How Long Will A 100Ah Battery Run A Fridge?
How do you work out how long a 100Ah battery will run a fridge?
In this post I look at the question: How Long will a 100 amp-hour battery run a fridge?
Like many complex questions, the answer depends on these smaller questions:
- How much energy does the fridge consume?
- What kind of battery is used, regular or deep-cycle?
- What battery technology, lead-acid or lithium?
A 100Ah lead-acid deep-cycle battery will run a fridge using 630kWh/year for 13.3 hours. 80% discharge is assumed, but at the recommended DoD of 50% the same 100Ah battery will run the fridge for 8.3 hours. A lithium iron phosphate 100Ah battery will run the fridge for 15.8 hours at 95% discharge.
Fridges are a special case. Like heat-pumps, freezers and air-conditioners, they have compressors with motors which start and stop.
When an electrical motor starts up it draws a surge current from the supply 3 times greater than normal running current. At other times the motor is not running, and sometimes it runs.
This means the power needed by a fridge changes over time, unlike a purely resistive load. For this reason it’s difficult to say exactly what power a fridge takes.
The easiest way to work out how long a battery will run a fridge is to calculate the fridge energy required over time in kilowatt-hours (kWh).
How many watts does a refrigerator use?
We need to differentiate between watts and watt-hours. Fridges are rated in watts according to their size and may range from 200W to 1000W.
A typical fridge may be rated at about 400 watts while a fridge freezer about 6oo watts, but what does this tell us? A far more useful figure to work with is the fridge annual energy consumption, either per year, month, day, or hour.
When calculating battery usage it’s more convenient to talk in terms of hours – how to find the hourly energy usage? Easy – from the fridge label:
First step is to find out how much energy a fridge uses a year. Courtesy: Penn State, e-education Institute
Discover your solar saving potential
I’m going to use a fridge annual energy consumption of 630kWh from the above label to work how long a 100Ah battery would run it.
How much electricity does an old fridge use?
In my own tests I found that old fridges can take up to twice as much power as a new one. See fridge tests here.
All moving parts wear over time and compressors are no exception. Also, electrical components become less efficient and degrade over time. Another important part of the efficiency equation is Power Factor.
Power Factor is the ratio between volts and current in an AC circuit. Fridge PF gets worse for older models.
Power factor is a ratio expressed as a decimal less than 1. The lower it is, then the more current has to be supplied for the same power.
Older fridges have lower power factors which adds to overall wear to push up the energy consumption.
How many amp hours do I need to run a fridge?
An conventional AC fridge needs an inverter to run from a battery. Inverters also have losses that need to be supplied with power, but for this post I’m not taking this into account.
Inverter efficiency can vary hugely and depends on it’s loading. If fully loaded at it’s rated capacity the efficiency could be 95%.
A fridge load will go up and down as the compressor cycles, so at times the inverter efficiency may average 60 to 70%.
When sizing a battery to run a fridge it’s best to add 25% capacity to account for the above.
I prefer to convert battery amp-hours to watt-hours. That way we can compare like with like, energy consumed in watt-hours vs battery capacity in watt-hours.
Before doing this, I need to talk about battery types.
What is the difference between deep cycle battery and car battery?
A regular 12 volt car battery have an average capacity of 50Ah. They will run a fridge but I don’t recommend it.
Car batteries are designed to deliver hundreds of amps quickly to crank the engine. After the engine starts, the alternator immediately works hard at charging it back up up.
A car battery rarely gets discharged more than 10% and they shouldn’t be left in a deep state of discharge.
How do deep cycle batteries work?
The internal plates are fabricated in a different way in deep-cycle batteries, allowing them to deliver medium currents for hours at a time.
The deep-cycle batteries used for RVs are often called ‘leisure‘ batteries, and although they can deliver quite hefty currents, they can’t deliver several hundred amps quickly.
Deep-cycle batteries can be discharged up to 80% but manufacturers recommend a maximum of 50% to maintain long life.
Is there a difference between deep cycle and marine battery?
A marine battery offers the best of both worlds – cranking power for starting boat engines, and the ability for delivering sustained currents for lights and pumps when the engines aren’t running.
I use a Varta 90Ah marine battery with an inverter for an emergency backup power supply at my home.
How Long Will A 100Ah Lead-Acid Deep-Cycle Battery Run A Fridge?
How Long Will A 100Ah Lead-Acid Battery Run A Fridge
We have all we need to answer the question now.
- Average fridge energy consumption = 630kWh/year = 1.73kWh/day = 72 watts/hour
- Battery capacity available = 80% x 100Ah = 80Ah
- Battery capacity available in watt-hours = 80Ah x 12 volts = 960 watts
- Hours of fridge run time =960/72 = 13.33 hours
Table – How long will a 100Ah battery run a fridge with energy consumption 200kWh/yr to 1000kWh/yr?
|
|
How long will a 12V 100Ah run fridges – annual energy consumption 200kWh to 1000kWh |
|
|
Annual Fridge Energy Consumption (kWh) |
Lead-acid deep-cycle (discharge to 80% of capacity) – hours run time |
Lithium iron phosphate (discharge to 95%) – hours run time |
|
200kWh/yr |
42 hours |
50 hours |
|
300 |
28 |
33.3 |
|
400 |
21 |
25 |
|
500 |
17 |
20 |
|
600 |
14.5 |
17.2 |
|
700 |
12 |
14.3 |
|
800 |
10.5 |
12.5 |
|
900 |
9.4 |
11.2 |
|
1000 |
8.4 |
10 |
Lithium iron phosphate battery vs lead acid battery
IMO lithium iron phosphate are far superior to lead-acid batteries. They are more expensive but this is more than compensated for by the longer life.
LiFeP04 batteries:
- are inherently deep-cycle – up to 95% (100% in some cases)
- weight just 30% of similar capacity lead-acid battery
- more expensive than standard deep-cycle lead-acid
- charge/discharge cycle life – 2000 (if discharge limited to 80%)
Calculator – Enter your fridge annual energy consumption to see run time with 100Ah battery
How Long Will A 100Ah Battery Run A mini Fridge?
A mini fridge consumes on average 250kWh per year. If we consider using a 100Ah deep-cycle lead-acid battery then the calculation is:
250kWh/year = 28.5 watts per hour
Battery capacity available in watt-hours = 80Ah (80% discharge) x 12 volts = 960 watts
Mini fridge run time = 960 watts battery capacity / 28.5 fridge watts/hr = 33.7 hours
How Long Will A 100Ah Battery Run A TV?
How long can you run a TV on a deep cycle battery?
A 50 inch LED TV takes about a constant 100 watts, so the run time calculation is:
Battery capacity available in watt-hours = 80Ah (80% discharge) x 12 volts = 960 watts
50 LED TV run time = 960 watts battery capacity / 100 TV watts/hr = 9.6 hours
How Long Will A 100Ah Battery Run A Laptop?
The average laptop uses about 50 watts of power. The calculation for run time using a 100Ah battery is:
Battery capacity available in watt-hours = 80Ah (80% discharge) x 12 volts = 960 watts
Laptop run time = 960 watts battery capacity / 50 laptop watts/hr = 19.2 hours
How Long Will A 100Ah Battery Run A Computer?
How long can a 100ah battery run a computer?
Desktop computers power consumption varies depending on the peripherals in use and how hard the CPU is working. In general the average desktop computer draws about 180 watts.
Run time calculation:
Battery capacity available in watt-hours = 80Ah (80% discharge) x 12 volts = 960 watts
Desktop computer run time = 960 watts battery capacity / 180 computer watts/hr = 5.33 hours
