This three-part series addresses powering freezers and refrigerators off batteries using a power inverter. The first part gave an overview of the topic; the second part dealt with inverter sizing and gave you a number of design guidelines for the required inverter power output.
In this third part article, I give you a more detailed answer to the question: how to size the battery pack for powering freezers and fridges.
To give you a good answer, I did some market research and compiled a database of around 100 new freezer and refrigerator models. I studied the energy use numbers, inverter efficiency and usable battery capacity. All of which, with a bit of math, gave me the results needed.
Summary: Freezers and fridges run 3 to 10 hours on a 60Ah car battery, 6 to 20 hours on a 100Ah leisure battery, and take 150Ah to 400Ah of deep-cycle battery capacity for a day of runtime. Chest freezers and freezerless refrigerators are most efficient and use the least battery capacity, while large side-by-side and French-door refrigerator-freezers use the most.
We will start with battery pack size and runtime tables for freezers and fridges of different type and size. Then, I will give you formulas to calculate the pack size and runtime, which you can use if you know the energy use [kWh] number for the freezer or fridge you are planning to use.
What size battery pack?
The table below gives you the battery pack capacity in amp-hours (Ah) that you need to reserve to power different size and type freezers and fridges for one day (24h). I calculated the pack sizes assuming 12V AGM or flooded lead-acid batteries and 2021 level energy efficiency for the devices.
|BATTERY PACK SIZE|
FOR 24H RUNTIME
|Assumed: 50% DoD,|
90% inv. eff.,
typ. energy rating 2021
The table shows that freezers and refrigerators need anywhere from 150Ah to 400Ah of battery capacity for each day of autonomous runtime – this means runtime between recharges.
How long will it run?
The table below shows you how many hours will different size and type freezers and refrigerators run on 100Ah of 12V flooded lead-acid or AGM battery capacity (nominal). This is basically the same data as in the first table, but put the other way around.
|RUNTIME PER 100AH |
of 12V battery capacity
|Assumed: 50% DoD,|
90% inv. eff.,
typ. energy rating 2021
The table shows that 100Ah of 12V battery capacity – i.e. a typical small leisure&marine battery – will run a freezer or refrigerator for 6 to 20 hours. Large fridge-freezers will run below 10 h and freezerless refrigerators and small-medium chest freezers closer to 20 h.
I compiled the tables using typical energy efficiencies for new appliances in each class, 2021 level. If you have an older or particularly inefficient fridge or freezer, you may have to adjust the amp-hours upwards.
The amp-hours in the tables refer to nominal battery capacity and assume 12V flooded lead-acid or AGM batteries, which are the two common battery types for off-grid inverter power systems. Further, I assumed you get 50% of the nominal capacity out of the battery, which is OK for deep-cycle AGM or flooded lead-acid batteries in good condition. But note that this would be too taxing for regular automotive “starter” batteries or worn deep-cycle batteries.
Note also that high ambient temperature, frequent door opening or lots of warm/unfrozen food input will all increase energy consumption from the Energy Guide numbers, and require more amp-hours.
To verify my assumptions as well as the energy use numbers reported by manufacturers, I also did actual runtime tests in a few setups.
Below you see a graph of one of the tests, where I powered a medium-sized chest freezer with a 12V, 100Ah deep-cycle AGM battery. Plots of battery voltage and current draw of the inverter display a typical freezer compressor power cycling. The open-circuit voltage drops steadily from 12.8V (100% charge) to 11.75V (20% charge), at which point I disconnected the freezer.
The test confirmed the manufacturer energy use numbers and my assumptions. Although the measured runtime of 22h is longer than predicted in the table for the 11 cu-ft freezer, the explanation is the very deep 80% discharge I used in the test. The table assumes 50% discharge, which about the maximum recommended in normal cycle use. Had I stopped at 50%, I would have ended quite close to the 16 hours indicated in the table.
What the numbers show
The practical conclusion we can draw from the tables is that a single 12V, 100Ah deep-cycle leisure battery will easily power a reasonably sized freezer or fridge through an afternoon of tailgating or an RV/boat day trip.
But substantially more than 100Ah is needed at off-grid properties: to keep a fridge or freezer running through a few cloudy/calm days, battery packs from 400Ah up to 1500Ah are needed.
The average automotive lead-acid battery is smaller at 50Ah to 75Ah and has “starter” characteristics; it is designed for maximal cranking amps, not max amp-hours out. With a car battery, you can count on getting only 1/3 of the runtimes in the second table without damaging the battery.
Difference in appliances: The most energy-efficient cooling appliances – chest freezers and freezerless fridges – will take only 150Ah to 200Ah per day. The biggest energy hogs are side-by-side and French door refrigerator-freezers, requiring from 250Ah to 400Ah of battery capacity each day. Upright freezers and refrigerators with a top or bottom freezer stand somewhere in the middle.
How to calculate battery pack size?
The battery sizing and runtime tables above are good for a ballpark estimate, but they assume a typical energy efficiency for the freezer or fridge and a good-condition deep-cycle battery.
If you know the freezer or fridge specs, you can get a more accurate battery size estimate by calculating it yourself with the following formula:
Battery Ah = Runtime [h] * Energy use [kWh/a] / 50
For example, the battery capacity needed to get a 100h (~4d) runtime for an energy-efficient 250 kWh/a chest freezer is 100 x 250 / 50 = 500Ah. Note these are again the nominal amp-hours that read on the side of the battery, not the actual consumed Ah.
I derived this formula from a more detailed one by making a few reasonable assumptions:
- 12V AGM or flooded lead-acid batteries
- Depth of Discharge 50%
- Inverter efficiency 90%
- Typical use profile (ambient temperature, openings, etc)
How to calculate runtime?
As with the battery size, you can get more accurate runtime estimates than those in the table by calculating them yourself. You can do this by using the battery nominal amp-hours and freezer/fridge energy use.
The formula is essentially the one above, but flipped the other way around:
Runtime [h] = 50 x Battery Ah / Energy use [kWh/a]
E.g. a 200Ah 12V leisure battery will run a 500 kWh/a refrigerator-freezer for 50 x 200 / 500 = 20 hours.
This formula rests on the same assumptions as the battery size formula: 12V deep-cycle AGM or flooded lead-acid batteries, 50% discharge, 90% inverter efficiency and typical appliance use profile.
In this article, we learned how to dimension the battery capacity for running your cold storage – the battery size and runtime tables gave us a rough idea for different freezers and refrigerators, and the formulas provided a means to calculate these more accurately based on the fridge or freezer specs.
Looking at the numbers, we concluded that a single car or leisure battery may just provide the cold for a day trip, but will not get you much further than that. Powering fridges and freezers on an off-grid property for days requires massive battery packs from several hundred to a few thousand amp-hours.
This should not be too surprising: refrigerators and freezers are always on and one of the biggest household electricity consumers, and take no less off the grid.
Q & A
Before we go, let us apply rules presented in this article by answering a few common questions on battery sizing for freezers and refrigerators:
How long will a deep cycle battery power a mini fridge?
Most mini fridges will run a bit under 20 hours on a typical 100Ah, 12V deep cycle leisure/marine battery.
Mini fridges have capacities of only 1 to 5 cu.ft, but are typically not very energy efficient: at around 275 kWh on average, their annual electricity use is at the level of much larger (freezerless) fridges. This yearly number translates to a mean running power of around 30W.
A 12V deep cycle battery (AGM or flooded lead acid) with 100Ah nominal capacity can give out around 50 Ah or 600 Wh in normal cycle use (50% depth of discharge). With an inverter efficiency of 90%, this is enough for around 17h of the 30W mini fridge load.
How many batteries does it take to run a freezer or a refrigerator?
Any freezer or fridge can be run off a single 12V car battery at least for some time, so in principle one battery is enough.
However, reaching a practical 20+-hour runtime for a freezer or a fridge requires a battery pack with multiple batteries. A battery pack for a three-day runtime, for example, could consist of 4 to 20 individual 12V, 100Ah deep-cycle batteries. The pack amp-hours should be chosen based on the appliance energy use and the desired runtime, see table and formulas above.
What size battery to run a fridge?
A fridge can be run on a single 12V battery with anywhere from 50Ah to 200Ah of nominal capacity.
However, the battery should be sized to give a certain runtime which depends on your application: it may be 6h for an afternoon’s trip, but 72h at an off-grid cabin. In the former case, a single 50Ah battery may be enough, but the latter requires a pack of multiple 100Ah or 200Ah batteries. See tables and formulas above for sizing guidance.
How long will a 100Ah battery run a fridge?
A 100Ah deep-cycle battery will typically power a refrigerator for anywhere from 6 to 17 hours before needing recharging. The longest runtimes are for freezerless fridges and the shortest for large side-by-side and French-door refrigerator-freezers.
These runtimes can be extended by around 30% if the battery is run completely flat, but this will damage the battery and is not recommended. For more information, see the tables and formulas above.