# Electronic – Sizing a solar panel and battery for a simple solar application

batteriespowersolar cell

I am a novice. I want to correctly size a solar panel and battery for the following application. I will be powering 2, 6 watt cfl lights and a 40 watt incandescent light. I am planning on using 400 watt inverter from Northern Tool. I need the inverter because of the need for the 40 watt incandescent bulb which will be used as a heat source. The 40 watt bulb will be on 24 hours a day when the temperature is below 32 F. The two 6 watt cfl lights will run a maximum of 5 hours a day year round. I have done research on the internet, but so far some of the information I have obtained is contradictory. I live in southwestern Pennsylvania. What I need to know is the size of the solar panel in watts and the size of the battery Ah.

Using a 120 VAC inverter and a light bulb to keep your pipes from freezing is just a dumb idea.

If you need 40W worth of heat in an enclosed space, just select a bunch of power resistors that add up to that amount, screw them to a heatsink and run them directly from your 12 V battery. For example, a 15-ohm resistor will dissipate about 10W @ 12V, use four of them.

You can also get CFL lamps that run directly from 12V.

Let's say you need a run time of 48 hours, and you have a minimum of 6 hours to recharge.

Your load is drawing a daily average of 3.33 (heater) + 5/24 × 1 (lights) = about 3.5 A. Multiply by 48 hours to get the battery capacity: 170 Ah

Batteries aren't perfectly efficient, so it'll take about 170 / 0.8 = 212.5 Ah to recharge it.

If you want to drive the load and recharge the battery in 6 hours, you need a panel capacity of about 40 A @ 12 V, or 480 W.