Friday, October 1, 2010

A Watt is a Watt

If you’re thinking about buying your first solar panels or a generator to recharge a battery bank for daily electric power during an emergency or for a remote or off-grid area, here is some information about ‘watts’ that will help you understand them so you can make informed decisions.

I have talked with a number of people about my solar power set-up and find that many have a misunderstanding about watts and 12 volt power. Here’s a quick explanation of solar panel watts and storage battery watts and what it means to you. However this is text book conditions and actual results and battery age can alter the following but it still is a good guideline for understanding watts.

Solar Panels:
Solar Panels produce DC power to be used immediately or stored in batteries. Their power generation is commonly called out in ‘watts’ which is short for ‘watt hours’. Watt Hours means the total number of watts produced, available or useable in one hour or 60 minutes.

Batteries store DC power that is commonly called out in ‘Amp Capacity’ or sometimes ‘Watt Hours’. Again, Watt Hours means the total number of watts available or useable in one hour. If you don’t have the battery watts known but have the amps known you can calculate it yourself if you know the amps and volts. Formula: amps x volts = watts

An important battery note; Some battery manufacturers recommend you only use or discharge a battery to just 50% while some others recommend just 30% of the batteries total amp or watt capacity and then you should recharge them. My RV/Marine 12 volt storage battery has a 100 amp capacity and they recommend I only discharge to 50% or just 50 amps. OK great so what does that mean and how do I use this information? 50amps x 12volts = 600 watts available for me to use before I need to recharge it.

Examples of watt consumption:
My kitchen TV, a 13 inch flat panel, is rated at 23 watts running on 110 volts. That means it uses 23 watts per hour when on. But wait, I’m running it on a 12 volt battery not 110 volt grid power! So how many 12volt watts does it use? Here’s where a ‘Watt is a Watt’. There is no difference between 1 watt of 110v power and 1 watt of 12v power, a watt is a watt regardless of the voltage source!

If I only run the TV from my battery with 600 watts of useable storage capacity I could theoretically watch my TV for 26 hours (600watts divided by 23 watts = 26 hours) before the battery must be recharged. Obviously I am running an inverter to raise the 12 volts to 110 volts and there is some power or watts consumed by the inverter (generally 10%) for its work but we will leave that out for the example. (You would need to know the inverter watt draw for an accurate summary of your needs including its idle time. Mine a 150 watt inverter is using .35 amps sitting idle or doing nothing 24 hours a day).

Another example: A 100 watt light bulb uses the same watt scenario to run. From my battery with 600 watts in it I could run that 100 watt light bulb using an inverter for 6 hours.

Another example: A kitchen mixer that has a 475 watt motor will use 475 watts in an hour while running constantly for an hour. From my battery that would be 1¼ hours of run time available before I have to recharge the battery.

The same will apply to 110 volt well pump and other electric motors if you know their wattage consumption. Normally that information is on its name/information plate.

Regardless of the voltage source, be it 110v, 48v, 24v, 12v or 1.5v a watt is a watt. All watts are the same regardless of what voltage source it comes from.

Hope this helps! YM


  1. Don't forget about inverter efficiency when calculating watts.

    I was surprised (appalled!) at the 85% mine was giving me, in addition to the higher power draw of the appliance due to additional transformer heating using a MSW inverter.

    I have read similar manufacturer data regarding recommended 50% and 30% discharge before charging. Sometimes, unless you pay close attention, discharging according to a "voltage level" can be erroneous since the battery can recover. Are you using a coulomb counter (AH meter)?

  2. I was also surprised with inverters and the power hey comsume. Mine uses more watts than the TV and radio it runs!
    When it's time for an upgrade I will pay close attention to the inverter under load consumption and at idle consumption.

    For now I use the controllers digital voltage meter to judge the amount of discharge. Here's another area that when I upgrade it will have an amp counter for both in and out of the battery.