Practically speaking, it seems just going with an AC cooktop is the way to go. I have a Duxtop one that shows it uses 1800W. My West Marine inverter puts out a modified sine wave at 1500VA (4000VA surge max 15 seconds). Not sure how to convert the two, but the cooktop does not heat when powered by the inverter, so I assume output of the inverter is insufficient. Is there a cooktop that would work with my inverter? Any recommendations?
I'd get a 2400W pure sine wave inverter, like the victron 3000/120 (3000VA). You will have to upgrade your electrical system to match (cables, fuses, possibly battery bank size). Voltage drop is the enemy here.
Anytime you power sensitive electronics, a pure sine wave is preferred. Even the modified sine wave inverters can have a pretty rough ac output compared to shore power or a generator. You need an inverter that can react quickly to the switching on off of the cooktop to maintain a steady voltage, so oversizing helps in this respect. The 1800W cooktop could draw up to 1900-2100VA (volts-amperes) continuously due to the nature of the internal electronics.
When it comes to sizing inverters, it's the loads VA that matters, not the watts. Watts only apply to simple resistive loads, like a water heater.
A 1000W microwave will draw around 1600VA (power factor of microwaves is around 0.5 to 0.75)
The power factor of a quality induction cooktop should be 0.95 or higher (assuming they have power factor correction circuitry).
Recommended setup:
Mid grade hobb like duxton or changbert
Victron 3000/120 inverter
4/0 or (smaller if paired) conductors from the battery to the inverter (note the abyc requirements for fusing parallel conductors)
Class t fuses
300Ah or larger lifepo4 bank, recommend around 600Ah. Check the continuous rating of the cells and the bms.
Charging system that can keep up (alternator, solar, etc...)
Low loss connections /Crimps everywhere (see Will Prose
video on youtube)
Yes, a simple change can snowball quickly.
Why lifepo4? Because they maintain a much higher voltage under load compared to lead batteries (they have lower internal resistance). You'll typically see around 13.1 to 13.3 volts under 100A with lifepo4, whereas you'd see 11 to 11.5V under same load with a lead acid battery bank, and much more internal heat build up in the lead batteries, reducing efficiency.
Going to induction for an off grid boat is something I would do only after modernizing the rest of the electrical system, it should be the last step. Building a boat to use only electric heating is not cheap, it's much cheaper to keep using gas UNLESs you are crusing the world and don't want to deal with all the different types of connections or you are in a very hot climate and don't want to heat the boat as much when you cook or put a lot of moisture in the boat 9very cold climate).
You're looking at at least $8000-$10 000 including batteries, wiring, fuses, bus bars, solar, alternator, regulator, etc... and about 2 weeks of work, if you know what you're doing. Plus electricians time.
In the pacific NW, 400-600Ah of lithium and 600 watts of solar will serve most under 40 ft boats well and be self sufficient for power off the grid from late April to early September. On really good days, you can even heat hot water tank as a dump load instead of waisting solar after the batteries are full. This would work for short simple meals on induction (coffee, eggs, soup) anything you can cook under 5 to 10 mins. Longer than that and you'd need more batteries and solar or keep the propane stove for the more intricate cheffy meals.
The other important consideration when sizing the dc system is the efficiency and daily draw of your refrigeration sytem. Some under 40ft boats draw 60Ah, other 150Ah a day just to make ice and keep the beer cold. We know we draw 4.5 A on a 15/14 min duty cycle in the summer, A bit more if the starboard side is in full sun in the afternoon (up to 15 min on, 8 min off).... there's more insulation to be done between the hull and ice box. 60Ah for beer, a pot of coffee on induction (kettle) costs about 6-7 Ah. Heating the hot water tank costs about 70-85Ah.
Here's a good test... can you run a hair dryer off the inverter on high setting for 12-15 minutes with nothing tripping , nothing getting hot (crimps, bus bars, fuses, battery switches, battery itself) and battery voltage staying above 11.5v? If the answer is no, then your electrical setup needs work before considering off grid induction stove use.
