# How to calculate the total time a battery can operate light?

hi,I am willing to just use solar panels to recharge my batteries and then to power my garden lights.  What I need is how do I know if my battery will be able to power my lights for atleast 24 hours.  How do I choose the solar panels to be able to charge the batteries. IF I know how to calculate for atleast one light and battery that should be enough for me to calculate for my whole system.

Thanks

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Chamburn (author) 3 years ago
well for example I already have a 12 volt 1 watt Led light and also a small sealed battery with 12v 1.3ah.
3 years ago
Watts = Volts x Amps
1 W = 12V x i, i = 1 W / 12 V, i = .083 Amps

Time = (Amp-hours / Amps) / (Safety Factor)
t = (1.3 Ah / .083 A) / 2, t = (15.6 hours) / 2, t = 7.8 hours

You should be able to maintain full brightness for ~ 8 hours depending on how efficient your driving circuitry is and you only run the LED off of the battery.

Do you understand the equations? Try different numbers and make sure you understand how different power lights and capacity batteries will effect the time you get.
1 year ago

can you explain how did you put the value for the safety factor?

1 year ago

It's an arbitrary number really. Mostly it takes into account any inefficiencies in the battery or circuit. It also would help extend the light if you had a couple of days in a row without enough sunlight. You can reduce that number at risk of not having enough power to get the light all the way through until the panels take over again.

Chamburn (author)  bwrussell3 years ago
Hi, yes I did understand the equations and already got all the required components but now I don't really know If I really need a transformer to power my 12 volt lighting system or can I just connect my lights directly to my battery or timer.
3 years ago
That depends on how the lighting is arranged and what the source is. Is it a single 12v light or multiple sources? What are the specs of the source? You can use any DCV as long as it meets the min required by the sources and you have the right resistance. The closer the input voltage to the required voltage the better though, any excess will be lost as heat through the resistor.
3 years ago
Using BWR's numbers, in other words, that little battery doesn't hold enough to carry you through a SINGLE night, so to begin with, you need a bigger battery - don't aim to flatten it overnight either, aim for say 25% of capacity, tops, so shoot for a 30Ah battery. Which you need to charge with the available daylight in say 10 hours, so you need 750mA@12V from your solar panel,
steveastrouk3 years ago
Start with your lights. How many volts and how much current do they draw ? How long will they be on for, in the worst case ?
Chamburn (author)  steveastrouk3 years ago
hi, Yes it did help however I need to know how to connect the lights together as Normally I would run a single wire then connect the lights to it but the problem is I don't really know how to make the connections to the main power cable where I would use parallel connection setting. How do I need to connect the lights so that in case one breaks down all the others kept on lighting.thanks
Josehf Murchison3 years ago
watts hours/watts/2

Watts are volts x amps.

Battery in watt hours (Storage capacity), light in watts and discharge till the light wont work (About Half to be safe).

You can do the same with amps.

Light in amps, battery in amp hours (Storage capacity) and discharge until the light wont work (About Half)

So a 5 amp light will stop working when running on a 10 amp hour battery in one hour.

Does this help?

Joe