# What should be the watts of solar panel and battery size

I want to light one 70watt bulb for 12 hours at night time. I want to know what should be the Watts of the solar panel and how much Ah battery will I require?

I searched many of the questions here and found this one relevant. But the question is how will I know the volts which you assumed here to be 12 V?

Powering 10watt light bulb for 24 hours using solar power

May I know if I have done the right calculation as below

Panel Watts req = 70 * 32 / 10

Sunshine hours=10

Assumed 12V, Battery Ah= 70W / 12V * 12h = 70Ah

I didn't see if you are using a 120 V bulb for an AC lamp. I assume it's an incandescent bulb. If this is the case you will need to step up the voltage .

The filament they use glows white hot to produce light (and heat) which is wasteful. I agree with @Dampmaskin that LEDs are more efficient.

70 W * 12 hours = 840 Wh. That is how much energy you need to take out of the battery every night and put back in every day. If you want the battery to last, you should pick a battery with twice that capacity. So that is around 1700 Wh. 1700 Wh / 12 V = around 142 Ah at 12V. Pretty big battery. You cannot discharge a battery 100% every day. It will not last long.

The solar panel needs to produce 840 Wh in about 5 hours. So that is 840 / 5 = 168 Watts. This is just a rule of thumb. The daily output of a solar panel is around 5 hours * rated power. Obviously there is some variation depending on location and season. You might want to look into data specific to your location.

So there you go. A 12V 142 Ah battery. A 168W solar panel. And a good solar charge controller with maximum power point tracking (MPPT).

70W * 12h = 840Wh

840Wh / 12V = 70Ah

This is the minimum capacity of your battery needed. I would suggest to use at least the 140Ah to get longer live time of your battery. Batteries don't like to be discharged to the bitter end.

All this has nothing to do with the solar panel.

Charging a battery needs always more power than you get back. Normally around factor 1.2 to 1.4

840Wh * 1.4 = 1176Wh

1176Wh / 10h = 117W

Your panel as to deliver at least 117W during your sunshine hours. How much energy is delivered by your panel is depending on the mounting position and is (almost) never the amount rated. But with a panel rated for 150 to 200W it could work.

You need 70 W * 12 h = 840 Wh from the battery. You are correct that for a 12 V battery, this is 70 Ah.

To get 840 Wh into the battery over the course of 10 hours, you need a 840 Wh / 10 h = 84 W output from the solar panels.

All assuming no loss, perfect sunshine, etc. Have you considered using LEDs instead of a 70 W bulb? That way you would get away with a much smaller battery and much smaller solar panels, and it will probably be much cheaper.