Sizing an inverter

[Riggsy]

I'm refitting my sons boat 46ft Narrowboat, currently called "Eaglet",

Going to upgrade the Inverter from the crappy Chinese inefficient 3KW horror currently fitted, to something more modern with an ECO mode to manage battery power.

He doesn't have much mains draw, The MAX is the electric oven at 1.6KW, (worst case.)

Other 240VAC is negligible (Play-Station etc), and the oven is unlikely to be used often (He lives on takeaways and beer, as did we all at his age 😁)

 

Is a 1600W inverter acceptable in this application, or should I guard band it and look at a 2000W option.

I ask as there is a bit of a step change in cost between the 2.

 

Basically, will the inverter stop if the draw is > 1.6KW, or will his pizza just take a bit longer to cook?

 

 

Thanks in advance 👍

 

Where we are Now:

Last Week:

 

[BEngo]

Look in detail at the continuous rating, not the headline on the box, and   establish if it is rated in W or VA.

 

A 1600 continuous rating should be fine for a 1600 W  oven, whether the rating is in W or VA . Beware of a VA rating if there are any big motors or other inductive loads around.

[Riggsy]

Just testing the new electric set-up!

 

4 minutes ago, BEngo said:

Look in detail at the continuous rating, not the headline on the box, and   establish if it is rated in W or VA.

 

A 1600 continuous rating should be fine for a 1600 W  oven, whether the rating is in W or VA . Beware of a VA rating if there are any big motors or other inductive loads around.

 

Thanks, The 1600W is rated at 25C, Its derated at a bastard rate as the temp rises 😞

 

 

Just asking if the Inverter will disable itself, or just limit the power at its output.

If the latter then I can live with it 🙂

 

Thanks

...Riggsy

[MtB]

Blimey that's a lot of windows. They would give me agoraphobia!! 

 

The 1.6kW inverter I'd suggest as the power factor for the oven will be near as dammit 1, and the quiescent current draw on a 1.6kW inverter will be less than that of a 2kW. 

 

 

 

[PeterF]

The inverter will not limit the power, it will heat up till it shuts down. If it limited the power with a fixed resistance like an oven it would have to let the voltage drop below 230V, which would not go down well with any voltage sensitive equipment. Make sure it is in a well ventilated location. An oven will be on full power as the oven warms up, but once up to temperature it will cycle on and off but at full power.

 

I see the VE BUS BMS mk2, do not forget that this needs a VE Bus inverter such as a Multiplus rather than one of the VE Direct inverters.

 

Two cautions with using the Victron Lithium Smart batteries, you may have this in hand, if so apologies. 1, the VE Bus BMS nor the batteries have inbuilt SOC calculation, you need a SmartShunt. 2, tje batteries do not have any form of internal BMS, the cells are direct to the battery terminals, consider what level of fusing you need in terms of the AIC, (Ampere Interrupting capacity), the maximum short circuit fault current that they can break (this is not the rated current), Mega fuses are not suitable. I ended up using Class T on my 300Ah Victron NG batteries bit Blue Sea MRBF may also be OK, but check.

[MtB]

2 minutes ago, PeterF said:

The inverter will not limit the power, it will heat up till it shuts down.

 

Thanks. 

 

An interesting point I had never realised or considered until you pointed this out. 

 

 

[IanD]

12 hours ago, MtB said:

Blimey that's a lot of windows. They would give me agoraphobia!! 

 

The 1.6kW inverter I'd suggest as the power factor for the oven will be near as dammit 1, and the quiescent current draw on a 1.6kW inverter will be less than that of a 2kW. 

 

For a 1.6kW oven you need an inverter with "2000" in the name (e.g. Multiplus 12/2000), these are rated at 2000VA or 1600W.

 

Don't forget that inverter efficiency drops near to full power, so they get hot and this wastes power -- that quoted "94% maximum" may be 88% or lower at full power.

 

So a bigger inverter (e.g. 12/3000) will be more efficient and run cooler and this will probably more than make up for the extra 4W no-load power.

 

 Costs more though... 😉

Edited August 1 by IanD

[cuthound]

11 hours ago, PeterF said:

The inverter will not limit the power, it will heat up till it shuts down. If it limited the power with a fixed resistance like an oven it would have to let the voltage drop below 230V, which would not go down well with any voltage sensitive equipment.

 

Many inverters will go into current limitation when they exceed their maximum output for more than a few seconds and then reduce the voltage to zero until the overload is removed.

[MtB]

1 hour ago, IanD said:

For a 1.6kW oven you need an inverter with "2000" in the name (e.g. Multiplus 12/2000), these are rated at 2000VA or 1600W.

 

Don't forget that inverter efficiency drops near to full power, so they get hot and this wastes power -- that quoted "94% maximum" may be 88% or lower at full power.

 

So a bigger inverter (e.g. 12/3000) will be more efficient and run cooler and this will probably more than make up for the extra 4W no-load power.

 

 Costs more though... 😉

 

I guess it depends how the ""1600W" rating is arrived at though. 

 

Someone as dense as me would probably assume an inverter rated at 1600W was capable of powering a 1600W resistive load as this is 1600VA too. Although perhaps you re saying a 2000W inverter also has a power factor of less than unity, i.e. 0.8 so it would still send 2000VA to the 1600W heater elements, with the current a bit out of phase? 

 

And a good brand name "ought" to be able to cope with an electric oven matching its rating I'd have thought. My own 1600W electric oven has a duty cycle of about 50%. Yes I spent an hour timing it a few years ago! (Can't remember why I needed to know.) 

 

 

 

 

 

[Slim]

Looking at the matter from a different angle.........

What will your son be doing whilst his pizza 😇cooks? Will he go around shutting down all other 230v devices ie fridge, tv, computer ? I doubt he will do so out of choice!

[MtB]

2 minutes ago, Slim said:

Looking at the matter from a different angle.........

What will your son be doing whilst his pizza 😇cooks? Will he go around shutting down all other 230v devices ie fridge, tv, computer ? I doubt he will do so out of choice!

 

Good point!

 

 

[jonathanA]

FWIW i think you will be very lucky if a 1600 W (or VA) inverter will run a 1600W appliance... both figures are likely to be subject to to bit of tolerance so you might be lucky or you might not.... in any case running the inverter flat out isn't going  to be good in the long term and as your battery voltage drops then the inverter will have to work harder getting hotter in the process... 

I would want to build in a bit of headroom and as MTB says he will doubtless have other stuff running....

 

As no one has mentioned it I raise the question of recharging the batteries after the pizzas have been cooked. It looks like you know what your doing but just worth mentioning it. Also not clear if you intend using lithium or LA batts.  Definitely worth looking at lithiums if you haven't already.

[PeterF]

He has a BMS for Victron lithiums on the board shown above.

[robtheplod]

some good kit on show there!   

[Jen-in-Wellies]

27 minutes ago, robtheplod said:

some good kit on show there!   

We like blue boxes. 

[blackrose]

Not sure if someone has already mentioned it, but the other things to think about when buying an inverter are whether you need a cheaper semi or pseudo sine wave or a more expensive pure sine wave unit. Running things like automatic washing machines requires pure sine wave. Microwaves prefer it too. My Sterling 2kW pure sine wave inverter runs my 1.6kW washing machine. I wouldn't try it with a smaller 1.6kW inverter.

 

Also it's worth thinking about neutral-earth bonding. Most of the remarkably cheap pure sine wave inverters you see on Amazon or eBay are not properly N-E bonded and will instead have a centre-tapped earth arrangement which may not be compatible with the boat's RCD or RCBO mains breakers.

Edited August 1 by blackrose

[IanD]

On 01/08/2026 at 11:02, MtB said:

 

I guess it depends how the ""1600W" rating is arrived at though. 

 

Someone as dense as me would probably assume an inverter rated at 1600W was capable of powering a 1600W resistive load as this is 1600VA too. Although perhaps you re saying a 2000W inverter also has a power factor of less than unity, i.e. 0.8 so it would still send 2000VA to the 1600W heater elements, with the current a bit out of phase? 

 

And a good brand name "ought" to be able to cope with an electric oven matching its rating I'd have thought. My own 1600W electric oven has a duty cycle of about 50%. Yes I spent an hour timing it a few years ago! (Can't remember why I needed to know.) 

 

My point was that most inverters have a headline "power rating" in the model number, which is what many people see but without realising that this is not actual power into a resistive load like an electric oven or kettle, it's a VA rating into a reactive load with PF=0.8.

 

So they don’t realise that to power a 1600W oven they need at least a "2000" (VA) inverter (1600W), not a "1600" (VA) inverter (1280W).

 

That also assumes they have nothing else drawing power while the oven is on.

 

So it's a good idea to get the next size up inverter e.g. a "3000" model, which will give some spare capacity and run cooler -- the cost difference is relatively small nowadays, as is the extra no-load power (4W).

[Riggsy]

On 01/08/2026 at 11:41, Slim said:

Looking at the matter from a different angle.........

What will your son be doing whilst his pizza 😇cooks? Will he go around shutting down all other 230v devices ie fridge, tv, computer ? I doubt he will do so out of choice!

Not much on the boat running at 240V. Just the Oven, Aux heater for Calorifier and a Playstation, PC really. Fridge etc are all 12V

On 01/08/2026 at 12:26, jonathanA said:

FWIW i think you will be very lucky if a 1600 W (or VA) inverter will run a 1600W appliance... both figures are likely to be subject to to bit of tolerance so you might be lucky or you might not.... in any case running the inverter flat out isn't going  to be good in the long term and as your battery voltage drops then the inverter will have to work harder getting hotter in the process... 

I would want to build in a bit of headroom and as MTB says he will doubtless have other stuff running....

 

As no one has mentioned it I raise the question of recharging the batteries after the pizzas have been cooked. It looks like you know what your doing but just worth mentioning it. Also not clear if you intend using lithium or LA batts.  Definitely worth looking at lithiums if you haven't already.

Thanks, I currently have a 300W Fogstar Li unit, soon to be followed by a 2nd one. They do have internal BMS, but I'm not relying on that.

Charging is via 2 x 460W solar panels in the summer, and the engine for the dull days. Thats all in and working.

 

[Tony Brooks]

5 minutes ago, Riggsy said:

Not much on the boat running at 240V. Just the Oven, Aux heater for Calorifier and a Playstation, PC really. Fridge etc are all 12V

 

What is this auxiliary heater for the calorifier? If it is a standard domestic immersion heater then it might well demand 3 to 4 kW (AI Google result), but there are lower power ones around. You need to clarify this before buying an inverter.  However, unless there is a shoreline I am far from sure trying to run an immersion heater from batteries is a good idea if you have lead acid batteries and without a lot of solar even with LPF batteries. It is the recharging that gets you, not so much the power draw.

[MtB]

1 hour ago, Riggsy said:

Not much on the boat running at 240V. Just the Oven, Aux heater for Calorifier and a Playstation, PC really.

 

I have to say that sounds like quite a lot to me. 

 

Oven 1,600W 

Calorifier heater 1,000W

Playstation 220W

PC 250W

 

Total 3,070W. Just over 3kW. 

 

 

 

 

[blackrose]

1 hour ago, MtB said:

I have to say that sounds like quite a lot to me. 

 

Oven 1,600W 

Calorifier heater 1,000W

Playstation 220W

PC 250W

 

Total 3,070W. Just over 3kW. 

 

Yes, it sounds like a lot to me too.

 

2 hours ago, Tony Brooks said:

What is this auxiliary heater for the calorifier? If it is a standard domestic immersion heater then it might well demand 3 to 4 kW (AI Google result), but there are lower power ones around. You need to clarify this before buying an inverter.  However, unless there is a shoreline I am far from sure trying to run an immersion heater from batteries is a good idea if you have lead acid batteries and without a lot of solar even with LPF batteries. It is the recharging that gets you, not so much the power draw.

 

I think it would be unusual for anyone to have a 3 to 4kW immersion heater on a boat, but I suppose you never know and the OP should check that first.

 

I run my 1kW immersion heater from my 560ah LFP batteries most days in summer if I'm on the boat (manually switched when the sun is shining). I have 2 x 455w solar panels, so if the OP has a similar output immersion heater I think his 2 x 460w panels should be fine for recharging 300ah (soon to be) 600ah LFP batteries. 

Edited Wednesday at 12:48 by blackrose

[PeterF]

7 hours ago, Riggsy said:

 

Thanks, I currently have a 300W Fogstar Li unit, soon to be followed by a 2nd one. They do have internal BMS, but I'm not relying on that.

Charging is via 2 x 460W solar panels in the summer, and the engine for the dull days. Thats all in and working.

 

Of you are using Fogstar batteries why do you have a Victron VE Bus BMS, the two are not compatible, the Victron BMSs only work with Victron batteries.

[Tony Brooks]

8 hours ago, Riggsy said:

Not much on the boat running at 240V. Just the Oven, Aux heater for Calorifier and a Playstation, PC really. Fridge etc are all 12V

Thanks, I currently have a 300W Fogstar Li unit, soon to be followed by a 2nd one. They do have internal BMS, but I'm not relying on that.

Charging is via 2 x 460W solar panels in the summer, and the engine for the dull days. Thats all in and working.

 

 

Are you sure about 300 Watts? Should that be Watt hours? I think that equates to something like 25 Ah. At least you should not suffer sulphation and you should get a fast recharge. Even 600W? batteries only gives you 50Ah from fully charged to fully discharged, so roughly 20 minutes of 1600W oven use, and even less when other stuff is in use.

 

You have got me doubting my calculations, but it all seems not very well-thought-out. If my calcs are correct then you seem to have a recipe for flat batteries, cold water, and partially cooked food when the BMS shuts down on excess discharge. You may be running the engine a lot more than you think, just to power the oven etc.

Edited Wednesday at 18:37 by Tony Brooks

[Jen-in-Wellies]

14 minutes ago, Tony Brooks said:

 

Are you sure about 300 Watts? Should that be Watt hours? I think that equates to something like 25 Ah. At least you should not suffer sulphation and you should get a fast recharge. Even 600W? batteries only gives you 50Ah from fully charged to fully discharged, so roughly 20 minutes of 1600W oven use, and even less when other stuff is in use.

 

You have got me doubting my calculations, but it all seems not very well-thought-out. If my calcs are correct then you seem to have a recipe for flat batteries, cold water, and partially cooked food when the BMS shuts down on excess discharge. You may be running the engine a lot more than you think, just to power the oven etc.

The minimum size batt Fogstar do is 100Ahr  so I suspect 300W should have read 300Ahr, which is one of their middling size batteries. Capable of discharge rates enough to feed a 3kVA inverter. 

[blackrose]

28 minutes ago, Tony Brooks said:

 

Are you sure about 300 Watts? Should that be Watt hours? I think that equates to something like 25 Ah. 

 

I assumed he meant 300ah