# Check My Calculation

55 Posts
Jan 20, 2009 09:25 pm
Check My Calculation

I need someone to check my calculation. I am using a 5w 110v bulb thru an inverter. 6hrs a day 7 days a week. Average 4 hrs of sun a day with 4 days of no sun at worse. 4 .350ma 12v solar panels and 115 ah battery run down to 50%. I have done the math and I think that will fly. What do you think? Maybe  someone who is running battery's need to give me some guidance.

220 Posts
Jan 21, 2009 01:33 pm
Re: Check My Calculation

hi max,

the first thing that came to mind when reading your post was, is there any way to go with a 12vdc bulb in your application? you should be able to find an automotive bulb with the proper ratings.

if the use of 12vdc bulbs is out of the question then, i would take actual readings with my meter to see what the measured input and output are. i'd especially be concerned with the metered inverter draw from the battery both idle and while powering the bulb.

we can test these low currents safely with our meter in line from battery output to inverter input. the measured inverter draw is really going to be the make or break on this setup. with your digital multimeter, use the 10amp option. (even the \$10 meters are surprisingly accurate) see what the current draw is on that inverter. we know our 5 watt bulb will account for about 420ma and anything over will be inverter losses. we may be disappointed to find that the measured draw on the inverter is much higher.

i would also check the pv production the same way-when it's nice and sunny can we see production in the approx 1amp or better range?

these readings (more so the inverter measurements) will give you much better raw data to work with.

we may be on the big size with that 110ah battery and would think a 25ah might work out better with the limited charging source available. (less self discharge losses and higher input to amphour ratios)

best regards, dave

351 Posts
Jan 21, 2009 03:23 pm
Re: Check My Calculation

I would take a look at landscape lighting. 5 watt 12vdc bulbs are availible in the MR11 floodlight style, although not every one stocks them. 4 watt and 7 watt wedge base bulbs are extremely common.

Feb 9, 2009 06:13 am
Re: Check My Calculation

5 watts x 6 hours a day / 12 volts nominal = 2.5 amphours per day.
(In retrospect, I should have included the fact that watts are watts. A watt doesn't know the difference between voltages. Besides when using an inverter, the 120 vac would not be in the equation because its actually coming from the batteries. Thats why I used 12 volts nominal in the equation above and not 120 or 110.)

115 amphour battery x 2.2% = 2.5 amphours.
(Again in retrospect. What your using from the batteries, I assume at night, is only within the top 5% of battery capacity. This shoud give you way more than 4 days without sunshine.)

4 - PV modules, .350 amps @ 12 volts.
4 hours of equivalent full rated charge per day.
4 x .350 x 4 = 5.6 amphours per day PV.

A surplus of 3.1 amphours PV per day. Not including inverter or wire loss inefficiencies.
(Last, these inefficiencies could add up to as much as a 15% loss. 15% of 3.1 still leaves a surplus of about .5 amphours.)

« Last Edit: Feb 22, 2009 11:28 am by Thomas Allen Schmidt »

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