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I am off grid using a Growatt 5000w inverter @ 240V (single phase). I want to know if the Chronomite SR15L 277V instant hot water heater can run on 240V power. This is because their 240V model will push the power limits of my inverter @4800W. Therefore, running the 277V model (4150W) at 240V would pull ~3595 watts (using ohms law) which is more reasonable for my needs.The power connections available: Ground, LINE and Neutral.

I’ve heard of running 277V strip heaters successfully on a 240V circuit, but obviously resulting in a lower output. The caution I’ve heard of regarding running on a different voltage are concerns with electric motors being used at a different voltage. What would be the concerns running the 277V instant hot water model on a 240V circuit?

https://www.chronomite.com/uploads/fileLibrary/Chronomite-Instant-Flow-SR-Electric-Tankless-Water-Heater-Low-Activation-Manual.pdf

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  • What is your primary house hot water source? Oct 11, 2023 at 20:01
  • @Harper-ReinstateMonica Currently have 20gal (75l) resist elec HWH. using ~8kWh/day during avg temps. much higher usage during winter 0-20F (~18C-7C). Oct 12, 2023 at 15:43

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If it's just resistive heat and (maybe) a bimetallic thermostat, and it draws 4150W at 277V, it should draw less at 240. Power is V^2/R, assuming constant resistance, so it should draw 240^2/277^2 times as much power. If my math is right that's about 75%, so on 240V you've got a 3025W heater. Somewhat less "instant".

If that's good enough for your purposes, and if my assumption that it's just a dumb heater are correct, it should work. If you were hoping to get all the wattage with lower voltage, you'd have to decrease the resistance and draw more amps.

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    No thermostat - it's controlled by a "flow switch" and that also seems unlikely to care what voltage it's switching. However, if the lower power does not warm the water enough for your purposes, you can't just slow down the flow to heat more - it shuts off below 0.35 GPM.
    – Ecnerwal
    Oct 11, 2023 at 17:04
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Consult with the factory but I don't see any reason it wouldn't work. The electronics will have a power supply flexible enough to handle voltage 15% lower than expected. Sizing could be a deal-killer, though.

Should be fine for tea

According to my math, the SR15L-277 will draw 13A (12.996A) on 240V. That will give it an effective power of 3120 watts.

The SR20L-277 will draw 17.32 amps at 240V and provide 4156 watts.

What concerns me here, however, is - if I was off-grid, with a 5000W inverter for my whole house, I wouldn't be prioritizing an auxiliary kitchen spigot for hot water for tea. Which worries me that you're trying to use this thing for your whole-house hot water supply, for everything from showers to dish washing. That's going to miss by an order of magnitude. Let's compare.

A British electric shower (with a notably low flow by American standards) draws 8500W, 9500W or 10,500W depending on model. My own rule of thumb is 40A (9600W) per GPM of flow you want. So your 13A setup is going to provide 1/3 GPM or 12 seconds to fill a teacup.

As you can see, the numbers just aren't working for bathing or washing dishes. You're off by an order of magnitude. Sizing errors are the classic blunder with tankless heaters. So many projects fail (with much more power than yours) because the designer thought they could scrimp on capacity to squeeze it into an available electric service.

Don't use batteries for large-scale resistive heating

Since you're coming off an inverter, you're coming off batteries, which are still pretty precious at >10 cents a watt-hour. And that problably came from a constrained source such as solar.

Outside of momentary uses like toasters, microwaves or induction stoves, it is absolute madness to trade away such precious power for heat at only 3.4 BTUs per watt-hour. That rather tepid 34,000 BTU British electric shower, you have to rack 1000 watt-hours of battery to run that thing for Six Minutes. That's a whole Group 27 battery or three Nissan Leaf v1 modules.

You need a physics cheat code: heat pumps. Heat pump water heaters are now standard products at sane prices. They chill the room they're in, so some clever ducting of that space might be useful (either to capture the useful air conditioning, or vent unwanted cold). They are tanked heaters, so they'll need some space and some weight (30 gallons = 250 pounds of water).

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  • Good thoughts. Current tank uses ~8kWh/day (avg 333 running watts). The tankless unit will run max 8min/day. kWh= 3100(W) × .1333 (h/day) / 1000(W/kW) = .41kWh/day. Tank would be used 8 hrs/week. Would go from ~240 to ~23 kWh/mo. Oct 12, 2023 at 16:05
  • Smallest HPWH is 36gal (Chinese manuf makes a 20 gal, but no data). Current WH is 20gal (~75l) so wouldn’t fit. House is 395sq ft (~36.7 sq m). Estimated energy usage rb.gy/5ul0r ~53.6kWh/mo. Still higher than on demand option. Also, electric resist comes on @37F (~2.8C), and water heater area will reach this or below often in winter). Propane or NG not practical here. Oct 12, 2023 at 16:22
  • Battery storage 13.3kWh. On demand will use ~3.8% battery/day. Oct 12, 2023 at 16:33
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    @staticGenerator the tankless you are specifying is far too small for showers or hand washing. It uses so much less energy because it makes so much less hot water. I just want to make sure you're clear on that. Oct 13, 2023 at 4:27
  • Thanks, as you can see I considered all suggestions you offered, and they are appreciated. Life off grid away from city life has great rewards as well as many challenges! The tankless is for heating small amounts only. Oct 13, 2023 at 20:41

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