When visiting the sites of many different solar water heaters one finds amazing claims of how much electricity is saved and what the believed payback period is.  Very few of these claims have the underlying calculations available so that potential customers can determine the reasonableness of the assumptions being used as well as accuracy of the calculations. 

This means that even the least impressive system can look amazing should the wrong assumptions be used or even wrong calculation be performed.

It is for this reason I've included the full assumptions as well as basis of calculations on this sheet and this will allow the user to determine reasonabless for themselves.  In addition, by providing the assumptions and basis of calculations it gives the user a better understanding of how the values are arrived at.

As you will see all assumptions are based on very conservative estimates and all values used in the calculation were obtained from independent tests performed by the University of Stellenbosch, South Africa's leading solar research institute.  

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Performance of Gilder Solar Geyser based on Results of Stellenbosch University Test (note that 3 metre heat pipes were used in test)
Watts of Electricity Saved when heating up until the following times:		12pm	4pm
Temperature at 12pm and 4pm respectively (Degrees Celcius)		43	52
Temperature at 7am (Degrees Celcius)		20	20
Temperature Differential (Degrees Celcius)		23	32
Litres including capacity in 48m of 32mm pipes less empty space in geyser		130	130
Joules of energy required to heat 1 liter of water by 1 degree Celcius		4180	4180
Total Joules of Heat Energy added and retained by the water water taking into account all losses		12,498,200	17,388,800
Joules of energy per Watt/hr		3,600	3,600
Watts of energy absorbed and retained by Gilder Solar for time period including inefficiencies		3,472	4,830
Therefore the electricty consumption in KWH saved outright per day on sunny days would be			4.83
Price per kwh			R 0.50
Therefore at 50 cents per kwh the amount of electricity saved per day of reasonable sunshine			R 2.42
Anticipated Electricity Savings over time with very conservative assumptions below		KWH	Rand Value
Assumption of on average 50% reasonable weather (usually most other solar water heater companies use a 70% good weather assumption)		50.00%	50.00%
Average Days per month		30.42	30.42
KWH energy savings per day up till 4pm as per calculation above		4.83	4.83
Electricity saving per month with 50% good weather assumption		73.46	R 36.73
Electricity saving per year		881.52	R 440.76
Megawatt hours saved per year from the grid		0.88
Efficiency of Gilder Solar Geyser within the 20 - 43 degree band based on Stellenbosch test
First were going to calculate the maximum surface area exposed to the sun for systems with 3m long pipes
Surface area Pipes		1.63
Length		3.00
Diameter pipes		0.034
Quantity of Pipes		16.00
Maximum Surface area of tank exposed to sun		0.35
Width of Tank		1.00
Height of tank		0.35
Maximum surface area of system exposed to the sun in m2		1.98
Average Watts per hour per m2 from 7am to 12pm according to Stellenbosch Test (ranged from 0 up to 1000 Watts)		500
Therefore based on surface area of tank exposed to the sun the solar radiation received by the tank per hour in Watts		991
Number of hours exposed to sun		5
Total watt/hours of the suns energy that reached the tank surface area		4955
Watts of energy absorbed and retained by Gilder Solar for time period including inefficiencies (see calc above)		3,472
Therefore the efficiency of Gilder Solar Geyser in the temperature band 20 - 43 degrees Celcius		70.07%
Cost of Gilder Solar Geyser per Watt of energy saving when compared to Photovoltaic Panels
Watts per hour generated by system given 1000watts/m2/hr solar radiation at 70% efficiency (as calculated above)		1,388.69
Retail cost of system including VAT with low volume production		R 2300
Cost per watt/hr of energy capacity of Gilder Solar Geyser		R 1.66
Average cost per Watt of PV panel assuming Monocrystaline panel (excluding cost of battery storage)		$4.00
Current Exchange Rate R/$		R 10.00
Cost in Rand per Watt for a PV panel		R 40.00
Ratio of Cost of Gilder Solar Geyser to equivalent capacity in PV panels excluding cost of battery storage for PV		0.041
Number of times PV is more expensive than the Gilder Solar Geyser per Watt of electricity saving		24
Recoupment period in years for the Gilder Solar Geyser based on the conservative cost and savings as stated above and assuming electricity price increase in line with interest (i.e. electricity price increase in line with interest on cost of geyser)		5.3 years