Puzzled by a/c water heating from PV excess power (iBoost, eddi etc)

[mrjohnson99]

I've just asked Marlec, who make iBoost, and got a non-answer. I've read the manual and understand the basics behind the unit.

Not an expert in any way, so expecting to be corrected.

I'm planning to put in a 2.4kw system with a 5kwh battery.

First principles first. V = IR. P = IV. So P = I squared x R. So heat from an immersion heater resistance is proportional to the current squared, up to the maximum rating of the heater. In round figures, a 3kw immersion (a/c) is 12A at 240V. I know it's not quite that simple for a/c.

Doesn't this mean if I can only spare 1.5kw for the heater it produces a quarter of the heat? At 0.75kw a sixteenth of the heat? Won't that mean very poor efficiency with a 3kw immersion?

iBoost says it will work with any immersion up to 3kW. Would I get more heating overall with a 2kW or 1kw immersion? I don't need rapid heating at all. The tank is currently heated by gas for an hour in the morning, so this would be substituted by solar heating the previous day. I have a 120l tank which, by my calculations, needs around 5.5 kwh for a 40C temperature increase.

My head hurts!

Thanks in advance,

Martyn

 

[efficientPV]

Most are just guessing what they actually need to heat water. Quick rule of thumb is 1KWH is needed to heat 6 gallons, that includes heat loss. Heat loss is between 1KWH to1.5KWH a day depending on temperature. I have 120V heaters operating on 60V and each tank sees only about 500W as I an extracting only excess power and the elements follow that 1/4 rule. Tall tanks are the way to go because water easily stratifies and lower section at lower temp ahs very low heat loss. The upper 15 gallons suffices for many applications and there is enough time for recovery. There are power meters to record daily use. I use one with a donut to slip one wire thru. The voltage section of the meter just has a 120V plug. With that I just double the wattage to get the real value. Only having to remove one wire makes things easier and a little safer than making 4 connections. Heating only lower section of a tank with PV makes for a slow recovery. There is often no need to heat the entire tank. There are smart systems with a middle heater and it learns your daily use and heats the appropriate sections for lower energy use.