WHAT YOU MUST ASK BEFORE PURCHASE?
 
Key data and characteristics of equipment which you must know

Customer often feels lost at selection od the components. He is buried with large number of data about
device or gear which he needs to purchase. Each seller mostly stresses advantages of his product.
There is very rarely friend who knows something about renewable power supply systems. Under those
conditions, purchase of expensive equipment could become lottery.

However, for every device there is several key data which greatly determine its operation and contribution
to general quality and durability of the system. Hereafter we cite most important data about various devices
which is relevent to request from reseller and explanations of meanings and importance of those data.

Solar modules


a) a) What are rated power tolerances of the module? What is the real power of the module?




At comparison of different modules, most important is to value real amount of energy which our module produces. Many factors affect the production, but most important is the power of module. At assessment, it is important to compare minimum power of module, not its rated power. For example, rated power of module is 100 W. If the manufacturer specify that the power tolerance is -10%, then with you can count only on 90 W. If another manufacturer specify power tolerance 0%, that module can realy count with all 100 W which is significant saving. Besides that, modules with lower tolerances from rated values, are suitable for applying in bigger systems, where modules are often connected strings. How much is tolerance from rated power, (what is minimum power of the new module), everyone can simply check, and that is one of essential index of general manufacturer quality.

b) What is the power-loss of the module due to temperature increase?
  Power of solar modules decrease with temperature growth, and increases with temperature decrease. In summer sun radiation is much stronger and annual loss of energy is bigger if the power dependency of the temperature is more expressed.For example, loss of power is 0,6%/oC (typical value for most modules). In summer module usually warm up to 65oC. Rated power of module is declared at 25oC. Our module will for temperature rise of 40oC, lose 24% of power (40 x 0.6 =24). Some other module, with temperature coefficient 0,2% under same conditions will lose just 8% of power, respectively three times less.

c) Manufacture completely controls all phases of production process from raw silicon to final module?
  Such manufacturer can guarranty quality and control characteristic tolerance from rated values.
 
Wind turbines

a) What is warranty period?
  Wind power turbines during operaton are exposed to extremely hard conditions (gail force hits,
vibrations, ice accumulation etc.). Elimination of failure is expensive because of complicated
approach and working conditions (mounting on mast).

b) What are references and manufacturers credit and how long is he present on the market?
  Attested and proven technology with strong references significantly decreases probability of failure occurence.
 
Solar charge controllers

a) Could controller charge battery with significantly higher current than the one that solar modules deliver      (MPPT tehnology)?
 

Almost all solar charge controllers on todays market, does not permit solar modules to work with voltage
at which the power is maximal. During charge, controller connects solar module directly to battery.
Therefore module voltage is forced (reduced) to battery voltage, and current of the module stays
about the same. Since the electric power is equal to product of current and voltage, the result is
loss of power which is proporcional to reduction of module voltage. Sequent is great loss of energy
(up to 30%), in solar systems.

MPPT charge controllers provide power loss elimination (Maximum Power Point Tracking)by keeping
always solar module voltage at the point at which the power is maximum. For maximum power,
solar module voltage is higher than battery voltage. MPPT charge controller acts as an transformer
connected between solar modules and battery. During transfer of power from module to battery,
charging current increases proportionally to difference between solar module and battery voltage.
Boost of charging current in relation to other type of charge controllers is bigger if the battery is more
empty (more charging when more is needed).

Until now, MPPT controllers (because of high price), have been applyed only in large solar systems
where saving on number of modules justify the investment in controller. Finally, by year 2005, price
barrier fall down, so that in the smallest solar systems(above 80W), usage of this revolutionary charge
controllers is justified.

b) What are permited ranges of solar module voltage and battery voltage?
  Wider voltage borders of charge controller provide easer adjustment to requirements of the system.
Bigger power solar modules often have voltage different from 12V respectively 24V. Battery voltage
is (in case of extending solar system), suggested to be increased because of saving on price of
charge controller, fuses, switches and cross section of cable.
 
Batteries

a) Diagram of Cycle number of the battery?
  Continuous charging and discharging (especially deep discharge), drastically reduce lifetime of
each battery. Number of charging-discharging cycles during battery lifetime is most important
data for evaluation of battery operation. Number of cycles is smaller if the depth of discharge
is bigger. Dependance of possible cycle number regarding to depth of discharge is shown
by diagram. Ask reseller to show you such diagram.

b) What is the real capacity of the battery?
  Battery capacity depends on the speed by which battery is discharged. If the discharge time is
shorter, available capacity is smaller. Because of that every manufacturer beside capacity should
notice the discharge time (h). Solar batteries are usually declared with 100h discharge,
(for exp, 300Ah, C100). High quality professionall batteries are declared often with 10h discharge
(for. exp. 300Ah, C10). Relation between 100h and 10h capacity is about 1,25
(if the 10h capacity is 200Ah, that battery has 100h capacity of about 250Ah).

c) What is the lifetime of the battery? What is the weight (mass) of the battery?
  Lifetime of quality batteries is about 10 to 15 years. In solar systems, lifetime could dramatically
decrease If adequate battery with appropriate charge-discharge cycle number is not chosen.
Greater weight of the battery (with same capacity) regularly demostrate the bigger operation
capability, and often the quality also.
 
Inverters

a) Is output voltage waveform pure sine or some other?
  Grid voltage, for which are estimated all our AC loads is pure sine waveform.
That kind of waveform is free of distorsion and annoyance which would interference with load
operation. Beside sine wave, at the market are inverters with square waveform under different
names: modified sinewave, trapezoidal waveform and likewise. These inverters can also be quality
products, they have lower price, but before mounting it is advisable to check that your load will not
have problems with this kind of inverters.

b) What is the continuous power of the inverter?
  Continuous power provide evaluation of load supplying possibilities in normal operation. This data
certainly have to be stated because many products on the market are declared with power for
restricted operation time of 30min or even shorter. Correct manufacturer will notice continuous
power and also short-time power. In any case, careful investigation of way that the inverter power is
declared needs to be done.