Solar Panels!

[Lui]

Hi everyone!

I need to install solar panels to get 2 kwh of electricity a day!(in average).

how many square meters of solar panel I need to get that 2 kwh per day?

I live in London with cloudy sky!

thanks in advance for your reply.

[Jonny]

A quick look at what's available local to me seems like 240W panel is most powerful one at a size of 1.7m x 1m with huge variation in price for what seems like the same panel type (i.e. Mono Crystalline). So with full sunlight required you would need say 9 panels for 2kW and if cloudy probably double. I don't believe they are overly efficient in low light but I'm not an expert here so in practice might be different. Still that's a large area to cover, hope  you have a big roof

[kaindub]

Luis
A couple of things you need to figure in the equation
What time of year is this power required? If it's for mid winter you need a bigger array
Is your array able to be angled to face the sun on the worst day?
Can you do the calculations?
Look up the internet and find out the minimum sunshine in the worst month in London.
By simple trigonometry you can work out the % of power your array will receive due to the angle
You can then work out from the number of hours of sun what power one panel will produce in a day.
Work out how many panels you need.

There may be some days when you never generate sufficient power.

You may need batteries to store power for several days for when there is not enough sun.

[Lightages]

First have a look at these solar irradiation maps:
http://solargis.info/doc/71

I live in one of the best places in the world for solar power, the Atacama Desert in Chile. The solar irradiation here is over 3kWh/m2. Depending on where you live in GB the solar irradiation average is around 900 or lower. This implies you need 3 to 4 times more panels than I would need for the same energy collection. In reality this will depend on the minimum solar conditions you might experience and still get the minimum solar production you need. The maps are for yearly average. My minimums are very close to my maximums. In your country the minimums are probably less than 1/10 of the maximums.

I have 16 x 205 watt panels oriented basically in an optimal direction. I use around 10kWH per day and my batteries are charged using around 1/3 of the available solar radiation per day. Around 1/4 of the rest of the available solar radiation goes to maintaining the battery charge and supplying the demands during the daytime. This might sound like I have an excess of panels, but no I don't.

If you are running a solar system as the only source of power you need to have batteries of course. The batteries need to be sized based on your daily consumption without sun and for autonomy without sun for a couple of days and so that the batteries do not drop below 50% charge. How long you want the batteries to not go below 50% charge when it is cloudy is based on a couple of things. The first is how long is a normal period of cloudy days. The second is if you prefer to have a generator as a backup for the sun.  The costs of having a big enough battery bank for every contingency can far outway the cost of a generator and its fuel. Once you have the battery bank size established you then need to size the panels needed to charge them properly and also supply your load.

The batteries most used, by far, in solar installations are lead acid types. They can be actual liquid acid and lead plates types where you need to maintain water levels, or they can be GEL or Absorbed Glass Mat batteries that do not need maintenance. Each of these type have a minimum charge current rate. This minimum rate is needed to mix the electrolyte so that you do not just charge the layer of electrolyte touching the lead plate surfaces. The size of your solar panel array is dependent mostly on this rather than on your actual energy consumption requirements per day. It is almost always the fact that you will have more panels than you need just to charge the batteries correctly. Don't follow this rule and your batteries will die a quick death from undercharging and sulphation. You can also run a battery system of 12 volts, 24 volts, or 48 volts. The higher the voltage of the battery bank the more panels you need in watts to be able to produce the required current.

So now you know what is the minimum of panel output you need, you need to account for inefficiencies. The panels will not perform as per their rating. They will usually be working closer to 80% of what the rating is depending on temperature and how clean you keep them. A good charge controller will pass maybe 90% to your batteries. Your batteries will also only provide around 95% of the charge put into them back to the load. If you are using an inverter to convert your DC battery voltage to 220V then there is another efficiency of around 90-95%.

So those are the basics of calculating a solar energy system. It is much more complicated in just asking the question "how many square meters of solar panel I need to get that 2 kwh per day?"