In this article we will talk about the relationship that must exist between the number of solar panels and batteries to achieve a good design for the system.
The appearance of a large number of online stores for the sale of photovoltaic solar energy products and the incessant offers in cheap solar kits represents a great competitiveness in the photovoltaic sector that a priori, should benefit the private customer since in this way they can access very competitive prices that were previously reserved only to professionals in the sector.
Too Many Solar Panels and Few Batteries – One Mistake!
But unlike the professionals in the sector, the private customer does not know how to differentiate between a good solar kit design and a bad design , between good batteries or poor quality batteries. So we only have to trust the seller who advises us and possibly compare the results with other sellers to get an idea of whether they are cheating us and if the seller is trustworthy or not.
But what happens if what one seller tells us or another is nothing like? What if, in addition to not looking alike, the prices are very different from each other?
The result is that we find ourselves lost in a price war between online stores that may not benefit us at all.
The trend of online stores is to lower costs , but in most cases costs are reduced to the detriment of quality. And this is not at all beneficial for the end customer who ends up trusting the seller who gives him the most confidence, but who finally competes FORCED for the low price MARKET , I give you several examples:
1.- MANY solar panels and FEW batteries.
The great competition between online stores has favored the offering of photovoltaic solar kits with an excessive design of solar panels that supposedly can cover a very high consumption demand and with very small capacity batteries for that promised consumption.
The reason for these offers is very easy to understand , batteries are very expensive and solar panels are very cheap. Assuming that 100% of the consumption was carried out during the hours of sun and coinciding with the solar production, in an ideal case in summer we could assume that no energy is consumed from the batteries and the design would be correct. But in reality:
The result of this bad design is also very easy to understand . The actual consumption is partly carried out at night, the solar production of the panels in winter is practically half that of the summer, in addition to the existence of cloudy days, they mean very high depths of discharge of the battery, reducing the useful life of batteries to a few years .
Let’s see a very common practical case with an estimated consumption of about 3500Wh / day with use for the whole year.
With a power of 1600Wp installed solar panels, which would be 5 panels of 320W or 6 panels of 270W, we have the following solar production:
|Zone 1||6368 Wh / day||2944 Wh / day|
|Zone 2||7904 Wh / day||3232 Wh / day|
|Zone 3||8384 Wh / day||4160 Wh / day|
|Zone 4||8560 Wh / day||4512 Wh / day|
|Zone 5||8256 Wh / day||5104 Wh / day|
* Official data from Photovoltaic Geographical Information System (PVGIS) calculated with total losses of the Photovoltaic system (temperature, wiring, solar panels, etc.) of 24.7%.
- Zone 1: Pontevedra, A Coruña, Oviedo, Santander, Bilbao, Vitoria, San Sebastián
- Zone 2: Ourense, Lugo, Burgos, Palencia, Valladolid, Pamplona
- Zone 3: León, Zamora, Salamanca, Segovia, Soria, Logroño, Huesca, Lleida, Girona
- Zone 4: Ávila, Madrid, Toledo, Guadalajara, Ciudad Real, Córdoba, Castellón, Valencia, Palma de Mallorca, Jaén, Granada, Málada, Cádiz, Murcia
- Zone 5: Cáceres, Badajoz, Huelva, Seville, Albacete, Alicante, Almeria, Santa Cruz de Tenerife, Las Palmas, Ceuta, Melilla
We choose Madrid as an example and we see that solar production in July can reach up to 8,560Wh / day , but in December, on the other hand, practically half 4,512Wh / day.
Can we say then that the solar kit will cover a consumption of 8,560Wh / day?
Obviously the answer is NO . Since such a high consumption in December is not possible. The correct thing would be to say that solar production will cover 3,500Wh / day because the worst radiation conditions are in December where we can cover that consumption. Obviously in summer the consumption during the day may be higher.
What battery capacity will be necessary to cover the consumption of 3,500Wh / day?
- 3,500Wh of consumption x 3 days of autonomy = 10,500Wh; To use only 50% of the battery we multiply that value x 2 = 21,000Wh.
- A 21,000Wh battery at 24v we simply divide 21,000wh / 24v = 875Ah (from battery to 24V)
Therefore, at least we should put a battery at 24V of about 875Ah, we would choose a stationary battery with the closest real capacity: Hoppecke 6 OPzs stationary battery of 900Ah (we would need 2 12V units to form the 24V system)
The quick way to perform the calculation would be to multiply the daily consumption x 6, therefore we would have:
3,500Wh x 6 = 21,000Wh; with 24V battery it would be 21,000W / 24V = 875Ah (24V battery)
What do we find on the internet?
Unfortunately we find everything. I give you an example:
Solar kit for daily consumption 16,000Wh / day consisting of:
- 10 solar panels of 320W (which are 3200Wp) that as we have seen would produce about 16,000Wh / day in July in Madrid.
- Battery made up of 8 6V (48V) elements with 550Ah capacity; what they suppose 26,400wh; which are 48V x 550Ah
IMPOSSIBLE CONSUMPTION: As we have explained previously. 16,000Wh / day would be possible to consume only one day in July when consumption is 100% daytime. If we have part of the consumption at night, we will consume from the battery and each day we will discharge the battery more and more. This consumption in winter would be impossible.
UNDER-SIZED BATTERY: With the quick calculation, the battery that we have seen before, it should be 16,000Wh x 6 = 96,000Wh and with the battery at 48V it would be 96,000wh / 48V = 2000Ah. Which is practically a quarter more than what this solar kit includes . In other words, the battery of this design does not have a day of autonomy , so the daily deep discharges will be practically 80% -100% if our consumption is 16,000Wh / day. As a result, this battery will die in about 2 years.
VERY HIGH DISCHARGE DEPTHS: Battery manufacturers provide us with the number of life cycles of batteries . And they tell us that the greater the depth of discharge of the battery, the fewer life cycles they have. Take for example Hoppecke stationary batteries , possibly the best brand of stationary batteries on the market:
- 8000 cycles for depths of discharge of 20%. About 20 years.
- 3000 cycles for depths of discharge of 50%. About 8 years.
- 1500 cycles for depths of discharge of 80%. About 4 years.
EXCESSIVE CHARGE CURRENT RATE: In addition, manufacturers also tell us that the maximum charge current of the battery should be of the order of 10% of the battery capacity at C10.
1600Wp of solar panels will charge a 24V battery with a 66A current rate . A 600Ah battery in C10 would be the right thing to do . Like the hoppekce 6 OPzS 600 from 900Ah in C100.
3200wp of solar panels will charge a 48V battery with a 66A current rate . Therefore a 550Ah battery in C100 that will have a capacity of about 400Ah in C10 will withstand an excessive daily charge current rate .
You don’t know what C100, C20, C10 means? Here we explain it to you: Meaning of battery capacity C100, C20, C10
The result is having to change one or more cells of a stationary battery after 2 years, which should last at least 15 years. In addition, this new glass must work in the company of its already worn out brothers with the consequent drop in battery performance. And of course, the uncertainty of when the old glasses will fall and when it will be more profitable to change the entire stationary battery or keep changing 2-volt glasses.
The calls and inquiries received with solar installations with 2-3 years of life (when the price war began on the internet) that have had to change one or more stationary battery cells are increasingly constant. Normally low quality stationary batteries such as U-power, or the low range of the Tab, Tudor, etc. that normally accompany this type of bad designs oriented to lower costs .
2.- Low quality batteries a little cheaper than good batteries.
Another fictitious savings that is happening is: As we are not sure which batteries are good and which are bad, and since sellers provide us with offers with very different brands and models at very different prices, the tendency of people is to go cheap . Since if I do not trust anyone, then the cheapest is better and in the future we will see.
The result of these decisions is that open traction lead-acid monoblock batteries are used, that is, car batteries, for solar installations. Also with capacities much lower than those recommended for the consumption carried out. Obviously these batteries die after a year or 2 years of life. Even worse are some solutions of expanding the batteries by connecting new units in parallel to make up for the poor performance of batteries that are half dead due to poor choice and misuse.
Use 12v monoblock batteries in parallel configurations. Do not connect batteries in parallel to achieve the desired capacity in order to reduce battery costs a bit. When the desired capacity cannot be reached with monoblock batteries, it is necessary to choose stationary batteries with 2v elements with capacities up to more than 4000Ah.
Choose batteries such as U-power with doubtful reliability to really get a little cheaper compared to the best batteries on the market such as Hoppecke, Bae, Hawker, etc. Again the result is the premature death of a glass and the doubt of changing the battery whole or replace glasses little by little while they die and we work old glasses with new glasses, reducing the performance of the new glasses and the entire set. Really a bad investment.
3.- Deceptive Solar Panel Offers
It is very common to see offers for 24v solar panels that are actually 60 cells . The trick is in the price, since 60-cell solar panels cost around € 0.6 / Wp while 24V solar panels cost around € 0.8 / Wp. But the result of the operation is to buy MPPT regulators more expensive than PWM. Or if we do not know this fact, it will suppose a bad design that will not charge solar batteries well: 24V solar panels that are not
There are still offers at incredible prices of solar panels on the internet , ebay, amazon, from manufacturers that are no longer seen in the market. 32 solar cell solar panels are not 12v panels and are not suitable for charging solar batteries, like the luxor solar panel offerings that are possibly 20 years old. In addition, it is not convenient to buy solar panels from brands that have disappeared in the market such as photowatt, isofoton, siliken, etc. since we will not have any guarantee.
It is very easy to discover a bad design of a solar kit, here we leave you 3 easy points:
To calculate the battery:
- Multiply the daily consumption that the solar kit can cover x 6: Ex: 3500wh x 6 = 21,000Wh
- Divide this value by the voltage of the solar kit battery: Ex: 3500wh / 24V = 875Ah
The result obtained will be for a battery with 3 days of autonomy and with daily discharge depths of the order of 20% and without exceeding the discharge depths of 50%. So the life expectancy of this battery will be the maximum possible.
To calculate the charging current of solar panels with MPPT Regulators:
- Multiply the power of each solar panel x the number of panels: Ex: 5 panels of 320W are 5 x 320W = 1600Wp (peak watt)
- Divide this value by the voltage of the solar kit battery: Ex: 1600wp / 24V = 66A will be the charging current. * Do not confuse with the output current of the solar generator.
The result obtained will be the maximum current rate that the battery will withstand, therefore the battery should have 10 times more capacity expressed in C10.
Batteries in parallel:
- If you see a solar kit with a 12V inverter and several 12V batteries, it means that they will be connected in parallel
- 24V inverters with more than 2 12V batteries will also be in parallel
- In summary, with 12V monoblock batteries:
- 12V inverter can only have 1 12V battery
- 24V inverter can only have 2 12V batteries
- 48V inverter can only have 4 12V batteries
- With stationary batteries with 2V cells:
- 12V inverter can only have 6 2V elements
- 24V inverter can only have 12 2V elements
- 48V inverter can only have 24 2V elements
Clues to discover a BAD DESIGN
If you see a solar kit with a design of batteries in parallel RUN OUT !!!
If you see an ODD number of batteries, please REPORT IT!
Look at the voltage of each battery and multiply it by the number of batteries; the inverter MUST be of that voltage, if it is not BAD!
If you see a lot of solar panels in cheap solar kits start to doubt!
Multiply the consumption that the solar kit can cover x 6, and divide the result by the battery voltage. The capacity of each of the batteries must reach that number calculated to have 3 days of autonomy with a depth of discharge of 50%. Or about 4 days with discharge depths of 60%. If the capacity of the solar kit batteries is much lower, it is WRONG!
If you don’t know your consumption , use our consumption calculator with an instant budget . As you introduce the electrical appliances, solar kits designed for both holiday use and permanent use appear .