May 19, 2012, 8:31 AM
Post #5 of 8
(Previous post lost during editing... oooops)
Re: [Kimpatsu Hekigan] How to size a solar PV power system?
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. . .
The reason I bought it was to quantify the power consumption of my computer set-up in anticipation to switching it to off-grid solar power, i.e., no CFE involved, except as a "last-resort" back-up.
It appears that my set-up uses about 115 Watts with everything running and about 25 W when "asleep" or on stand-by. Average total power consumption in a 24-hr period has been about 1,700 W thus far.
What would be my first step in sizing a solar PV power system with batteries for this level of consumption? What questions should I be asking?
This is an experiment for me, so the economics of such a system, like the time needed to recover my investment, are not that important. Reliability, however, is important.
Any thoughts or ideas would be much appreciated.
There is actually not enough information to answer your questions:
~ We need to know how many cloudy days in succession you can have.
~ We need to know your lowest daily temperature in your battery storage area.
~ We need to know what kind of battery you intend to buy - 12V or 24V? Lead Acid? Flooded Lead Acid?
~ Helpful to know how many hours of good sunshine you get on the shortest days of the year...
~ Your "1700 Watts per day " for computing does not make sense... Power usage per day is in Watt-Hours, not just Watts.
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Let's assume either 1 or 2 cloudy days in succession:
Let's assume your lowest daily (battery area) temperature is 60ºF or 16ºC:
Let's assume (an aggressive) 6 sunny hours per day**:
Let's assume you are buying 12V lead acid batteries:
Let's assume you meant 1700 W-hr is what you actually measured:
**Mexico is actually rated for just 4-5 sunny hours per day for making good power, during the winter, on sunny days...
All the assumptions translate to:
You running your computer in the "ON"-mode for 18.6 hours per day (for 115 W as "ON" and 5.4 hours of 25W in "sleep" mode). and
If you assume 6 sunny hours per day (that you do not adjust the panel position to keep them pointed at the sun throughout the day), and 365 sunny days a year, then you would need roughly 16-18 hours of battery power during low sun intensity on the panels per sunny day. and
1700 Watt-hr per day of 120V power consumption translates to 14.2 amp-hr of current used at 120V. Assume a crummy inverter that is only 80% efficient, yields 18A-hr at 120V used during the 24 hr cycle.
This translates to 180 Amp-hr used at 12V.
~ Typical lead-acid batteries actually operate at 12.65V for a full charge.
~ If you discharge more than 50% of a lead acid battery's amp-hr rating, you dramatically shorten battery life. Modest battery life is maintained by keeping discharges to less than 40% of battery amp-hr ratings (maintaining 60% of full charge).
~ To keep things simple, lets assume a power factor of 1.00
This means that for every sunny day, you need
180 Amp-hr x 2.5 = 450 Amp-hr of 12V battery backup per day.
If you have one cloudy day - and no power generated - translates to 24 hours with little or no power generated - which translates to roughly 450 Amp-hours of power used by your computing during that time (one day without sun) with minor (20%) efficiency losses.
The 450 Amp-hours translates to roughly 5,400 Watt-hours of battery backup to give really solid protection and good battery life for one Not-Sunny day...
Two dark days conservatively translates to 900 Amp-hours or 10,800 Watt-hours of theoretical needed 12V battery storage capacity.
Solar panels do generate power before 9:00 AM and after 3:00 PM, and they do generate some power on cloudy days, so, making these adjustments to the theoretical values, tweaks them down to:
360 Amp-hr = 4,350 Watt-hr for one cloudy day of typical battery storage capacity
720 Amp-hr = 8,700 Watt-hr for one cloudy day of typical battery storage capacity
Using low capacity 200 A-h batteries would mean you need 2 batteries for 1 cloudy day / 4 batteries for 2 cloudy days.
Using higher capacity 500 A-hr batteries would mean roughly 1 battery for 1 cloudy day, and 2 batteries for 3 cloudy days.
Since discharging batteries below 50% of their capacity permanently damages them, I would go with overcapacity (an extra battery) - or plan to really cut back your power usage (no sleep mode - and limited hours per day) to withstand longer sieges of cloudy days....
So, on every cloudy day you might want to cut back your computer usage and completely drop the (unnecessary?) sleep mode -when not using it: Do a TOTAL shut down by unplugging the computer or switching it off totally using a power strip.
Gotta know how many cloudy days you can get in succession and your lowest daily temp. to do any better...
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(This post was edited by YucaLandia on May 19, 2012, 10:52 AM)