Microprocessor controlled load sharing on EU3000is

dadams
dadams Registered Users Posts: 14
Greetings folks,

Wow, this forum is full of good info!!!

I've just installed my first solar system with 1kw of panels and a Tristar MPPT60 controller.
So far, having a bunch of fun with it.

I need some advice from the knowledgeable sages on here please.

I'm planning to replace my old 5kw Chinese generator with a pair of Honda EU300is units.

In order to run them most efficiently, I want to have one generator running the base load of the house with the second one kicking in when power requirements demand it.
So, at night we would have one running lights / fans and TV etc, when it reaches 70% load or someone switches on the kettle / microwave, the second one starts and shares the load for however long it's required.

I can do all the control start stop etc with an Arduino microprocessor. My concern is, everywhere I've looked, I see you have to have both generators running when connected in parallel.

Is this actually the case? I'm hoping I will be able to start and stop one unit as and when I require the extra power without breaking anything.
Secondly, what happens when the second generator starts and it's running at full rpm for a while to warm it up. Would I have to disconnect the parallel cable while this happens? Honda say they should be started with the parallel cable connected, is this a technical reason or a safety reason?

Can anyone confirm this on the EU3000is please?

Comments

  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    These are inverter generators. They have an inverter inside which sets the frequency and voltage. Therefore, the frequency doesn't depend on the rotation speed. The generator can run slower or faster depending on the power needs. That's how they conserve fuel (and perhaps one 6500i generator may not be any worse than the pair of 3000i :confused:). If you run them without syncing their inverters they may damage each other or loads, so they have to be synced at all times.

    Starting a generator takes quite a bit of time. If you start it when a load is detected, what is going to power the load until the generator starts?
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    One downside of my 3000i is that it ALWAYS needs the choke pulled on to start, the amount is temperature dependant.
    It's best to start them with the ECO switch off to get it warmed up.

    these 2 items will need a custom install of some sort.
    As NG said maybe a 6500 or similar size would be better.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    NorthGuy wrote: »
    These are inverter generators. They have an inverter inside which sets the frequency and voltage. Therefore, the frequency doesn't depend on the rotation speed. The generator can run slower or faster depending on the power needs. That's how they conserve fuel (and perhaps one 6500i generator may not be any worse than the pair of 3000i :confused:). If you run them without syncing their inverters they may damage each other or loads, so they have to be synced at all times.

    Starting a generator takes quite a bit of time. If you start it when a load is detected, what is going to power the load until the generator starts?


    Hi Northguy, thanks for the input and valid comments.
    I've been looking at the 6500i and it was my first choice till I discovered a discussion about it having two alternators being driven by one engine. And, they are not linked in parallel. So, it's designed to supply two 3000 watt loads rather than one 6000 watt load. I'm speaking under correction here but that's what I picked up from the post.
    If someone here is running their 6500i connected in to their breaker box and pulling full load off of it then I'm all ears.
    I kind of like the idea of having two smaller generators backing each other up. Most of the time only one unit will be required, especially late at night when it's only running a few fans and lights. I can write code for the arduino controller so it always starts the generator with the least run time hours first. Thereby, eliminating the problem of the primary generator clocking up far more hours than the secondary.
    I really need to know if the secondary eu3000is, when connected in parallel, via the Honda parallel cable, is capable of being started as and when needed. From Honda's videos, it sounds like both need to be running from the start. Which would blow me out the water before I leave the blocks.
    Does anyone run two eu3000is's and have any experience with them?

    Thanks and regards

    Dave
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    westbranch wrote: »
    One downside of my 3000i is that it ALWAYS needs the choke pulled on to start, the amount is temperature dependant.
    It's best to start them with the ECO switch off to get it warmed up.

    these 2 items will need a custom install of some sort.
    As NG said maybe a 6500 or similar size would be better.

    Greetings Westbranch, thanks for your input.

    Yes, I'm actually looking forward to hacking / re-enginerring (to a certain extent) the eu3000is. I intend to put them in a sound proof box with good fans and ventilation to make them as quiet as possible.
    I already know they are really quiet gensets, but I live in a complex and really want to make them as quiet as possible so I don't feel guilty running them all night.

    I can servo operate the choke according to the temperature of the engine block. Easily enough done with a temperature sensor, servo and microcontroller. Also, starting them out of eco mode is a pretty simple thing to control.
    I just need to find someone who is already running a pair of these eu3000is gensets and confirm that the secondary or slave unit can be started and stopped as necessary. From Honda's videos, it appears the parallel cable needs to be connected then both gensets need to be started in the same mode. eg, eco or full throttle. I can't find anywhere if starting and stopping one unit as required would damage anything.
    I can setup the wifes ipad or phone so she can start the second unit before she uses a heavy load appliance. Or, supply the extra load from my inverter and batteries while the second unit starts.
    There are many ways to overcome the startup delay, just need to know the gensets are capable of essentially intermittent coupling.
    Anyone with experience on these particular units would really be a big help for me.
    Also, anyone with experience on running a eu6500i would be a great help.
    I would still prefer to run the two eu3000is units as it would just be more fun to build it!!!

    Thanks in advance for any input guys.

    Really appreciate it.

    Dave
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    Hi dadams,

    Well, Solar and batteries, with an occasional genset run is very very quiet.

    Seems to me that you may be starting in the middle of this project.

    However if you like to tinker, and it seems that you might, buying some EU gensets might be a good place to begin in the middle of things.

    Use an EU6500isa here, and have two EU3000is/as, but have never done the kludge of tying the two together. This doubles the number of parts involved, and reduces reliability. AND hacking the 3000s will, almost certainly void their warranty.

    An EU 6500 at light load WILL burn more fuel than one EU3000 at that same load. Many tradeoffs, but it is not at all clear what you are trying to do with this system ... sounds like it might be a backup system to a grid-powered residence (?). Have Fun! Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    NorthGuy wrote: »
    These are inverter generators. They have an inverter inside which sets the frequency and voltage. Therefore, the frequency doesn't depend on the rotation speed. The generator can run slower or faster depending on the power needs. That's how they conserve fuel (and perhaps one 6500i generator may not be any worse than the pair of 3000i :confused:). If you run them without syncing their inverters they may damage each other or loads, so they have to be synced at all times.

    Starting a generator takes quite a bit of time. If you start it when a load is detected, what is going to power the load until the generator starts?
    Your exactly right, as usual. The instructions say if you don't have a green light on BOTH Generators, shut them both down and re-start. Could it work the other way, maybe if you still have a warranty left .... lol. I don't know if the receptacles is hot on both generators when connected with only one running.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Microprocessor controlled load sharing on EU3000is

    I believe that for the eu family--Both AC outlet sets are hot. However (for example) the eu2000i family can be paralleled by connecting the two gensets in parallel to a single 30 amp outlet. You cannot draw 30 amps from either genset 20 amp outlet.

    This eu2000i companion has the 30 amp outlet built in (when paralleled with a second eu2000i).

    You might also look at the Honda em4000sx -- It sounds like a nice unit and Chris Olsen (past poster here) has been very happy with his. Full electronic engine controller (with automatic mixture control) and very good cold weather performance (with ~70 watt light to warm unit up in deep icebox north environments). Be careful, there are lots of non-SX units out there too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    Vic wrote: »
    Hi dadams,

    Well, Solar and batteries, with an occasional genset run is very very quiet.

    Seems to me that you may be starting in the middle of this project.

    However if you like to tinker, and it seems that you might, buying some EU gensets might be a good place to begin in the middle of things.

    Use an EU6500isa here, and have two EU3000is/as, but have never done the kludge of tying the two together. This doubles the number of parts involved, and reduces reliability. AND hacking the 3000s will, almost certainly void their warranty.

    An EU 6500 at light load WILL burn more fuel than one EU3000 at that same load. Many tradeoffs, but it is not at all clear what you are trying to do with this system ... sounds like it might be a backup system to a grid-powered residence (?). Have Fun! Vic

    Hi Vic,

    Yes, I live in Africa and although our home is powered from the grid, we have constant load shedding and power outages. At least two a week. (And we're lucky, some folks have 5 outages a week for 8 hours a day)
    So, I started by installing the solar panels, inverter and batteries to power lights, fans and computers.
    Then I got bit by the bug. :-)
    As I've said earlier. Replacing the old Chinese genset which although pretty reliable, has seen better days and just loves to destroy electronics.
    So looking for a clever, fun and quiet way to power my home during the frequent outages.
    If the EU2000i had electric start I would be tempted to go for those but alas...
    So, I'm looking for an interesting and economical solution to powering my home for the two nights a week the grid is dead.

    Thanks all for the feedback so far

    Dave
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is

    I must be able to start and stop one unit surely.
    When you start them and they're connected in parallel, one of the generators starts after the other anyway. So what's the difference if I stop and start one whenever I feel like it?
    Amounts to the same thing surely?

    Dave
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    Is it possible to rewire those mandatory loads to another Distribution box and only have those loads powered by the 3000i? You would need a transfer switch.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    I've been looking at the 6500i and it was my first choice till I discovered a discussion about it having two alternators being driven by one engine. And, they are not linked in parallel. So, it's designed to supply two 3000 watt loads rather than one 6000 watt load. I'm speaking under correction here but that's what I picked up from the post.

    I have a eu6500i and a eu2000i. I used the 6500 to build a house (air compressors, table saws, etc.) I rarely use the 6500 now, it is larger than I need and compared to the 2000 it is a gas hog.

    The eu6500 can put its full power into the 240 volt outlet. At 120 volts it has a 30 amp twist lock outlet and a couple of 20 amp outlets. You can draw full power at 120 volts, but the most you can get out of one outlet is 3600 watts. Building the house we used only the 120 outlets, but we drew full power at times.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    I must be able to start and stop one unit surely.
    When you start them and they're connected in parallel, one of the generators starts after the other anyway. So what's the difference if I stop and start one whenever I feel like it?
    Amounts to the same thing surely?

    Dave


    Yes, exactly, I would think so.

    I have 2 eu2000is. One is the companion model with 30amp 120V plug. I have run them in parallel a few times. They are pull start and they obviously are not started simultaneously. Not practical to shut then off simultaneously either. I have run them dry of fuel before storage and when I do that one always shuts down many minutes before the other. No ill effects that I can tell...
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    You are right that the paralleled gensets must be able to handle one or the other stopped/started at different times - first when you start them up then when one of them runs out of fuel before the other one. However, if one stops the other may shut off its inverter. A test would easily answer that.

    However, I think you are trying to get too complex to solve a problem that can be solved a little easier. I'm going to suggest doing something I've done two different times. Size your inverter to handle your loads - maybe using two, one for base loads, the other for the other loads when you are awake and home. Then monitor your battery voltage to fire up the genset if it gets too low. The genset should be sized to about 70-80% of its continuous rating to handle the max current from the battery charger, which gets you the best efficiency per gallon burned. Once there is a good charge, shut the genset off again.

    I first did this with a big genset that I made more efficient by adding a huge battery bank so I only had to run the genset for a few hours twice a day instead of 24/7. Then I did it again with the same inverter but using a Prius as the "generator" (link in my signature for details) and as the battery. The Prius has the intelligence to start itself, charge its battery, then shut off. This later setup is my primary power source when the utility power goes out. Day to day a smaller battery bank handles the many brief blips/sags we often get.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    vtmaps wrote: »
    I have a eu6500i and a eu2000i. I used the 6500 to build a house (air compressors, table saws, etc.) I rarely use the 6500 now, it is larger than I need and compared to the 2000 it is a gas hog.

    The eu6500 can put its full power into the 240 volt outlet. At 120 volts it has a 30 amp twist lock outlet and a couple of 20 amp outlets. You can draw full power at 120 volts, but the most you can get out of one outlet is 3600 watts. Building the house we used only the 120 outlets, but we drew full power at times.

    --vtMaps

    Aah, ok so that clears up one question. We run on 240v in Zimbabwe so from what you're telling me, I'll be able to pull full load off the generator if I connect it in to my change over switch.
    Ok, so that makes it an option but I still feel it would be more efficient to run two eu3000's on an as needed basis.
    A lot of my load will be very low as it'll be during the night running a tv, few lights and bedroom fans.
    Also, in my neck of the woods, buying two eu3000's works out US$800 cheaper than buying an eu6500...

    Thanks for the feedback.
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    techntrek wrote: »
    You are right that the paralleled gensets must be able to handle one or the other stopped/started at different times - first when you start them up then when one of them runs out of fuel before the other one. However, if one stops the other may shut off its inverter. A test would easily answer that.

    However, I think you are trying to get too complex to solve a problem that can be solved a little easier. I'm going to suggest doing something I've done two different times. Size your inverter to handle your loads - maybe using two, one for base loads, the other for the other loads when you are awake and home. Then monitor your battery voltage to fire up the genset if it gets too low. The genset should be sized to about 70-80% of its continuous rating to handle the max current from the battery charger, which gets you the best efficiency per gallon burned. Once there is a good charge, shut the genset off again.

    I first did this with a big genset that I made more efficient by adding a huge battery bank so I only had to run the genset for a few hours twice a day instead of 24/7. Then I did it again with the same inverter but using a Prius as the "generator" (link in my signature for details) and as the battery. The Prius has the intelligence to start itself, charge its battery, then shut off. This later setup is my primary power source when the utility power goes out. Day to day a smaller battery bank handles the many brief blips/sags we often get.

    Hey techntrek, thanks for that info, your idea is a goodun but I'm trying to avoid large battery banks. At the moment I've got things running on a 700ah bank and for light loads or intermittent heavy loads, it works just fine. I'm nervous of buying loads of batteries because in my part of the world, they are hellish expensive and they don't last as long as I would like. Still, I guess that's the same everywhere :-)

    I have a eu1000 (best genset I've ever owned) and I know someone nearby who also has one. Does anyone know if the inverter technology in the 1000 is sufficiently similar to the 3000 to enable me to run a test? If it works with the 1000's would it be safe to assume it'll work with the 3000's ?
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is

    One thing I've just thought of, in all the Honda videos I've watched, they start and stop them with no load.
    Only once running do they connect the load.
    What I'm proposing is having one unit kick in with the other already under load.

    Hmm, questions questions....
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    One thing I've just thought of, in all the Honda videos I've watched, they start and stop them with no load.
    Only once running do they connect the load.
    What I'm proposing is having one unit kick in with the other already under load.

    Hmm, questions questions....
    In case you missed this.

    The instructions that come with the connecting cord say if you don't have a green light on BOTH Generators, shut them BOTH down and re-start. It would be a easy test to find out. Trying to get them to sync when one is under load may not be all that easy. It would be easy to drop the load from it and on the inverter , start the other and re-apply the load to both. Trying to make it a seamless progressive thing would be nice though. I'd say if it was that easy, someone would already be doing it.

    I use a time delay cube on a A/C and a timed relay on the ECO, The delay holds the load for 5 seconds and the timed one shot relay turns the ECO off for 10 seconds, then back on. That gives the A/C time to start and then drop back to ECO on.

    There might be a way on say a Outback GTFX to use the Generator support feature and the AGS or Aux relay, start one manually and let the inverter control the other Generator automatically.

    I have a pair of EU2000's in the end I found that having one on the right Inverter and one with a stand alone charger works best for me. I might lose a little in conversion, but I control what and when. With a EU3000 you could just start and stop one with a LVS or CSR and be done.
    .
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    One thing I've just thought of, in all the Honda videos I've watched, they start and stop them with no load.
    Only once running do they connect the load.
    What I'm proposing is having one unit kick in with the other already under load.

    Hmm, questions questions....

    Have you tried the Yahoo 'Honda eu2000i' forum? Mostly about the eu2000, but other honda generators also. Your question has come up there, but I can't remember the answer :cry:

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    mtdoc wrote: »
    Yes, exactly, I would think so.

    I have 2 eu2000is. One is the companion model with 30amp 120V plug. I have run them in parallel a few times. They are pull start and they obviously are not started simultaneously. Not practical to shut then off simultaneously either. I have run them dry of fuel before storage and when I do that one always shuts down many minutes before the other. No ill effects that I can tell...

    Hey mtdoc,

    Thanks for the input :-)

    Have you ever had one unit run out of fuel, re-fuelled it and restarted it without removing the load from the other unit that's still running?
    Coz that's essentially what I'm trying to do.
    Please say yes, please say yes!

    LOL

    Thanks

    Dave
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is
    In case you missed this.

    The instructions that come with the connecting cord say if you don't have a green light on BOTH Generators, shut them BOTH down and re-start. It would be a easy test to find out. Trying to get them to sync when one is under load may not be all that easy. It would be easy to drop the load from it and on the inverter , start the other and re-apply the load to both. Trying to make it a seamless progressive thing would be nice though. I'd say if it was that easy, someone would already be doing it.

    I use a time delay cube on a A/C and a timed relay on the ECO, The delay holds the load for 5 seconds and the timed one shot relay turns the ECO off for 10 seconds, then back on. That gives the A/C time to start and then drop back to ECO on.

    There might be a way on say a Outback GTFX to use the Generator support feature and the AGS or Aux relay, start one manually and let the inverter control the other Generator automatically.

    I have a pair of EU2000's in the end I found that having one on the right Inverter and one with a stand alone charger works best for me. I might lose a little in conversion, but I control what and when. With a EU3000 you could just start and stop one with a LVS or CSR and be done.
    .

    Now that's a great idea, something along the lines of a grid tied inverter or an inverter with multiple inputs providing a single output to the house breaker box.
    Anyone know of such a creature?
    Would need to share the load on the inputs rather than switching between them.

    Dave
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    Now that's a great idea, something along the lines of a grid tied inverter or an inverter with multiple inputs providing a single output to the house breaker box.
    Anyone know of such a creature?
    Would need to share the load on the inputs rather than switching between them.

    Dave
    Outback GTFX is grid interactive. It will sell to grid or stand alone. It only has one input, but you could add a transfer switch. It'll do Generator Support seamless while connected to any AC input. The Inputs on some hybrid's are Bi-directional unless you have one with two inputs and I don't know the ones that have it. The Xantrex does, but it's generator support is really slow to engage. When you switch the Input to Generator the sell automatically turns off on the Outback.
    .
  • dadams
    dadams Registered Users Posts: 14
    Re: Microprocessor controlled load sharing on EU3000is

    Hi Everyone,

    Well, it looks like what I want to do is entirely possible.
    Please watch this Youtube video.
    http://www.youtube.com/watch?v=t0Q0TSwKBNI
    And it appears it's really simple as well, just plug your eu series generator in to any clean source of ac and it will synchronise on startup.
    Well, that's what the video shows anyway.
    Anyone got any comments?
    I notice he's not even using parallel cables and it works. Amazing.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is

    Well, you can't deny that he's doing it and it seems to synchronize to the Inverter. The question is what happens when it starts to back drive the Inverter ?? You then have uncontrolled voltage heading back into the Batteries, if I understand it correctly. Since the Honda can vary it's output, maybe that's not a issue.

    It could open some possibilities for some folks. If you look at the E-bay combiner cords there is nothing that would make the sync of two generators, so it has to be internal to the generator inverter.

    .
  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    Hey mtdoc,

    Thanks for the input :-)

    Have you ever had one unit run out of fuel, re-fuelled it and restarted it without removing the load from the other unit that's still running?
    Coz that's essentially what I'm trying to do.
    Please say yes, please say yes!

    LOL

    Thanks

    Dave

    Sorry, I can't recall whether I've ever done that. If you don't get anyone responding with first hand experience here or on the Yahoo eu2000i group let me know. I can try it sometime - though it may be a few weeks beforeI can get to it.
  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Microprocessor controlled load sharing on EU3000is
    dadams wrote: »
    Hey techntrek, thanks for that info, your idea is a goodun but I'm trying to avoid large battery banks. At the moment I've got things running on a 700ah bank and for light loads or intermittent heavy loads, it works just fine. I'm nervous of buying loads of batteries because in my part of the world, they are hellish expensive and they don't last as long as I would like.

    Sorry, I didn't mean to suggest that you needed a large battery bank. The traction battery in the Prius is only roughly 1500 watts, although that means it cycles on-off fairly often. I think your current battery bank is large enough.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is