Inverter Formulas

Frankiep

Guest
I've had several additions into my Motorhome .Among them a Sterling 1800w Inverter. So what info i'm after is, information put as simply as possible the formula to work out for E.G. Using the inverter, how long approx should i get ,let us say from a 500watt heater,or any other appliances when i know the wattage of it.It is fitted with 2x100ah Leisure Batterys It does have a 120 watt VeeTech Solar panel on top ,regulator etc, but i'm not looking for any calculations relating to that. Using the inverter, and im plugged into a mains socket in vehicle, i then presume that 230v or 240v will be used in the calculating formula. Grateful for help.
 
Simple Amps = watts divided by volts

Thus 500 watt heater 500 / 12 = 41 amps

2 x 100 AHR batteries to 50% discharge is 2 hours !

As the inverter also has a loss the time will be slightly shorter

If you use 1800 watts 1800 / 12 = 150 amps hope you have thick cables :scared:
 
500(W)/12(V) = Battery Draw(A) = 41.66 A

So just divide your wattage by 12 to get the battery side current.(240v is not used in the formula because that gives your appliance side current - you are only interested in what is being taken out of the battery)

Add 5% for inverter losses

41.66 x 1.05 = 43.75 A

You have 200Ah of capacity of which you should only use about 100.

So you can use your current of 43.75A for approx 100/43.75 = 2.28 hours

In practice your batteries may be in less than perfect condition eg 80% and they won't be optimally charged say 80% so you may only get 0.8x0.8 of this time ie 0.64 x 2.28 = 1.46 hours.

Dividing by 12 to get the current and comparing it to your amp hour capacity is only a guide because your amp hour capacity assumes new batteries perfectly charged which is very rarely the case. 500W or 44A is probably too much to take out of 2 leisure batteries on a regular basis without really caning them and reducing their life. For a 1800w inverter, you should have at least 4 leisure batteries of 110Ah to sensibly use any appliance approaching 1000w.
 
Also, if you have hammered the batteries, unless you have installed really heavy cable from your starter batt via a relay to your leisure batteries, the resistance of the wire will reduce the charge into the leisure batts and it could take quite a long journey to recharge them or loads of sunshine. Certainly not what you get when you need a heater.

I ONLY use electric heating on the odd occasion I use hookup.

Gas heating or hot water bottles when wilding.
 
Leisure batteries do not like being used for large current drops. If you want that kind of current draw then think of changing the LB for either starter batteries or marine use dual starter/leisure ones. I used a 1000W invertor to power a 700w microwave in my embryonic days of campervanning and regretted abusing the LB to that extent - it soon failed!

As for inverter inefficiencies - I think 5% is being fairly conservative - a 1000W inverter JUST powered a 700W microwave through losses.
 
A "700W" microwave uses far more than 700W. There is a big start up surge and also the continuous power draw is more like 1000W to 1200W depending on model. They only say 700W because that is the cooking power so your struggles to power it in that respect, were not to do with inverter inefficiency.

Inverters typically lose 5 to 10% but I said 5% in this example as it was a Stirling inverter and those things are top of the range and pretty well constructed.
 
A 700w microwave is the power output of the microwave magnetron.

My home one is 1490w input, 900w out = 60% efficient the rest is the heat you feel from the top vents when it's on.

So a 700w device with the same efficiency would be about 1170w input, allow some for the inverter inefficiency call it about 1300w from your batts. About 108 Amps!!

The inverter probably dropped it's voltage so that the microwave takes longer, still hammers the batts.

I use starter type batts to run my inverter. I've got 310 Ah in 3 batts but could do with at least one other.
 
Thanks for that firefox - I was just letting others know my experiences. I'll go with your between 5 and 10% rule :)
 
Our inverter has just popped its cloggs. We think it died because of the cold we experienced in France and Italy this year.

We have been looking at replacements and was surprised by some figures on the Waeco website.

A 2000 watt modified signwave inverter cost about £400 and had a consumption without load of <1.5 amps
The 2000 watt pure sign wave inverter costs about £2100 and had a consumption without load 0f <0.6 amps

I thought we has a 1600 watt pure signwave but on enquires to our supplier it turns out it is a modified sign wave type.

We have used this to run computers and other electrical equipment for 5 years now with no problems encountered untill we tried to run it at minus 10c one morning to watch TV on a Sky box on an Aire in France. It worked fine for three quarters of an hour, but then would not turn off. I checked it and could smell burning but by the time I was able to disconnect it it popped and smoke came out of it. It was not hot or overloaded but as soon as it popped the overload busser sounded for a short time.

Our leisure batteries were also behaving as if they were finished in these temperatures but on return to the uk and having them checked they are fine.

John
 
Good answers

I'd been given the impression that Leisure Batterys were the be all & end all for habitation needs & so thought that Inverters followed suit. Iv'e been over optomistic in my expectations of batterys and inverter . Although I know i did plunge in impetuously when i got them & didn't look into it beforehand instead of after ( how many times have i said that to myself) Amazing & interesting the the things you can learn off other people, never too old too learn as they say. I do have a 500watt Delonghi Bambino oil filled heater & at the moment if i wished to use it at home from plug in from mains on the frostguard setting i could. On the other hand it does have 6 settings so i'm presuming that if it was at some point used on one of the low settings ( frostguard or 1 if it was really freezing) through the inverter it shouldn't hammer batterys too much, with it having a thermostat (no timer). I can see that what firefox says having 4x110ah batterys would be better, i can't see there being the room . Although my 2x100ah Elecsolls are either side of the spare wheel & the vehicle is 24ft long , theyr'e located from outside at the rear. From the range of appliances that can be used (obviously with me not realising the demands on the amperage) i sometimes wonder was it worth bothering with an inverter. But ,here we are. Thanks to all for your answers ,iv'e got enough to keep me on the straight & narrow, unless i get too ambitous:juggle:
 
No probs! The only reason I have a 1000W inverter is to run things like electric drills or 200-300W items. I have very occasionally used a fan heater or grill on 700w setting for a short period of time say 10 mins. But I only have 2x110A batteries so it is not to be particularly recommended. Still it does give you that flexibility the odd time, but gas for cooking or heating is the way to go.
 
Frankiep said:
I'd been given the impression that Leisure Batterys were the be all & end all for habitation needs & so thought that Inverters followed suit. Iv'e been over optomistic in my expectations of batterys and inverter . Although I know i did plunge in impetuously when i got them & didn't look into it beforehand instead of after ( how many times have i said that to myself) Amazing & interesting the the things you can learn off other people, never too old too learn as they say. I do have a 500watt Delonghi Bambino oil filled heater & at the moment if i wished to use it at home from plug in from mains on the frostguard setting i could. On the other hand it does have 6 settings so i'm presuming that if it was at some point used on one of the low settings ( frostguard or 1 if it was really freezing) through the inverter it shouldn't hammer batterys too much, with it having a thermostat (no timer). I can see that what firefox says having 4x110ah batterys would be better, i can't see there being the room . Although my 2x100ah Elecsolls are either side of the spare wheel & the vehicle is 24ft long , theyr'e located from outside at the rear. From the range of appliances that can be used (obviously with me not realising the demands on the amperage) i sometimes wonder was it worth bothering with an inverter. But ,here we are. Thanks to all for your answers ,iv'e got enough to keep me on the straight & narrow, unless i get too ambitous:juggle:
Click to expand...

There are never any issues with the use of inverters, but to what means they are used for is the most important.
Sustaining equiment such as heaters is daft, due the the high end of consumption they take, so running such appliances over long periods just isn't realistic. However short stabs from items such as Microwaves and hair dryers are fine, but not without it's own problems.
The most important thing is batteries, batteries and um batteries. when battery banking, the general rule is to keep them as close together as poss and using a sensible diameter cable to link. The ideal situation is to avoid battery banking and have one large capacity battery, removing any resistance and power is direct from one battery source. A very important factor is to make sure the Inverter is as close to the batteries as you can get!!! It doesn't matter that the output (AC) is long, but the battery to inverter connection should have the least amount of resistance to maintain performance. You then move on-to how you replace/sustain the energy you have consumed. This varies with personal preference (Solar/upgraded split charging etc), but the importance is that you have that ability to put back quickly what you have taken out. Standard split charging forms do not cut it, as generally Motorhome manufacturers use small cable, coupled to a cheep automotive relay. With Solar, you only have to ask yourself when you use the van and where, as mother nature does have a hait of letting us down when we need her. If you use a Genny, why have you got an inverter????
Firefox is on the money when it comes to inveter deficency (5-10%), but that's a good quality product. Budget lines can be as much as 20% in terms the the energy consumption on top of the appliance you are trying to use.
The way I look at power in the motorhome, is like looking at a pyramid, foundation level is always your leisure batteries, second tier is power use, tier 3 is replacing the power, tier 4 is monitoring the power.

I have taken a while in perfecting my system to suit my family's use, but I have had no issues with using inverters and will continue to whilst the boss requires the use of hair plug-ins. I just wish she would have a Sinaed O'Connor cut, save all the hassle:p
 
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