Solar project for a large boat - Off-grid inverter selection

Hi. I have just read an interesting article by Jason Svarc dated July 10, 2017, which reviewed Victron Energy’s products - thank you very much for the information. I am currently in the process of fitting out a boat to be entirely off-grid. The set-up will be 9kW of PV via flexible panels and 25kWh of Lithium Iron Phosphate batteries. We will use three Victron Energy MPPT 250/85 charge controllers. I am currently considering the choice of inverter and at this point am likely to go with the Victron Energy Quattro 10kVA. I think that 10kVA(8kW) should be sufficient but is possible I might want a little more in the future (no more than an additional 4kW).

As Jason’s review is now over 18 months old I just wish to check whether there is any update on your general views regarding Victron Energy’s products. I would also value your opinion on the size & configuration of the inverter. I have the option to go with a single 10kVA inverter or I could instead choose to pair two 5kVA units. The latter set-up (as I understand it) would mean that I would then have the ability to add another 5kVA unit in the future should I decide that more capacity were needed. I am very much a lay-person, zero technical knowledge in this area, so would appreciate your thoughts on the pros and cons of combining multiple smaller units versus simply having one larger unit. A few things come to mind:

Simplicity: presumably one unit has to be simpler than two
Cost - higher to combine two smaller units of equivalent size to one larger unit
Space - takes up more space to combine smaller units
Performance (efficiency) - I understand that there is a very small diminution in electrical efficiency when combining inverters, but only very minor
Redundancy - perhaps better to have two 5kVA units versus one 10kVA unit just in sense that if one unit malfunctioned would still have the second unit and therefore still have some AC power?
Modularity/Scalability - my understanding is that I cannot subsequently add a 5kVA unit to a 10kVA unit as all of the units always need to be the same. Therefore would need to be three 5kVA units. Is this actually correct please? Could I not simply plug another inverter into one of the AC inputs of the Quattro 10kVA inverter?
No doubt other issues that I have not thought of…

As I believe Jason has lived in a solar off-grid home for many years I hope that he may have insights to offer. Of course, insights from everyone are very welcome! Your advice would be very much appreciated. Many thanks for your help.


@BHekman you might be interested in this one too.

Hi Maghnus,

In general I would go with one larger inverter as opposed to two smaller inverters as it is (as you stated) cheaper, easier and takes less space. That said it is really based on the total maximum load as one smaller 5kVA inverter may not be large enough to supply the energy demand of your boat (btw sounds like a nice boat).

Do you have what is known as a load table? This is a basic list of appliances and the estimated power rating and energy consumption of each appliance.
Using a load table you can determine the size of inverter required.

Also in regards to Victron, I would not change my review at all. I have used many of the new Victron products without any issues and if anything they have improved and added some great new features including software updates to improve monitoring.

Hope this help,

Hi Jason,

Thanks very much for your reply. Great to know that you are still very positive on Victron. Regarding the inverter, when I add up what is reasonably possible to be in use concurrently, e.g. two or three medium aircons (3-4kW?) plus always-on devices like fridge, etc… (500W?) I am pretty confident that 10kVA/8kW should provide sufficient AC power. The boat was built to be based in a marina with access to mains power, therefore no consideration was given by previous owners to any sort of energy budget. Basically it is kitted out pretty much like a regular house. If I was starting from scratch I have no doubt 8kW would be sufficient. Swapping out appliances with more efficient versions could be done but of course requires work and money so I am keen to try to stick with as much of what is there already as possible.

Would you agree with my view that peak usage for a regular small-medium house really need not ever be over 8kW please?

The Victron Quattro inverters come in five sizes - 3, 5, 8, 10 & 15kVA - therefore plenty of options. My question about using two smaller units of equivalent total size instead of simply one larger unit (i.e. two 5kVA units versus one 10kVA unit) really revolves around the benefits that this might bring in terms of A) redundancy (if I have two 5kVA inverters and one fails then I still have an inverter) and B) modularity (if I have two 5kVA units and in the future I decide that I need more than 10kVA inverter capacity then I can add more by connecting another 5kVA unit). I have read that it is not possible to parallel a 5kVA unit with a 10kVA unit as each unit always needs to be exactly the same. That said, I have also read that it might be possible to simply connect a 5kVA unit to a 10kVA unit via one of the AC inputs on the back of the larger unit.

Basically I’m trying to understand what the implications are of choosing one 10kVA unit versus two 5kVA units are with regard to possible future increase in inverter capacity requirement.

Thanks again,


Hi Maghnus,
I would definitely go with the single 8kW inverter over 2 x 5kVA inverters for a number of reasons - you can have battery balancing issues with 2 inverters, less efficient using 2 inverters, and more complexity. The Victron inverters are very reliable compared to cheaper chinese units so I consider the risk of failure to be very low (provided the inverter is mounted in a protected location out of the weather).

In terms of sizing the 8kW inverter sounds like the correct size for your application and demand, however inverters can derate power output in higher temperatures and it sounds like you are in a warm tropical climate? If you wanted to run all 3 aircon units at the same time on a hot day it may be problematic (what is the current draw of the aircons? Any more than 10A and it would be an issue). If you are looking to only improve efficency in the future and not add any more big loads then the 8kW inverter sounds like a good fit.

Hope this helps

Hi Jason,

Thanks very much - that’s extremely helpful.

Yes, subtropical climate - warm and humid for 4 or 5 months of the year, during that time daily temp range 27-35C/80-95F. For the other 7-8 months energy requirements drop dramatically - pretty much zero heating/cooling required. There are three sizes of aircons on the boat and these each draw 4-9A.

Tough call. 10kW would probably be ideal. That said, I guess the truth is that if unable to live on 8kW then probably means should change lifestyle and/or efficiency of lifestyle rather than increase size of inverter…

Thanks again,


Hi Maghnus, interesting thoughts. As you are saying, you are outfitting a larger boat. Most larger boats have a generator. I understand you want to use solar energy as much as possible, but since you use your aircons only part of the day and part of the season, would a reasonable size generator, say 6-8 kw, not be sufficient to cover those peak moments when you want your aircons to run? Victron has the Quattro range that is suitable for 2 AC sources. Personnally I would opt for the 2 x 5 kva with the possiblitity to upgrade in the future to 3 x 5 kva if you go full solar, it also provides you redundancy as you yourself concluded. On the other hand, if you consider the generator option, you might not need the 3 x 5 kva. Have you asked Victron themselves? I believe they also have some tools on their site to assist in configuring a system. At the moment I am also in the process of outfitting my 55 ft motoryacht Xanthiona with solar panels and a generator and am writing about how solar, grid and generator would interact with each other: (it has a google translate button). Good luck with your project. Regards, Jurjen Hoekstra

Thanks for the post @jurjenhoekstra, and welcome!

Hi Jurjen. Many thanks for your thoughts. Yes, certainly installing a generator would remove all issues regarding power availability. I may of course ultimately have to go down that path but for now will attempt to make it work purely with solar - if at all possible I wish to avoid having a generator due to environmental concerns, as well as cost and noise. The solar capture is substantial (9kW) and battery bank will provide for 25kWh of storage. I am pretty much limited to 9kW solar but could add to the battery bank in the future if required. With this in mind I feel it ought to be possible to live purely on solar. Will find out the answer soon! Thanks again, Maghnus