I want to run my water heater on solar power (Newbie)

Re: I want to run my water heater on solar power (Newbie)

Roughly, a full off grid power system will cost around $1 to $2+ per kWH. Using a heat pump type water heater will be 2x or more efficient than a pure resistive water heater... However, below ~55F, the heat pump types start losing efficiency (check specifications for details).

Guessing you are in the Nashville Tn area--Your average lows are not too bad--but your record lows (-10 to -17F in winter) would probably force you to use an antifreeze closed loop thermal hot water system for heating (unless you could shut it down during winter). Which makes the whole thing a bit more complex and costly (plus more maintenance).

If you can build/install a solar thermal system (open or closed loop), it will cost much less money to install and operate (although, like anything with plumbing, there will be leaks and maintenance).

Is there some reason you want a pure electric hot water system? With a battery bank, it does make it very expensive (like you have worked out).

There are a fair number of posts about solar hot water/solar thermal systems (and links) in the Working FAQ thread.

-Bill

Re: I want to run my water heater on solar power (Newbie)

Calculating the size panel/heat required... I have not done the calculations before, so below is a guess/estimate. But because solar thermal panels are not as expensive, you can oversize a bit without too much extra costs.

Say, 40 gallons of water per day, raise temperature from 40F to 130F:

• 130F-40F = 90F rise
• 90F rise * 8 lbs per gallon * 40 gallons per day = 28,800 BTU per day
• 28,800 BTU per day / 3.4 BTU per Watt*Hour = 8,470 WH per day

So, the standard solar PV + Battery estimate would be... 4 hours of sun minimum (9 months of year typical for many regions in North America), 0.52 end to end system efficiency for off grid system, battery for 1-3 days and 50% maximum discharge (longer life). Nominal calculation:

• 8,470 WH per day * 1/0.52 system eff * 1/4 hours of sun per day = 4,072 watt array minimum
• 8,470 WH per day * 1/48 volt battery bank * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max battery discharge = 830 AH @ 48 volt battery bank nominal

Note, the above calculations are rough numbers (8lbs per gallon for water, and 3.4 BTU/WH) but should be close enough for government work.

The above is assuming a resistance heater--If you go with a heat pump in a mild climate, it would use about 1/2 the power (or less)--so array+battery bank would be about 1/2 the size.

Same calculation for solar thermal... Typical thermal system is around 60-80% efficient:

• 8,470 WH per day * 1/0.60 eff * 1/1,000 WH per sq.meter * 1/4 hours per day = 3.5 sq meters
• 3.5 sqmeter * 10.7 sqft/sqmtr = 37.7 sq ft

So a 5ft by 8ft thermal solar array should be more than enough for a typical 9 month a year 40 gallon per day (with pretty cold input water) system.

The typical rule of thumb for solar thermal is around 1/2 to 1 sq ft of solar panel per gallon of hot water per day... Which seems about right (my estimate above is towards the 1 sq ft per gallon of water). Too large of panel and you may boil the water during hot summer weather in the solar loop. You might mount the panel with a "steeper slope" so that it gets less energy during summer and more during pre and post winter time when there is less sun and colder weather/water.

-Bill "my swag" B. (scientific wild a$$ guess:roll:)