electrical charging question

[deano]

Hi,

looking into the possibilty of running some lights of a 12v car battery and charging the battery with solar power.

the load will be 100w which if i've done my sum's right comes out at just under.5 of an amp ( 100/240 ).

The ligths will be on for between 3-6 hours, what wattage solar panel would i need to be able to keep the battery topped up and useable.

cheers for any info

deano

 

[geeza]

not sure but dont you need the sun to be out a fair bit also? If you could guarantee that then I would try and solar power my entire house. (damn Npower)

 

[Visage]

Hi,

looking into the possibilty of running some lights of a 12v car battery and charging the battery with solar power.

the load will be 100w which if i've done my sum's right comes out at just under.5 of an amp ( 100/240 ).

The ligths will be on for between 3-6 hours, what wattage solar panel would i need to be able to keep the battery topped up and useable.

cheers for any info

deano

Well, to make it easier we can work in Watt-hours. (which is synonymous with energy)

So a 100W bulb for 3-6 hours uses 300-600 Watt-hours of energy.

So divide that by the expected number of hours sunshine you;ll get (taking into acocunt the minimum level of sunlight needed) and there you go.

 

[happytechie]

if that's a 100watt bulb at 240v it's not the same amoubnt of energy as a 100W bulb at 12v if my memory of phiysics is correct?

HT

 

[Adam_151]

if that's a 100watt bulb at 240v it's not the same amoubnt of energy as a 100W bulb at 12v if my memory of phiysics is correct?

HT

It is the same, ignoreing any losses due to conversion, the watt is a measure of rate of energy transfer, 100w is 100 joules a second regardless of what voltage it is, of course to supply 100w at 12v you need a current 20 times greater than to do the same at 240v

Voltage is joules per coulumb, and current is coulumbs per second, multiply these and you get joules per second, or watts

 

[deano]

Well, to make it easier we can work in Watt-hours. (which is synonymous with energy)

So a 100W bulb for 3-6 hours uses 300-600 Watt-hours of energy.

So divide that by the expected number of hours sunshine you;ll get (taking into acocunt the minimum level of sunlight needed) and there you go.

so say 3hours =300watt hours and be generous at 5 hours sunshine i'd need 60watts

in reply to geeza,

have been away camping and using the inverta to run a few light bulbs in the tent so was just expanding the idea up to running the lights in the house, all bulbs are already energy saving so thought i could make it work failry easily. If i've got my above sums right then a 80w panel without any regulators is in te region of £300. So make's the idea uneconomical / to long to payback time. ( at current prices ).

Also got an old radiator in the back garden and an old imersion tank was thinking of warm water during the summer

cheers
deano

 

[Visage]

if that's a 100watt bulb at 240v it's not the same amoubnt of energy as a 100W bulb at 12v if my memory of phiysics is correct?

HT

Thats a good point.

100w @ 240V indicates a resistance of 576 ohms.

At 12V you'll only get 0.25W. Not bright at all.....

 

[Visage]

It is the same, ignoreing any losses due to conversion, the watt is a measure of rate of energy transfer, 100w is 100 joules a second regardless of what voltage it is, of course to supply 100w at 12v you need a current 20 times greater than to do the same at 240v

Voltage is joules per coulumb, and current is coulumbs per second, multiply these and you get joules per second, or watts

Yes, but current here is dependant on voltage and resistance.

Voltage will be fixed - since it will be whatever the storage battery is capable of supplying, as will resistance, since this is a physical property of the bulb.

 

[Mat]

You could always build a small wind generator?

A friend of mine has just done the same - built a fan to trickle charge a battery to run some LED's to light his tent. Works perfectly.