What is the demand factor for solar heaters?

[M.ABDULLATIEF]

I have power panel which feeding 10 solar heaters ( the electrical backup part in solar heater)
i need to know what is the demand factor i have to apply for this load in panel calculation

 

[beanland]

You will need to treat them as conventional electric water heaters because there can be times when there is no available solar heating, such as the dead of winter. For 10x different places, the NEC cites demand factors of 27% to 50%. It seems reasonable to use 50%. However, there is no specific guidance in the NEC for water heaters so any choice other than 100% runs a risk.

 

[ggunn]

You will need to treat them as conventional electric water heaters because there can be times when there is no available solar heating, such as the dead of winter. For 10x different places, the NEC cites demand factors of 27% to 50%. It seems reasonable to use 50%. However, there is no specific guidance in the NEC for water heaters so any choice other than 100% runs a risk.

Not to negate what you say about how one should treat the demand factor, but some solar water heating systems are so well insulated that they can still work very well when the ambient temperature is well below freezing.

 

[beanland]

GGUNN, I agree if the solar system uses a storage tank for the solar and a storage water heater there may be some real useful load diversity. However, it is not uncommon to use demand water heaters for backup.