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Using Generator and Inverter together?

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I'm planning to go off-grid for the first time, I think I have the refillable gas sorted for fridge and heating and thanks to the forum search function and have learned a lot about solar. I would initially get a 100w flexible panel and a 20a (MPPT not PWM) controller in case I need to add a second panel coupled to a 110ah battery that would be connected in parallel to the caravan battery which is the same size. I will change all the 12v and 240v bulbs to LED.  I will boil water and make toast using gas but would still like to be able to use the 800w microwave (max 8 mins) and Mrs Limecc the 2kw hairdryer (max 4 mins on 1kw heat setting).

I'm unsure whether to get a 1.5kw or 2kw pure sine inverter but because of quiescent losses it will be normally turned off when not in use. I have a Hyundai 3kw and Clarke 700w suitcase generator, the latter is quite compact and might be useful to prevent excessive battery drain when using the high power devices.

 

1. If the inverter and generator outputs are connected together to the caravan mains input, will they work together ok or will they be in competition regarding the voltage regulation?

2. Regarding my Fleetwood PDU2 caravan charger/distribution unit, I understand I need to turn it off to prevent losses from it trying to charge the onboard battery, but if I do this won't I lose the 12v to the Alde heating and lights?

 

Many thanks for your thoughts.

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1) Definately not, DONT DO THIS

 

2) Switch off the charger whilst using the inverter, not the master switch, maybe trip the circuit breaker for the charger only or have a seperate switch installed. This can be made automatic with a simple relay with changeover contacts, the charger would only be powered from EHU, when on inverter it's off. Ive done this on my Bailey and it works flawlessly.

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you cannot connect the output of an inverter or generator together. Because they produce am AC waveform there has to be someway of synchronising them.

 

It might be possible to connect two inverters or two generators in parallel but they'd have to be designed specifically for the purpose and they'd have an additional sync cable to ensure synchronicity.

 

An 800w microwave over will require a power supply capable of delivering around 1200-1400w to cope with the starting current. Even when used on a low power setting the unit switches on & off at full power but for shorter periods  so although the average power is less the peak power requirement is still the same. The hair dryer will take 1000w, so your small generator will be inadequate to power either of them.

 

an inverter producing 1000w will be about 85-90% efficient so will require 1100-1200 watts from the battery = 1200/12 = 100amps. which will require short stout cables connected directly to the battery to avoid voltage drop. You should be able get away with a 1500w inverter to power your microwave

 

It's never a good idea to connect an inverter to the caravan EHU inlet, use it as a separate device plugging your loads directly into the inverter itself.

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12 volt microwave and a haircut - sorted!

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1. It is possible to connect the output of generator and inverter together IF the inverter is designed for it. Most are not. Probably assume that the generator has a pure sine output.

Suggest you Google peukert and read the battery specs to see what effect this high draw has on battery capacity and how much energy you need to put back in to your batteries. Leisure batteries are usually rated at a C/20 discharge rate.

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A lower powered caravan microwave would only draw six amps.  This would use about 360 Watt-hours over fifteen minutes.  Or about 30 Amp-hours from the inverter.  Due to losses in the inverter this might equate to 36 Amp-hours from the battery.  That's a big chunk out of your battery's capacity!  A 1kw hair dryer run for four minutes will use 67 Watt-hours and that's 5.6 Amp-hours from the inverter or about 7 amp-hours from the battery.  Leisure batteries should not be taken below 50% if you want any life out of them.  So it's doable for an overnight, but not for a week!  a 100W solar panel in 12 hours of sunlight has a theoretical output of 1200 Watt-hours or 100 Amp-hours but you'd be lucky to get half that.  Even less if it's on the roof because it won't be pointed at the sun all day.

 

Where do you plan to go?

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This is a useful guide to what sort of power you'll get from a 100w solar panel - 

 

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The problem with running inverters is you are limited by how much is going back into the battery . Ideally you need about 600 w solar and 4 x 110 a batteries . Generators are expensive to buy need a supply of petrol which will stink the car or caravan out been there myself . The cost starts to out weigh using hook ups .

 

 

I have watched a number of installing different systems for wild camping and this one last night .

 

 

 

 

 

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32 minutes ago, matelodave said:

This is a useful guide to what sort of power you'll get from a 100w solar panel - 

Excellent video Dave!  I'm glad my solar is portable and can be pointed at the Sun.  It's only 60W but keeps the battery topped up when parked at home. i've set it to go up to 14.7V but it never gets above 14.  I'm 58 deg North!

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3 hours ago, kelper said:

A lower powered caravan microwave would only draw six amps.  This would use about 360 Watt-hours over fifteen minutes.  Or about 30 Amp-hours from the inverter.  Due to losses in the inverter this might equate to 36 Amp-hours from the battery.  That's a big chunk out of your battery's capacity!  A 1kw hair dryer run for four minutes will use 67 Watt-hours and that's 5.6 Amp-hours from the inverter or about 7 amp-hours from the battery.  Leisure batteries should not be taken below 50% if you want any life out of them.  So it's doable for an overnight, but not for a week!  a 100W solar panel in 12 hours of sunlight has a theoretical output of 1200 Watt-hours or 100 Amp-hours but you'd be lucky to get half that.  Even less if it's on the roof because it won't be pointed at the sun all day.

 

Where do you plan to go?

Have you calculated the reduction in voltage when running from 12V?

 

According to my calculations 6A x 230V gives a consumption of 1380W.

Assuming the the inverter is 100% efficient (none are) 1380W ÷ 12V would draw 115A.

That's a heck of a current draw, there's no way the proposed solar panel will make much difference while that's in use.

You'll basically kill the battery in one cooking session. 

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Posted (edited)
16 minutes ago, bspks said:

Have you calculated the reduction in voltage when running from 12V?

I don't understand your question? 

 

Mains voltage is 240 in the UK.

Edited by kelper

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We have been off-grid a few times so can do without the microwave or hairdryer. Never been a problem for us so I wonder how long you intend your off-grid stay?

Maybe it is long term?

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We have 200w of roof mounted solar, a 1500w (peak 3000w) inverter and a quality 115Ah battery.

I had hoped Mrs Bubble could use a 1000w hairdryer.  If we had a sunny day and she had short hair all would be well, we get about 15mins of hairdryer before the battery is down to about 50%. Unfortunately her hair is too long and wont dry in 15min.

The inverter is good when off grid for charging the battery, the TV, charging phones, the vacuum and providing its sunny getting Mrs B's hair 50% dry.

The caravan 'systems' run at about 0.8A even when nothing is turned on, so thats 19A per 24 hrs to put back in.

 

 

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I'd be looking for that 0.8A parasitic load!

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Posted (edited)

.

Edited by AJGalaxy2012

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Have a look here.

https://www.camperlands.co.uk/camping-inverters-leisure-batteries/?current=5&power_submit=Get+Rating

 

I think the calculations in the link are slightly different from other postings here because I don't think Ohms law applies to AC circuits.

 

I'm assuming the inverter is driven from the 12v caravan battery to power your AC devices. You will need the inverter fed 240v circuits isolated from the standard van circuit if you're also hooking up to 240v genny. 

 

There should be no reason why you can't be charging the battery at the same time from the genny but it won't keep up.

 

Although, if you have say a 1500VA generator, do you need an inverter at all. Just fire up the genny and run the appliances from the 240v van AC.

 

Info provided 'as-is'. DYOR.

 

Also when you run the genny, work out if you need an earth rod to ensure the RCD protection works flawlessly.

 

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Posted (edited)
1 hour ago, PandR said:

Also when you run the genny, work out if you need an earth rod to ensure the RCD protection works flawlessly.

Good advice!  If the caravan chassis became live it would not affect anyone inside the caravan but it would be hazard to anyone touching it from outside.  If not connected to the mains but with a generator powering the van the generator should be connected to an earth rod and so should the caravan. because it's sat on rubber tyres.  It might depend on how the steadies were attached.  OR the generator should be isolated from the ground.  With plastic plumbing much of the caravan metal work won't be earthed. 

 

This thread prompted me to check my caravan which is on house mains supply.  The gas hob and parts of the fridge are connected to the same earth as the 13A sockets.  The gas pipework is also fully earthed to the EHU.  But the chassis is not. 

Edited by kelper

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Ohm law does apply, so do other factors. The fact that they've decided to do the calcs at 10v rather than 12 is to take into account inverter losses

 

What they haven't taken into account is Puerkerts Law which allows you to calculate the loss of capacity when you drag more current from a battery. Most batteries are rated at the c/20 rate which means that a 100a/h battery will last 20 hours at 5amps. However if you pull 20amps out it wont last 5 hours but quite a bit less. Make it 50amps or more and the capacity will be even less - use this calculator to work it out http://www.csgnetwork.com/batterylifecalc.html

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2 minutes ago, kelper said:

Good advice!  If the caravan chassis became live it would not affect anyone inside the caravan but it would be hazard to anyone touching it from outside.  If not connected to the mains but with a generator powering the van the generator should be connected to an earth rod and so should the caravan. because it's sat on rubber tyres.  It might depend on how the steadies were attached.  OR the generator should be isolated from the ground.  With plastic plumbing much of the caravan metal work won't be earthed.

And then sometimes it's recommended to connected the Earth to the neutral...

1 minute ago, matelodave said:

Ohm law does apply, so do other factors. The fact that they've decided to do the calcs at 10v rather than 12 is to take into account inverter losses

 

What they haven't taken into account is Puerkerts Law which allows you to calculate the loss of capacity when you drag more current from a battery. Most batteries are rated at the c/20 rate which means that a 100a/h battery will last 20 hours at 5amps. However if you pull 20amps out it wont last 5 hours but quite a bit less. Make it 50amps or more and the capacity will be even less - use this calculator to work it out http://www.csgnetwork.com/batterylifecalc.html

Ohm's law only applies if the load is resistive, with regards to AC circuits  - I'm not sure whether microwaves are resistive or inductive.

 

Is there not also an issue with voltage in that in AC land, the voltage varies between 0 and 240v so you need to use an RMS voltage not 240v?

 

I hadn't spotted that their calculations were due to Inverter losses and not AC v DC differences...

 

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Posted (edited)
45 minutes ago, PandR said:

Is there not also an issue with voltage in that in AC land, the voltage varies between 0 and 240v so you need to use an RMS voltage not 240v?

 

UK mains voltage is about 240 (RMS); it can vary anywhere in the range 207 volts to 253 volts.  (230 +/=10%)  Suppliers try to keep in the top part.

Edited by kelper
error corrected

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3 hours ago, PandR said:

And then sometimes it's recommended to connected the Earth to the neutral...

Ohm's law only applies if the load is resistive, with regards to AC circuits  - I'm not sure whether microwaves are resistive or inductive.

 

Is there not also an issue with voltage in that in AC land, the voltage varies between 0 and 240v so you need to use an RMS voltage not 240v?

 

I hadn't spotted that their calculations were due to Inverter losses and not AC v DC differences...

 

The 340v peak of the ac sinewave ends up with 240v  is the RMS value which should be used for calculations.

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On 21/07/2019 at 09:36, matelodave said:

This is a useful guide to what sort of power you'll get from a 100w solar panel - 

 

Excellent info - my only observation is the hours of sunlight, I think, are taken as a yearly average.

My 'Googling' found approx double the hours of sunlight, ie. 6.7hrs, for June, July and August - so double the input to your battery for these months.

Its interesting, because as my earlier post - effective use of the hair dryer depends on the length of the wife's hair, the weather and the time of year, and of course use the dryer in the morning so there is the rest of the day for the battery to recharge.

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On 21/07/2019 at 12:05, kelper said:

I don't understand your question? 

 

Mains voltage is 240 in the UK.

I was trying to make sure you were working out the current draw, with the associated  volt drop per amp per metre, at 12V rather than mains voltage which is, incidentally, 230V (nominal) not 240V in the UK.

The voltage was changed in January 1995 so it's not a new thing (just a personal bugbear,  as an electrician,  of mine 😏)

Back on topic 115A at 12V will cause a massive volt drop at 12V unless really large cables, such as 25 or 35mm is used.

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7 minutes ago, bspks said:

I was trying to make sure you were working out the current draw, with the associated  volt drop per amp per metre, at 12V rather than mains voltage which is, incidentally, 230V (nominal) not 240V in the UK.

The voltage was changed in January 1995 so it's not a new thing (just a personal bugbear,  as an electrician,  of mine 😏)

Back on topic 115A at 12V will cause a massive volt drop at 12V unless really large cables, such as 25 or 35mm is used.

 

The nominal voltage was changed to 230V as part of EU harmonisation, defining tolerances which included all present voltages but, in practice, each country continued supplying power at their previous voltage so typically around 240V in the UK.

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11 minutes ago, bspks said:

I was trying to make sure you were working out the current draw, with the associated  volt drop per amp per metre, at 12V rather than mains voltage which is, incidentally, 230V (nominal) not 240V in the UK.

The voltage was changed in January 1995 so it's not a new thing (just a personal bugbear,  as an electrician,  of mine 😏)

Back on topic 115A at 12V will cause a massive volt drop at 12V unless really large cables, such as 25 or 35mm is used.

Uk voltage is officially 230V, yes, but do me a favour and measure it, in you home, now.  It's probably close to 250.

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