The short version. Total interior volume in cubic feet X 15 gives the estimated BTU required. Convert between KW and BTU
here. Chinese diesel heaters are pretty much 5KW no matter what they claim.
The longer, pre edit, version.
Here's a very detailed discussion of sizing your heater.
AMC Cliffy's Which One do I need As you work through the guide near the bottom you'll come to a chart for the area multiplier. I've also seen sources that say to use 15 for the multiplier. I think Sure Marine and others use the 15 multiplier. Where I disagree with this guide is to exclude cabinets, bunks and other built in spaces. Maybe OK for mild winters and occasional uses. But I lived aboard in Seattle for years and went through one of the colder winters for the area. Days of snow on the decks and docks, icy streets, below freezing. Those mentioned closed spaces were cold and affected the comfort of the boat negatively.
Since this is a thread on Chinese diesel heaters they are all rated in KW. You'll need to convert between KW and BTU. Lots of tools on line. Here's one
Stoves Online KW to BTU conversion.
Next dig around on You tube about these heaters. Decide which of the contributors you think are knowledgeable and truthful. I came to the conclusion that so call 8 KW Chinese heaters are 5 KW. Not necessarily a problem if you know that going in and if needed install multiple units.
Finally consider your use and acceptable level of comfort. My current boat has an Espar D8LC a true 8 KW, equals 27312 BTU. Indications are it heats a 45' boat well in relatively mild weather, say into the lower 50s at night. That's going to be good enough for our intended use. PNW late spring through early fall. My previous boat at 42 ft had a Hurricane Hydronic rated for 35,000 BTU, equals 10.255 KW. That heater worked very well down into freezing temps. I took advantage of zones which can be a big advantage. On a very cold night I could set the saloon and fwd cabin temps low keeping the aft cabin where we slept warmer. That way the heater didn't have to run as much meaning lower electric consumption during a cold night on the hook. A nice advantage to hydronic heat is it keeps your water heater up when it's running. Also, I don't have enough data yet, but it seems the total electric draw on similar nights was less for the Hurricane that for the Espar.
One more case, my live aboard in Seattle. I put in an air heater no longer available. Marketed as though it were made in Sweden. Here's a
press release from 2015 I don't recall which size I purchased. I recall it was not near enough for a 40 ft liveaboard. And it died the 1st winter. It does bear a remarkable resemblance to the Chinese heaters we now see.
A buddy with a 42 ft boat has the Hurricane but rarely uses it for his spring - fall cruising. He's installed a diesel drip heater in the saloon and they are happy with that.
Lots to consider. No simple one size fits all solution. I will say if I were going to heat a 40 - 45 ft boat for cold weather cruising with the currently available Chinese heaters I'd put in two or three. Greater capacity, possibility to set up zones and redundancy. Of course you'd need multiple exhaust ports and fuel supplies.
What is a good general rule of thumb for sizing a diesel heater?
