How to translate Kill A Watt meter information.

[TonyK]

I took the advice of some people here and bought a Kill A Watt meter to find out the energy usage of my small chest freezer to determine how big of a 12 volt battery bank I will need. I guess I’m a little slow here because I don’t know how to translate the information the Kill A Watt meter is giving me into battery amp hour size. My original belief that it was using 2 amps was wrong now that I've actually tested it with the Kill A Watt meter. The freezer is using 0.89 killowatt hours every 24 hours. It runs at 0.90 amps and 0.58 watts. Based on these readings how many amp hours of batteries would I need to use it every day, allowing for some cloudy days, if I only want to use 30% of the battery power each day? Is there a formula I can use to figure this out? Thanks for your help.

 

[CompMan2020]

Based on 890 watt hours a day, I don't see why you couldn't use a 300AH AGM battery bank. At 50% usage, that should give you 2 straight days of power without any issues.

 

[RCinFLA]

The more time you allow the Kill-A-Watt meter collect info the better the average. Adding new items, opening and taking stuff out can vary the daily total a bit. I wish the Kill-A-Watt meter kept a peak amperage reading.

0.9 kWH per day sounds about right for freezer.

Kill-A-Watt reads Watts and VA. Watts is true power, VA is apparent power. VA * power factor (PF) is true power. in watts. Check PF reading when compressor is running. Induction motor will have less then PF of 1. If you are reading 0.90 amps then VA (apparent power) = 117.6 VA.

Two peak power items to be watchful of is compressor startup surge and defroster cycle heater. They can have 3 to 8 amps of surge for short period of time. Your inverter has to be big enough to supply this surge current.

To your question, for a 12v inverter, assume about a 90% conversion efficiency to 120vac. 1 kWH per day A.C. with 90% conversion would be 1 KWH/0.9 = 1.11 kWH per day from battery.

If inverter input voltage average is 12.2 vdc and you consume 1.1 kWH per day then you are using 1,100 watts per day / 12.2v = 90 AH per day from 12v battery per day.

If you are using a 12v lead-acid battery do not use more then 70% of its rated capacity, preferrable less then 50%, if you want good battery longevity.

 

[TonyK]

The more time you allow the Kill-A-Watt meter collect info the better the average. Adding new items, opening and taking stuff out can vary the daily total a bit. I wish the Kill-A-Watt meter kept a peak amperage reading. 0.9 kWH per day sounds about right for freezer. Kill-A-Watt reads Watts and VA. Watts is true power, VA is apparent power. VA * power factor (PF) is true power. in watts. Check PF reading when compressor is running. Induction motor will have less then PF of 1. If you are reading 0.90 amps then VA (apparent power) = 117.6 VA. Two peak power items to be watchful of is compressor startup surge and defroster cycle heater. They can have 3 to 8 amps of surge for short period of time. Your inverter has to be big enough to supply this surge current. To your question, for a 12v inverter, assume about a 90% conversion efficiency to 120vac. 1 kWH per day A.C. with 90% conversion would be 1 KWH/0.9 = 1.11 kWH per day from battery.If inverter input voltage average is 12.2 vdc and you consume 1.1 kWH per day then you are using 1,100 watts per day / 12.2v = 90 AH per day from 12v battery per day.If you are using a 12v lead-acid battery do not use more then 70% of its rated capacity, preferrable less then 50%, if you want good battery longevity.

Thank you. This is all very helpful. Now I know how to figure this myself. One other question that you may know the answer to. I know I'm not supposed to mix old batteries with new. But what constitutes being old? Is a 2 month old battery considered old? What about a 4 month old battery? What's the time cut off for adding a new battery to an existing bank?

 

[gnubie]

It's more about how the battery has been used (or abused). A 2 month old battery that has been treated well and hardly cycled will still be very healthy and a good match with a new one. A 2 month old battery that has been deeply over-discharged repeatedly, charged at very high rates at high (or low) temperatures and suffered other types of abuse won't be.

 

[RCinFLA]

I agree it is more about how it was used. One time total discharge can make it toast. Will never fully recover.

When I buy new batteries I try not to get date codes that are greater then four months apart. I also carry a small pocket DVM to make sure they are not below 12.5 vdc indicating they have been sitting around for a while. Fully charged will be 12.7 -12.8 vdc at rested no load open circuit voltage.

Lead acid battery need to kept charged. Two to three months sitting partial discharged will sulfate harden to crystals which will not recharge. If you don't use them put them on a low current maintainer at 13.2 vdc float.