An 800va Inverter, 250w solar panel and 200AH Battery

Poster on Nairaland asked this question.

Re: Solar Energy, A Complement To FTA by BasedOnB1:34pm

Hello everyone, i need some quick advice here please.

I have a 800 VA sukam inverter, a 250 W panel, and a 30 amps PWM controller. My current 150 AH battery is long due for replacement, so i am thinking of replacing it with a new 200 AH. My concern is that the panel may take too long to charge the battery full, even though i don’t intend to discharge it to more than 50%. Am i just better off with a smaller battery, say a 150 AH or 100 AH?

I also need recommendations on reliable battery brands to buy (preferably from experience), i kinda believe the so called ‘telecom’ batteries are solid, even if they are more expensive.

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First of,  I think you are putting the cart before the horse. The first activity that should precede any offgrid setup is an energy audit of load that would run on the system. An audit of your load would help you in sizing your offgrid system which in turn would ensure their effectiveness at meeting your needs.

Things you need to find out about your load:

  • Total number of load (in watt) which would be on the system
  • Run time of which load in hours
  • Does any of the load have a starting current e.g some appliances like fridge and freezers requires about 5 times their running currents for a few seconds when you turn them on.


back to the question

Based on what you stated above. Your offgrid setup is composed of the following:

1 x 250w Solar Panel

1 x 30A PWM Solar charge controller

1 x 800VA Sukam Inverter

1 x 150A Battery.

Since we have no idea what the run time load on this system would be. We have to work on potential power generation, and amount of energy which could potentially be stored in the battery based on  what you generate.


Your Electricity Generation – All things Equal

Your 250w panel is most likely a 24v solar panel while your battery system is 12v. Although 24v solar panels can be used in charging a 12v battery, considerable amount of energy is wasted during the charge process especially when using a PWM controller. Here is why:

a 250w solar panel usually have the following Specifications:

Voltage Maximum Power of 30.5v

Current Maximum Power of 8.2A

30.5 * 8.2 will give you close to the 250w (Remember watt is made up of Voltage and Current)

A 12v battery requires between 14.4v – 13.5v to properly charge a battery. Hence your PWM controller would take 14.4v from the 30.5v your panel has to offer, and 8.2A current the panel is producing. 14.4 * 8.2 = 118.08v

118.08v is Max that can be used by your controller in the charging of your battery. The remaining 131.92w is wasted.

Kindly note that this calculation was done using ideal condition. In real life use, a 250w panel would rarely produce 250w. From my experience most panels produce about 77% of their rated power at pick sun hours. About 23% could be lost to either heat and / or wiring related loss.

In your case 118.08 x 0.77 = 90.9216.

Thus the reasonable number of watt to be expected from your solar panel is 91w. If we multiply this by the number of pick sun hours which is usually 5.

91 x 5 = 455. Thus the total number of watt you can expect from your 250w solar panel is 445wh which translate to about 37A for your battery.

From the above, we can establish that your battery will  get at most about 37A ( 445 / 12) everyday. Given your current setup.

My Advise.

Your system load and usage should be based on what you panel can produce back into the system. Although the standard recommendation is never to discharge your battery below 50% depth of discharge. In your case 50% of 200Ah battery is 100Ah. Unless you have an offgrid system that can put that about 120Ah into the battery. I would recommend you keep your daily load at 20A or 100wh (Just enough to power some energy efficient light bulbs)

If you buy a new battery and keep all aspect of your system as it is, then cycle the battery at 50% dod the first day. You will not be able to put that what you have used thus leaving the battery in a partial stage of charge. The second day would lead to a deeper discharge in less than 6 month that battery would be dead.

I did not include secondary charging option in my calculation because I do not know how good or how regular power in your location is.

What are your options

My advise is to hold up on buy any battery or equipment for your offgrid system until you have done proper audit of the appliance you intend to run on your system. You can procure a watt meter which cost about ₦5000 and is sold online in Nigeria. Use it and post the result in the comment section.