# In a solar power system, when the panel is off, does the battery determine the wattage?

I am considering putting together a solar system to power my wireless router when the power goes out. I just have one question that I cannot find the answer to:

Does the wattage of the solar panel matter in my case?

I will be using a 12 V battery and an inverter for after the sun goes down, and a 30 W solar panel seems to be enough to power the router during the day. So after the sun goes down, is the wattage then dependent on the battery? Forgive me if this is a stupid question, I could not find any comprehensive answers.

• The wattage (power draw) which can be supported at any given instant depends on the impedance of the battery, and can often be usefully approximated by multiplying its nominal voltage by rated capacity in amp hours and the "C" rating. The total energy which can be provided is the product of the capacity and effective voltage, which of course changes a bit during discharge. An inverter may be the simplest way of connecting a random appliance to a battery, but is probably not the most efficient, particularly for the relatively small load of a typical wifi device. Commented May 28, 2016 at 20:52
• @526rocks. Be sure to use a battery charger / regulator. Look for ones designed for solar cells as an input. They will charge the battery properly without chance of an overcharge. They will account for the variation in sunlight during the day.
– user105652
Commented May 28, 2016 at 21:30

Yes, the battery needs to produce enough power to run the router, and power is measured in watts. Wattage is a colloquial term which means power. Your solar panel and battery must produce at least as much power as the router uses. If they produce extra that's also fine, in fact that helps keep things reliable.

The thing is, most batteries aren't sold with power ratings. Instead, they are sold with a voltage and an amp-hour rating. They will also have either a C rating, or a rated current. Power is current times volts, so if the battery has a voltage and a current rating, multiply them together to get power. If the battery doesn't have a current rating, you can work it out by multiplying the capacity (in amp hours) with the C rating.

By the way, 30W sounds like a lot for a router, even if that's the DC power from the panel and the inverter is losing some of it in the process of converting to AC. If you check the actual power usage with a multimeter, you might find it's less than that.

Once you have worked out if the battery has enough power to run the router, you'll probably want to work out how long it can keep it running for. If you divide the power you are using by the voltage, you'll get the current required in Amps. Then you divide the capacity of the battery in Amp Hours by the current in amps to get a rough time in hours that it will last for.

• Thank you very much! Just to check my math, if I am using a 12v battery rated for 7 amps I will have 84 watts (power). I'll high ball it and say my router uses 15 watts, then it will draw 1.25 amps. Then my 7 Ah battery will last ~5.6 hours? Commented May 28, 2016 at 21:18
• Looks correct. You could get some more time out of the system by swapping the router's power supply and inverter for a dc/dc converter - it would be more efficient. Commented May 28, 2016 at 21:28
• Most home routers are 12 V DC. If yours is you can power it directly off the battery (through a fuse in a car-type in-line fuseholder). Commented May 28, 2016 at 23:36
• @526rocks My netgear 6300 is wired directly to my UPS battery and it's been running fine for months (like a laptop, it'll have it's own internal step down regulator, nothing in the router is going to be running at 12V natively - more like 5 or 3.3 - so you've likely got some margin, although the current draw will be higher at lower input voltages)
– Sam
Commented May 29, 2016 at 1:05