Add a Solar Panel to Keep Your Backup Battery Topped Up: A few weeks ago, I wrote about building a simple, home backup power system for charging phones during extended electricity outages. However, there's a limitation to any battery system, and that is that any battery power supply will eventually become depleted. I reckon to get a theoretical 13 days out of mine. What happens, in a catastrophic event, if grid power isn't restored at the end of 13 days? The answer is pretty simple. It's simple to harness the sun's rays and keep the battery topped up with an electricity-producing photovoltaic solar panel.