Which water heating element

[jamiegreen]

Currently I live completely off grid with a 2kw solar array (limited to 1.6kw by cc) and 6kwh of lifepo4 with diesel geni backup. I'm finding now coming into summer that when I'm out most of the day my batteries are constantly sitting in float and I'm wasting potential solar power. I've got a 250l hot water cylinder that supplies my home, at the moment I'm heating this via LPG, the 3kw element is too much for my victron 3kva inverter to run for long periods.

I had a choice of a 1.5kw or a 2kw element, I bought a 2kw element with the idea that it could turn on at say 98% battery and off at 80%, which would let the battery drain slightly and charge back up instead of sitting full all the time (not great for lifepo4?).. but also I don't want to degrade with too much cycling, but would cycling the top 20% really hurt it at all?

If I swap it for a 1.5kw element, taking into account they usually don't draw as much as they claim + inverter inefficiencies it may draw around the 1.6kw mark, but would have very little if any drain on the battery.. not sure if that would be better or worse.

Any opinions appreciated on which would be a better wattage element.

Thanks

 

[newbostonconst]

The 1.5kw element is what I use. In your case it is matched to the array size so power will move more directly from the array to the element without discharging the battery.

What are you using to trigger it?

 

[jamiegreen]

The 1.5kw element is what I use. In your case it is matched to the array size so power will move more directly from the array to the element without discharging the battery.

What are you using to trigger it?

Probably be using the AC out 2 on the victron inverter with a wizard set up to trigger using SOC. And another wizard to give household power priority to switch the immersion off if a household appliance overloads the inverter output. Maybe the 1.5kw would be a better idea, I was just thinking the 2kw would keep the battery from reaching 100% all the time I'm not using it.

 

[acdoctor]

About the LFP battery staying out of the knees will extend the life. If you have say 50% left in the morning. Don’t charge as full unless you see cells getting out of balance. Then you need to charge to the top to get BMS passive balance working.

 

[jamiegreen]

About the LFP battery staying out of the knees will extend the life. If you have say 50% left in the morning. Don’t charge as full unless you see cells getting out of balance. Then you need to charge to the top to get BMS passive balance working.

If my batteries are at 100% at say 8pm, generally by the next morning they'll be down to 85%.. this will drop more as I add more appliances in future. Only in the winter does the battery get full cycles, but they are grade A and in the past 6 months the BMS hasn't had to do any real balancing. I may well drop the top charge down a little.. think it's set around 27.8v right now with a max cutout of 28v. The immersion heater is a bit of a rock and a hard place for me here with either only using the solar at 1.5kw or using say.. 500w extra and cycling the top end of the battery repeatedly.

 

[AntronX]

If you have charge controller with solid state relay diversion load output then you can use it to turn on 48V DC water heater element. They are sold on ebay about $45. I used same method to divert excess solar power into resistor bank using homemade SSR.

 

[efficientPV]

Since you are talking about 1.5 and 2KW heating elements, you are probably talking about powering these with 120V. Any 240V heating element can be used and it will be 1/4 of the rayed power giving a lot more options.

 

[jamiegreen]

If you have charge controller with solid state relay diversion load output then you can use it to turn on 48V DC water heater element. They are sold on ebay about $45. I used same method to divert excess solar power into resistor bank using homemade SSR.

I'm going to use the inverts aux features to do this through its mains output, my solar is only running around 75v and the tank is a little way away from the cc so I don't want to have to run thick cables.

 

[jamiegreen]

Since you are talking about 1.5 and 2KW heating elements, you are probably talking about powering these with 120V. Any 240V heating element can be used and it will be 1/4 of the rayed power giving a lot more options.

Sorry should have specified, I'm in the UK so already using 240v.

 

[newbostonconst]

Since you are talking about 1.5 and 2KW heating elements, you are probably talking about powering these with 120V. Any 240V heating element can be used and it will be 1/4 of the rayed power giving a lot more options.

I use 120VAC on mine also....DC is harder to do then AC....cost more, harder to get, wears out quicker....Talking the Relay and wire....SSR's suck

I have excess capacity on my inverter so it is no issue.

 

[efficientPV]

With dual element heaters you can add a switch to use just one element for higher production outside of the two hours at noon or on cloudy day.