1)   Why should I use the photovoltaic system to heat water, if it achieves lower efficiency?

Yes, you are right, the efficiency of photovoltaic system (PV system) is about 15 – 18 %, whereas the efficiency of solar thermal system can be up to 85%, but it is the efficiency in converting solar radiation into usable (thermal or electrical) energy from a square metre. It means that if you use the PV system, you need to cover a bigger part of the roof with solar panels to get the same amount of energy, but it doesn’t mean that you have to pay more.




2)   Do I have to have a special water heater in order to use Solar Kerberos? We’ve had a water heater for only two years and it’d be a waste to throw it away!

No, Solar Kerberos is designed so that it can be used basically with any common type of water heater with a capacity of 100 to 200L.


3)   What if the sun is not shining – does it mean we won’t have hot water?

It certainly doesn’t – if there is not enough sunlight, Solar Kerberos will use electrical power supplied from the grid to heat water. Everything is done automatically, you don’t have to do anything. Besides, you can look at the well-arranged screen and see how much solar energy Solar Kerberos used (and thus saved) and how much energy it took from the electricity grid.


4)   How is Solar Kerberos installed? My water heater is on the ground floor and the photovoltaic modules will be on the roof, will any building work be necessary?

No, not really. There exist systems of PV modules installation for every kind of roofing material. And the biggest advantage in using PV modules is the fact that the modules are connected to the control unit only by two flexible cables. Therefore it isn’t necessary to do any cutting or plastering, etc. Furthermore, transmission losses are quite insignificant, so it means that PV modules can be installed at a greater distance from the water heater (on the roof of a garage or a garden shed, etc.) than in case of using the solar thermal system. Using solar thermal system requires that between the collectors and the storage tank there should be two well insulated copper pipes distributing the heat transfer fluid.


5)   Can Solar Kerberos be used with a “wet” heating element as well?

Certainly – using a “wet” heating element doesn’t mean that the conductors at voltage are in the water (it is used, or it was used in some boilers, or some of you may remember the “self-boiling cups” made in the USSR), it only means that if the element has to be changed, it is necessary to empty the water heater. If you use a so-called “dry” heating element, the only thing you need to change is the ceramic insulator, and the flange, which the insulator is inserted in, remains in the water heater full of water. However, using a “wet” heating element is not a problem. By the way, there are certainly more wet heating elements than dry ones in the world, apart from water heaters they can be also found in washing machines, dishwashers and kettles.



6)   I’ve read somewhere that if the water heater is connected to direct current, it may lead to electrolysis and damage to the water heater, is it true?

Your question is related to the previous one – we have indeed heard some such rumour, but you don’t have to worry. If electric current, be it direct or alternating current, got into the water, it would result in a failure, which would be dealt with by a safety component (fuse, circuit breaker, residual-current device).


7)   What is MPPT?

MPPT stands for Maximum Power Point Tracking – it is a feature the inverter in Solar Kerberos has and which enables it to apply proper load to PV modules no matter how much sunlight there is, thus getting maximum possible power. It is a technique commonly used in photovoltaics – it is practically impossible to get a PV inverter not using the MPPT system. Were I to explain it in layman’s terms, I’d compare it to a beautiful old sailing ship with a lot of sails. If there is a breeze, the captain (the MPPT control circuit) will give the order to raise all available sails (it will adjust the load applied to PV modules) and the ship will sail as fast as possible, given the current wind. But if there is a strong wind which might break the masts or cause other damage, the captain gives the order to lower some sails to let the ship sail fast and safely at the same time. The MPPT inverter in Solar Kerberos works similarly – it adjusts the load applied to PV modules. If Solar Kerberos didn’t have the MPPT inverter, you could picture it as a ship under full sail – if there is a maximum wind, both ships will sail at the same speed, but if there is not a maximum wind, then the ship featuring MPPT sails will go faster, and all it cost was just initially an extra set of sails.


8)   Does Solar Kerberos have an inverter or does it have to be used with a DC heating element?

Neither one nor the other. Solar Kerberos features a special DC/DC inverter with MPPT, so the output is direct current. However, a DC heating element does not exist – it always is only resistive load, and it doesn’t matter which kind of current passes through it. It’s the same as with light bulbs (you may remember old Edison’s light bulbs with a filament of tungsten wire, nowadays called “heat balls”, etc. :) ) Those were connected to alternating current in lights and to direct current in torches – it always is a resistance wire, which is heated as the electric current flows in it.


9)   Can I connect the PV modules directly to the water heater, if their combined voltage is 230V?

No, never try doing this! Although I’ve said that the heating element can be connected to either AC or DC, the thermostat and the thermal cutoff in the water heater cannot be connected to direct current. Direct current tends to draw so-called “electric arcs” – this is used for welding and this is also what made František Křižík famous. However, an electric arc can do considerable harm to thermostats and thermal cutoffs – it can burn electrical contacts, generate high temperature, melt plastics and it doesn’t usually need more than one attempt to do it. That’s why special DC relays or semiconductors are used for disconnecting, as electric arcs tend to form on these occasions. There are special water heaters, which can be connected to direct current flowing from PV modules, however they do not have the MPPT and thus their power decreases by about 40%, especially when there is not enough sunlight.


10)   What are the operating costs of Solar Kerberos?

Basically, there are none – you just install it and it starts working. The only costs are associated with standby consumption, which is about 2W. For comparison, the solar thermal systems need: power supply to the control unit, circulation pump, and what’s more non-freezing heat transfer fluid has to be regularly changed, checked and re-filled.


11)   How does the photovoltaic system work in winter?

Oddly enough, better than in summer – well, at least as far as maximum power is concerned, this is really higher in winter than in summer. Of course, there is less sunshine in winter and the sun is much lower, so the total volume of production is obviously lower but by no means insignificant.  Photovoltaic modules can generate electric power using the sunlight.


12)   What happens if I do not use the hot water? Will the modules get overheated?

Nothing happens. After heating water to the set temperature, Solar Kerberos switches the energy to charging, e.g. it can charge rechargeable batteries, supply power to various appliances or it can be left unplugged. Zero consumption doesn’t do any harm to photovoltaic modules and that’s another advantage over solar thermal collectors.


13)   Does Solar Kerberos need to be connected to electricity grid in order to work properly? I was unpleasantly surprised that the solar thermal system my friend has doesn’t work when there is a power cut!

No, it doesn’t. If the sun is not shining, Solar Kerberos uses the electric power generated by the photovoltaic modules. If it is not connected to the grid, it just means that at night you won’t be able to see anything on the screen or to change the settings. Furthermore, in the event of a power cut you can use Solar Kerberos as a solar backup generator – you can use it for charging a backup battery, which will provide, via a DC/AC inverter, power for essential appliances, such as gas boilers, circulation pumps, etc.

14)   How do I know how much solar energy was used to heat the water and how much energy was taken from the electricity grid?

Right on the Solar Kerberos screen – there are two “electricity meters”, which show energy consumption in kilowatt hours. You can also find there information about instant electric power, voltage and current produced by the solar panels, the inverter efficiency and other interesting data. If you use a system, which doesn’t have the MPP tracking inverter, or the solar thermal system, you just have to believe...



15)   An electrician connected Kerberos, the sun is shining, but Kerberos is not working – in the Converter menu I can see that there is 200V on panels, but the output is zero. What did we do wrong?

Nothing, everything is alright. Kerberos monitors situation in the sky for the first ten minutes and turns itself on when it is sure that it is not only a hole through clouds that is shining on the panels. It is a common means in the photovoltaics that increases the lifetime of such devices. Also in the morning, when the sun rises and in the evening, when the sun sets, there are always a few minutes of the solar power on the edge when, in case of an unsophisticated device, it cannot decide whether to be on or off and switches itself on and  off in fast intervals. This decreases the lifetime of some of the elements significantly and so our device is prevented from such operation by break.  Ten minutes in the whole-day time is not a lot, moreover, the break is during the edge – and thus there is very little solar power, so the loss is minimal and the lifetime of the device is significantly increased.   

When you install Kerberos for the first time, the sun may be high in the sky and there is a lot of power, however, Kerberos needs to make sure about the situation independently, thus there is a ten-minute pause after each turn on. Of course, if the sun is overshadowed by clouds at that time and the voltage drops under 120V, or if you disconnect the panels, ten minutes is counted again.