Use of Inverters

Submitted: Thursday, Jun 19, 2014 at 21:32
ThreadID: 108322 Views:1888 Replies:2 FollowUps:4
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Trying to do favour for daughter for demonstration purposes. I have a Dick Smith 300w inverter and she wants to run 2 of( at least) 14watt heater lamps for melting fragrance waxes off it using my deep cycle battery(of course). Just wondering if anyone can give me a rough idea how long she could expect before low voltage alarm comes up on the inverter. The battery is a 110ah deep cycle. Am running a shiort test on it right now at home with just one, but all I have to go by is voltage drop. I do know that inverters use a fair amount of energy running themselves, but would like a fair idea before i say should be ok to use for at least 4 hrs no problems
Thanks for help and understanding.
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Reply By: Allan B (Member, SunCoast) - Thursday, Jun 19, 2014 at 22:55

Thursday, Jun 19, 2014 at 22:55
Kev,
The battery drain will be about 4 Amps. If you allow for a safe 48Ah drawdown of the battery then it will last 12 hours. That is assuming that the battery is in good condition and really does have 110Ah of capacity.
So you should have no trouble in achieving 4 hours use.
Cheers
Allan

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Follow Up By: kevmac....(WA) - Thursday, Jun 19, 2014 at 23:02

Thursday, Jun 19, 2014 at 23:02
didnt think would have any probs.... just wanted a second opinion other than my other personality lol
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Reply By: Green Rv - Friday, Jun 20, 2014 at 07:52

Friday, Jun 20, 2014 at 07:52
hi kevmac
would like to know how your test went
and i would like someone to correct me if i am wrong

watts divided by volts = amps

so to work this out

inverter draw
300w divided by 240v = 1.25amps

heater draw
14 x 2 = 28w
28w divided by 12v = 2.3 amps

2.3amp + 1.25amp = 3.55 amps total

then as previous reply 3.55amps x hrs = amp/hrs

if this is wrong would someone like to show the maths

thanks
AnswerID: 534663

Follow Up By: Allan B (Member, SunCoast) - Friday, Jun 20, 2014 at 08:24

Friday, Jun 20, 2014 at 08:24
The inverter 300 Watts is its maximum rating, i.e. the most that it can supply. It does not draw 300W by itself. It does draw a small amount even when not loaded but not much. It does however have some inefficiency when converting the energy from 12v to 240v, possibly as much as 80%.

Accordingly, my calculation contains that factor plus some further allowance just to be on the safe side. Experience teaches discretion.

Other than that, your "300w divided by 240v = 1.25 amps" provides the "240v" current not a "12v" current. It cannot be directly added to the heater "12v" heater draw of 2.3 amps. 300 Watts draw on a 12v battery would actually be 25 Amps.
Cheers
Allan

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Follow Up By: B1B2 - Friday, Jun 20, 2014 at 12:08

Friday, Jun 20, 2014 at 12:08
G'day Kevmac,
All very accurate calculations.
When I switch on a 150w inverter, my Caravan Prostar solar regulator shows 0.3A load with a battery voltage reading 13.7v.
Plug in a 40w incandescent lamp and it uses 3.8A. The voltage drops to 13.3v.
I tried a 12watt fluoro just for my amusement and it used 1.2A, as it would be a tricky thing to calculate.
A 300w inverter would use a bit more. It's always better to match your load to the inverter to reduce losses.
Batteries are a dark art, temperature affect your capacity substantially, and as the voltage drops the Amps will increase.

Cheers,
Bill
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Follow Up By: Allan B (Member, SunCoast) - Friday, Jun 20, 2014 at 13:32

Friday, Jun 20, 2014 at 13:32
Actually Bill, my calculations were not "very accurate". They were maybe realistic approximations and allowed for a less-than-perfect situation.

Your 0.3A measurement of the inverter quiescent current is typical for a 150W inverter and represents about 4 Watts which is not too bad and varies according to their design and build quality. Larger rated inverters would have proportionately higher quiescent current.

Your measurements of the 40W incandescent lamp reveal an inverter efficiency of 79%, typical of a 150W inverter. Larger sizes of good quality can perform with higher efficiency.
Cheers
Allan

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