell (actually Deka/East Penn) FLA deep cycle golf cart batteries, 2x6V, less than $180 per 200+AH pair from BatteriesPlus or Sam's Club

Yes heavy, but take care of them can last 6-8 years.

My fridge uses less than 9 watts per hour, so roughly 200 watts over 24 hours ( 17 amps). I could theoretically go a regular 2 night trip without solar and stay above 50% amperage. Lighting etc brings me to about 300 watts per day, so 50 watts and good sun works out ok.


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Re: Portable Solar Hookup Question

Postby John61CT » Sun Dec 03, 2017 3:38 pm

Socal Tom wrote:My fridge uses less than 9 watts per hour, so roughly 200 watts over 24 hours ( 17 amps). I could theoretically go a regular 2 night trip without solar and stay above 50% amperage. Lighting etc brings me to about 300 watts per day, so 50 watts and good sun works out ok.

Watts per hour or day has no meaning, nor does amps per any time period.

Perhaps you mean watt hours and amp hours, wH and AH per time period?

Even very efficient 12V compressor fridges burn 3-4 amps while the compressor is running.

In very hot weather running as a freezer @15°F the duty cycle is a much higher percentage than as a fridge @38° in cooler ambient temps.

Say from 15A per day up to 60A.

I usually size banks to go without inputs for 3 days, of course for lead trying to never go below 50%.

FWIW
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Portable Solar Hookup Question

Postby Socal Tom » Sun Dec 03, 2017 4:15 pm

John61CT wrote:
Socal Tom wrote:My fridge uses less than 9 watts per hour, so roughly 200 watts over 24 hours ( 17 amps). I could theoretically go a regular 2 night trip without solar and stay above 50% amperage. Lighting etc brings me to about 300 watts per day, so 50 watts and good sun works out ok.

Watts per hour or day has no meaning, nor does amps per any time period.

Perhaps you mean watt hours and amp hours, wH and AH per time period?

Even very efficient 12V compressor fridges burn 3-4 amps while the compressor is running.

In very hot weather running as a freezer @15°F the duty cycle is a much higher percentage than as a fridge @38° in cooler ambient temps.

Say from 15A per day up to 60A.

I usually size banks to go without inputs for 3 days, of course for lead trying to never go below 50%.

FWIW
watts per hour is the same as watt hours. Watts and amps both have standard definitions.
From Wikipedia
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length.[6][3][7]

The watt (symbol: W) is a derived unit of power in the International System of Units (SI) defined as 1 joule per second[1] and can be used to quantify the rate of energy transfer. Power has dimensions of
M
L
2
T

3
{\displaystyle {\mathsf {ML}}^{2}{\mathsf {T}}^{-3}}


Since I gave a unit of time to go with the amps/watts it should all make sense.


Watt hours give a rate of usage. Watts are the total. It’s similar to miles, 55 miles per hour is a rate of speed, however if I drive 55 mph for 2 hours, I’ve gone 110 miles.


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Re: Portable Solar Hookup Question

Postby low277 » Sun Dec 03, 2017 9:53 pm

:) Amps and volts are inversely proportional but watts are watts!!!! Way back when we used "P" to indicate watts.
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Re: Portable Solar Hookup Question

Postby John61CT » Sun Dec 03, 2017 11:31 pm

Both watts and amps are analagous to mph, have the time element already, maybe gallons per hour is better, rate of flow, may constantly vary over time, an instantaneous snapshot. Difference is with amps you need to know voltage.

Watt-hours and amp hours are a static quantity of energy, like gallons.

Capacity held by storage, or energy used by a load over a period of time.

So a 12V fridge while compressor is on burns at a *rate* of 4A.

20% duty cycle (compressor averages 12min running per hour) that's .8 AH per hour or 19.2 AH per day.

Bigger temp differential or less insulation might be a 70% duty cycle, means 2.8 AH per hour 67.2 AH per day.

I don't care what Wikipedia or anyone says, 1 AH is the energy consumed by something running at an average of 1A **for** one hour.

Saying 1A **per** hour is not at all the the same thing, and in fact is meaningless, maybe 'increased by 1A per hour as temps rose', implies acceleration or something.

In any case, nitpicking aside, this fridge running off a 50W panel will destroy the 87AH batt pretty quickly if used for fulltime boondocking.

Lead batts **must** get recharged back to 100% Full (not 99%) at least a few times per week or they will last a small fraction of their lifetime treated properly.
Especially if not actually designed for deep cycling.

Yes there is one exception but at $500 per 100AH not practical for most.
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Re: Portable Solar Hookup Question

Postby Socal Tom » Mon Dec 04, 2017 11:10 am

John61CT wrote:Both watts and amps are analagous to mph, have the time element already, maybe gallons per hour is better, rate of flow, may constantly vary over time, an instantaneous snapshot. Difference is with amps you need to know voltage.

Watt-hours and amp hours are a static quantity of energy, like gallons.

Capacity held by storage, or energy used by a load over a period of time.

So a 12V fridge while compressor is on burns at a *rate* of 4A.

20% duty cycle (compressor averages 12min running per hour) that's .8 AH per hour or 19.2 AH per day.

Bigger temp differential or less insulation might be a 70% duty cycle, means 2.8 AH per hour 67.2 AH per day.

I don't care what Wikipedia or anyone says, 1 AH is the energy consumed by something running at an average of 1A **for** one hour.

Saying 1A **per** hour is not at all the the same thing, and in fact is meaningless, maybe 'increased by 1A per hour as temps rose', implies acceleration or something.

In any case, nitpicking aside, this fridge running off a 50W panel will destroy the 87AH batt pretty quickly if used for fulltime boondocking.

Lead batts **must** get recharged back to 100% Full (not 99%) at least a few times per week or they will last a small fraction of their lifetime treated properly.
Especially if not actually designed for deep cycling.

Yes there is one exception but at $500 per 100AH not practical for most.

Ok, I'm done defining terms, at this point its just frustrating. Not everyone is full time boon docking, so the best options are not the same under different circumstances.
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Re: Portable Solar Hookup Question

Postby John61CT » Mon Dec 04, 2017 11:30 am

Yes, if you know you can get to shore power every few days, it completely changes the calculations.

Just increasing the bank AH capacity can completely eliminate any need for solar.

But having a system designed to be self-sufficient off grid of course gives greater flexibility.

The challenge of getting to 100% Full at least every 2-3 days means the panel to bank ratio needs to be targeted to worst-case cloudy conditions.

Or, many people are fine with just replacing their bank more frequently.
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