# Check my numbers

Solar Expert Posts: 127 ✭✭
The company I work for makes glass that is used in assembling PV panels (Corning Inc). I was asking around trying to see if there was a way I could get panels as a test bed sort of thing. That isn't happening but it turns out that one of the Scientists that works on the project just installed a 5Kw grid tie system and since I am looking for a company/installer to work with I approached him and we had a long conversation. At some point as I am describing my plans (off grid/ battery storage/inverter system) he tells me about installing one of the GE heat pump water heaters. I have seen them and they are efficient and all but he was making a point of saying how good they would be for an off gris system. Where you could set it up to heat your water during the day when you are generating energy and how much cheaper it would be than using propane. His point of energy storage was understandable but my point to him was that the energy the a PV system generated and used for the heat pump would only be energy that would be usable as hot water not as wattage that I would need for lights and such at night. Unless I am missing something. Here are the numbers.

I am looking to installing a 3-5Kw of panels .he exact Ah of batteries is not firm but maybe a day or 2 expecting to use 2-3Kw a day (maybe more). The heat pumps rating is 1850KwH per year or (1850/365=5Kwh per day). Doesn't this mean that most all if not all of the wattage I could generate would be used to make hot water using a 3-5Kw system? Or am I missing something?

• Banned Posts: 17,615 ✭✭✭
Re: Check my numbers

The short cut answer: 5 kW * 4 hours * 50% overall system efficiency = 10 kW hours AC possible from the panels.

In terms of batteries 5 kW of array would charge up about 800 Amp hours @ 48 Volts. At 25% DOD that would be roughly 9.6 kW hours DC, pretty close to the expected AC panel harvest.
• Solar Expert Posts: 127 ✭✭
Re: Check my numbers

So I could make hot water but not have lights at night? Is that correct?
• Banned Posts: 17,615 ✭✭✭
Re: Check my numbers

Based on those numbers you'd still have about 4kW hours to spare per day.

The cheapest way to heat water off-grid is direct solar.

The heat pump water heaters don't always work that well because they are climate dependent. As for it being cheaper than propane ... weigh the extra cost of the heater & enough PV to power it against the propane. It doesn't look very economical here, but fuel could be higher where you are.

BTW, that "extra" 4kW hours per day is nearly 2 times what I use off grid in total.
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: Check my numbers

The cheapest way to heat water off-grid is direct solar.
This is one of the greatest truths in solar energy. I am there, and I am doing it. Have been for years and wouldn't have it any other way!
• Solar Expert Posts: 1,071 ✭✭✭✭
Re: Check my numbers

The amount of electric you use in the heatpump waterheater is dependent on how much hot water you use. I have the Ge geospribg waterheater with a utility meter on it to monitor the power usage. It used around 497 kwh the first year. It has 2 other advantages. It dehumidifies the air and cools of my utility room and kitchen. ;)solarvic:D PS today it is hot here and I am washing clothes, running the dishwasher, and going to take a long shower so I can keep my house cooled down without running the air conditioner.
• Solar Expert Posts: 1,484 ✭✭✭✭
Re: Check my numbers

Heating water with solar electric is inefficient whether it is direct heating element or heat pump. Heat pumps are about 200% better on delivered BTU's for kWH of consumed electric. Most claim 200% to 250% which is due to additional tank insulation on most heat pump hot water heaters compared to normal low cost direct element hot water heaters.

From GE spec sheets, its 50 gallon unit can recover 8 gallons per hour solely using heat pump with ambient temp of 68 degs F and hot water of 120 degs F running 550 watts of power.

For the 8 gallons of reheating data, this translates to a BTU/hour generation of 3,328 BTU's/hour rate, consuming 550 watt-hours.

By comparison a direct electric heating element of 4500 watt electric element generates 15,365 BTU's per hour, consuming 4500 watt-hours.
This is 4.6 times BTU's/hr. with 8.2 times the kWH's consumed.

Heat pump is 1.8 times more efficient in BTU's per kWH, but has low raw BTU delivery rate resulting in longer recovery time, 22% of direct electric element. This is fine if you have a large enough reserve tank and not too many folks taking showers in short span of time. 8 gallons is a pretty skimpy shower, way below my daughter's consumption amount. Maybe resonable if you dilute 120 deg water down to 90-95 degs F.

For a direct solar hot water heater you can achieve 40-50% efficiency on the 1kW/m^2 or about 450 watts (1550 BTU's/hr) per sq meter of collector. SolarPV will be about 10-12% overall efficiency for the 1 kW/m^2 collector area or about 110 watts (375 BTU's/hr). With a heat pump that would yield about 2x or 750 BTU's/hr. This is about half what a direct solar hot water panel will yield.
• Solar Expert Posts: 2,397 ✭✭✭✭
Re: Check my numbers

I have a single 4x10 direct flat plat panel, family of 4 mostly, 80 gallons of 150F pre storage, 50 gallon gas heater backup set at 110f in summer for 130 gallons of hot water available. It provide all the hot water for our needs, small pump to panels runs very little in summer.