How can we power Christmas LED lights without mains supply
My question is what specification of car battery would we need to get to power about 600 lights for a month, hopefully without having to charge the battery too often.


Here's how you work it out.


Total the lamp power consumption in watts (W).
Divide this by the voltage to find the current in amps (A).
Multiply this by the run hours to calculate the daily ampere-hours (Ah).
Multiply this by the number of days between recharges to get the total Ah battery capacity required.
Get a battery with about 1.5 times that capacity so that you don't need to discharge it fully (never a good thing) and to de-rate it for low temperature operation.


e.g. 


600 x 0.75 W lamps -> 450 W.
450 W / 12 V = 37.5 A.
At 8 hours per day that's 8 x 37.5 = 300 Ah.


Let's do a reality check at this point. Here's a car battery:



Figure 1. A 12 V, 80 Ah car battery.

As you can see, with 600 of these lights things are well out of hand as you would need five or six of these.

Run the calculations yourself with specifications from the lamps and see how you get on. Don't forget that if you blink or fade the lights that you reduce power consumption proportionally.