power usage for Chinese Diesel heater

Bit of a long one but hopefully some body will touch on something that i am missing,

I currently have .......
2 x 100 watt solar panels (not a lot of sun in winter i know)
2 x 110ahr Varta (A28's from memory) fitted 2018 and seem ok
1 x 8kw (well it said 8kw on the link when i bought it) chinese diesel heater fitted august 2021
1 Victron bluetooth mppt 20a controller fitted around October / November 2021

Problem im having is lately if i leave the chinese diesel heater on all night on the lowest setting it keeps the mh nice and warm on a frosty night, but the pair of batteries are reading 11.9v in the morning, fully charged the day before at 2pm and not a lot of use regards lights (all led bulbs fitted) tv on for a few hours but its led and uses around 25w from the label info on the back,

Usually i would have no need to plug in at all before fitting the heater,

About a month agoI took the batteries out and gave both a full charge with the car smart charger and let the pair of batteries sit for the week in the shed, both read 12.6v after sitting for a week with nothing connected,

This time i plan to connect a 10 watt bulb and leave it 12 hours to see what the battery is reading and do the same to the 2nd one to compare incase one of the batteries is failing and draging the other down,

Next plan is to get an inline amp meter and see what the heater is actually drawing,

Any suggestions as to what else i can do to find why the batteries are so down in the morning,

Not a lot of sunshing in the wintery mornings so not expecting the solar to do much in fairness,

Robert, if I'm reading you correct, you are saying that Chinese heaters are only good for sunnier climates not much use in ireland,

Diesel heaters certainly have their use. I run two 24/7 at home. But I have said on here many times I don't like how they don't switch off on a stat for Motorhome use. Constantly drawing power, @JJF will I'm sure have the figures. I suppose more batteries are required and another solar panel

I think the propex gas heaters are more beneficial to a motorhome as they go on and off on a stat. But at six times the price, maybe another battery and solar panel is a better option.

My webasto heater on diesel will draw 17/18 amps on startup from glow plug for a few minutes, then drop down to 8 amps for 5-10 minutes, and finally settle at 4/5amps. My van is small and heats up very quickly a larger van will have a longer intermediate draw.
If you are heating for 8 hours, in winter, you will have used up most of one of the lead batteries. Also all those little milliamps working away in the background add up in a 24hour day.

Great info. Have never seen it highlighted before that these heaters are such heavy users of precious amps from the leisure batteries

Thanks for all info lads,

I know whats causing the problem now and can do something about it,

I was thinking that a pair of charged110ahr batteries should last the night easily for the diesel heater but ya learn something new everyday

Sure yeah can't bate the auld pot belly stove

I’ve had me derv heater on all day today and it’s running on solar batts are sitting at 12.7 and are at 98% I’m going out now to plug the van in for an hour now that the sun is down last night I turned the heater of around 9.00 and the batts where still good without plugin

Just to diverse a bit, are the heaters gravity fed or can they suck the diesel up from below

Thanks, just toying with the idea at the moment