electric boilers?

farmwi

Spent a lot of time and math thinking about this question.
Short summary. Cheapest might not be what the customer wants.

Insulation (but he didn't ask for it.)
Storage, lots of it, 3 days or more. Hot water, insulated tank/box, copper HEX. (he didn't ask for it)
Solar Heat Panels are 3x more efficient at collecting heat that Electric per square foot and cheaper. (he didn't ask for it)
Set solar heat panels at winter angle, and you'll still have too much in the summer, they will need covers or need to be drained or a fluid that won't boil, or you need a way to waste heat.
High efficiency boiler, but maybe if the need is large enough or off grid a goal? A cogen natural gas generator. (didn't ask for it, but does he want a generator? Make power store the heat.) One type. https://www.controlledair.com/yanmar-chp-micro-cogen/
A cheaper off grid solution, pellet stove boiler, put heat in storage, oft considered CO2 neutral depending on pellet source.

Solar Electric Heat, via battery storage, by far most expensive. Least solar input during the winter, snow on panels, clouds in sky, least sun length of day, during the greatest need for heat. If they are going to heat with solar electric, use heat storage.
Most off grid storage are 48v nominal of any size. Many on grid without batteries are either individual inverters or string with voltages up to 600v. High voltage strings can start inverting with less solar input. High voltage DC batteries are starting to show up. Battery considerations. Lose 15% of pwr while charging and and again while discharging slowly, if your discharging quickly, say to heat a house, a 30% loss (unless you absorb heat from the batteries) and a rapid lifespan reduction.
Still wants to, Heat Pump (+ CoGen, make power for heatpump and collect heat from making power).
How to get even less heat, (+ CoGen and electric boiler)
Even less heat, Solar panels + batteries + electric boiler, maybe house insurance for batteries.

Hot_water_fan

Heat loss comes first. Slant Fin doubled checked by fuel usage.

PV panels + air-to-water heat pump (or electric boiler depending on comfort-level/availability) + gas backup seems the easiest route. Gas backup needn't be condensing as it shouldn't run that much ideally. No need for storage.

LDT

Amory Loving said it best decades ago, using (electricity) to heat...is like cutting butter with a chainsaw. Electric boilers went out years ago - the all electric alternative nowadays is the heat pump; much more efficient.

Jamie Hall

To reiterate. Step 1. Heat loss. What is your power requirement (BTUh or kilowatts) to heat the place. Step 2. If there are existing emitters in place (hot water heat) what temperature water will they require to provide that much power. Step 3. Can you meet that temperature requirement with an air to water heat pump. Step 4. If the answer to Step 3 is yes, how much solar power can the proposed (or maximum feasible) array harvest? For guidance, in your area one normally figures on requiring a minimum of 8 to 10 times the power capacity in the solar array relative to the power demand in the load if the project is to be truly net zero or off grid; if you don't have that capacity, bluntly, the solar array becomes fancy shingles with a green tint. Step 5 Determine your battery storage. Again, a rough guide for net zero or off grid is a capacity, in kilowatt hours, of 24 times the solar array output, in kilowatts. Step 6. Now spec. out your inverter(s) and switchgear.

If the answer to Step 3 above is no, install gas or oil sized to handle the load from Step 1. There's no harm to adding air to air heat pumps for the shoulder seasons, the advantage of which is they will also give you AC.

Hot_water_fan

For guidance, in your area one normally figures on requiring a minimum of 8 to 10 times the power capacity in the solar array relative to the power demand in the load if the project is to be truly net zero or off grid; if you don't have that capacity, bluntly, the solar array becomes fancy shingles with a green tint.

@Jamie Hall you give great advice on the Wall, however, this is way off base.
First, you need to accept that solar panels do not need to supply every kWh for a house. Just because solar covers 50% of the usage does not mean its impact is nothing but a "fancy shingle."
Second, a house with solar does not need to be off-grid or net zero. The vast majority (probably 99%) are not. Focusing on 100% storage requirements is unnecessary unless stated. Even if storage is included, it needn't cover the whole load. Some utilities actually subsidize grid connected batteries because it helps them.

ArthurPeabody

A resistance heater doesn't care whether the electricity is AC or DC. Thus, inverting the output of the solar panels wastes energy. Doing that would require the heater having a separate circuit from everything else, but for this large an installation it would be worth it.

I'm not a professional - I joined for some advice on my home AC, stayed for Mr Holohan's stories - but I am a physicist who has lived off the grid (in a location I had no choice) and figured out how to make everything DC (but heated with wood, both space and water.)

I think an honest assessment will deter your customer. Solar water heaters and heat pumps are a better idea, even if it also has an electric boiler, which would be smaller.

yesimon

Most off grid storage are 48v nominal of any size. Many on grid without batteries are either individual inverters or string with voltages up to 600v. High voltage strings can start inverting with less solar input. High voltage DC batteries are starting to show up. Battery considerations. Lose 15% of pwr while charging and and again while discharging slowly, if your discharging quickly, say to heat a house, a 30% loss (unless you absorb heat from the batteries) and a rapid lifespan reduction.

Good overall points but a few corrections on solar/batteries. Solar panel voltage is fairly constant regardless of irradiance so the benefits of lower activation voltage ratio for string inverters compared to microinverters is very minor. Almost all home batteries are now lithium-iron-phosphate (LFP) chemistry. It has ~92% round-trip efficiency similar to other lithium batteries. Due to its robust olivine crystal structure, LFP efficiency and battery degradation is not harmed by high speed charging/discharging up to 4C, equivalent to a full discharge in 15 minutes. A typical residential battery will have discharge rates much lower than that.

GroundUp

I'll try to stay on topic instead of bickering about details irrelevant to the question asked. Bottom line is this: yes, electric boilers can be great. I have hundreds of them out there saving their owners money every day versus propane. Heat loss calc will determine the size needed. A look at the oil bills for the past few years will get an idea on how many gallons are used per season. Multiply that number by 112,000 and you have approximate BTU usage for the year. Divide that number by 85,000 to determine gallons of propane used, or by 3412 to determine kWh of electricity used. Multiply those two numbers by their respective cost per unit, and the result will give you the approximate annual cost comparison.

For example: 500 gallons of oil is equivalent to roughly 56M BTU
56M divided by 85,000 is 659 gallons of propane
56M divided by 3412 is 16,413 kWh of power

If propane is $3/gal, it costs $1977 per year
If power is $.12/kWh, it costs $1970 per year

Insert your own variables for local pricing, and you have your answer.