Running a fridge of an invertor

ZootHornRollo

Good morning,

I was hoping someone with some electrical knowledge might be able to help me.

I have a mobile catering unit that I would like to leave on site however I would need to leave the fridge and freezer working overnight. There is no electrical hook up and I run a 4kw generator each day, running for around 9 hours per day.

I am running a 250w fridge, small domestic freezer, a strip light and a waterpump off the generator.

I am trying to work out a way to leave the fridge and freezer running over night and wondered if a leisure battery (or two) with an invertor might be the answer? And if so, how big a battery and invertor would be required? Would there be enough time to charge the batteries whilst the generator is running?

Thanks in advance.

G.

DoaM

1. You need to work out the total power usage required by all the electrical loads. (You say a 250W fridge, what power ratings are the other devices?)

2. You'll need an invertor that can supply the total requirement plus, say, 25% overhead.

3. You'll need to see the efficiency rating of the invertor (e.g. 80%) and then choose a battery supply that can provide for the total invertor load.

Some example calculations:

1. Total load (of fridge, freezer, water pump etc.) is 1000W
2. Total invertor output required is 1,250W (25% overhead)
3. Efficiency is 80% so invertor supply rating is 1,250/0.8 = 1,562W, so let's say 1600W (1.6KW)

Therefore, in theory, an 8KWh battery supply would supply the invertor (and thus the loads) for 5 hours.

This is very rough-and-ready, just to give an idea of the considerations.

kwikbreaks

Couple of points to add. Fridges and freezers don't run continuously so although you need to size the inverter for peak the battery pack needn't be.

I'd add considerably more than 25% for the inverter rating because start up power required will exceed running load.

Batteries store pathetically small amounts of power. A typical modest sized leisure battery may be 100amp hour so at 12v that's 1.2kwh or just 75w per hour over 16hours. So I'd guess you would need two or three of those at least.

I have a plug in meter which I got years back from a Maplin bargain bin that you can put in line with fridges and the like. If you don't want to make big and expensive errors try to see if there are such things still on sale.

Ballpark. Fridge and freezer will be maybe 20% duty cycle so maybe a couple of 100ah leisure batteries would do and even if they ran out stuff wouldn't thaw in an hour or two.

It's all down to calculations really.

psychic_teabag

I wonder if the freezer can take care of itself overnight ..? If it's set to its coldest setting, and all space is filled with freezer blocks, then how much does the temperature actually rise overnight ?

You could possibly manage the peak power by having a timeswitch which explicitly offers to power to the fridge and freezer alternately. Perhaps switching back and forth every half hour or so. So each device might have to wait up to half an hour between requesting cooling actually actually getting it, but as noted, since they have a relatively short duty cycle, they should on average receive sufficient power to keep the contents cool. So as long as they are set to relatively cold settings, it won't matter if they rise very slightly above their upper threshold before cooling resumes.

Frozen_up_north

Beware that lead acid batteries have a quite short life if discharged below about 50% of their rated capacity, your 100Ah batteries in reality should be considered to be 50Ah.

Ordinary “caravan shop” leisure batteries are poor compared to absorbed glass mat (AGM) types. Be prepared to spend quite a lot on batteries such as the Varta Professional AGM types.

Sorry to put a dampener on your ideas, but it looks like a very costly solution compared to a mains hookup.

ZootHornRollo

Thanks for the replies guys.

I'm really just testing feasibility at this stage.
I am moving further away from my pitch which makes towing back n forth an expensive task so it is an investment that won't take long to recoup.
I have no option for an electric hook up at all.

I wouldn't I tend I. Running the light or water pump with the battery but no harm including them as it would only add extra headroom.

The light is 65w, the waterpump 48w and the freezer 200w

So... That's a total of 563w.
When buying generators I was always told fridges/freezers use 3 times the power so that will be 1463w peek.
So a 2.5kw investor would give plenty headroom?
I can get a varta agm 230 ah battery. Do we think that's enough?

Invertor £200
Battery £250
Plus decent charger... £150
Changeover switch £30

Speaking of charging... How long would it take to charge the above battery with a 15amp charger?

ZootHornRollo

The alternative is a local storage yard and £100 per month plus electric or towikg back n forth at around an extra £50 per week in fuel.
... So it doesn't seem that expensive ��

Heedtheadvice

I would just add so far that the motor start times are quite difficult for inverters to handle. You should allow for it to supply a peak current several times the nominal running loads.



Additionally rather than a modified sine wave (crude) inverter go for a pure sine wave inverter that better supports motor starts. They are more expensive but will also support any low power electronic devices.


Do not skimp on quality with low cost items, consider cost of food lost as well as value of business lost in case of failure!


You can get specific answers once you come back with specific data (already requested) on the loads.

ZootHornRollo

I'll take your advice re. pure sine.

That would take the invertor cost up to around £400

ZootHornRollo

Appliance loads are in post above

Gloomendoom

A gas fridge and freezer might be a solution. Mind you, they aren't cheap.