Off-grid Solar Power system

Re: Off-grid Solar Power system

Welcome to the forum Khan.

We have been getting a few people form Pakistan lately looking to install good sized solar systems--I guess there is something going on there with an 'energy crisis'?

Your numbers look pretty reasonable... And I would suggest that you find some sort of Kill-a-Watt type or similar Watt-Hour meter to confirm your measurements (or, if you have utility power, you can read your meter/check your bill).

Off grid solar power (especially the batteries) are not cheap--And batteries need replacing every 3-7+ years--So an over sized system can cost you a lot in battery maintenance and an undersized system can kill batteries in months or less.

One of our first recommendation is conservation. Some appliances use power even when turned off (digital video recorders, various electronics, etc.). So, measuring their load (on and "off"), how much power the appliances use per 24 hours (such as a refrigerator--They cycle on and off), etc... Sometimes it makes sense to only turn things on when used (my clothes washer and drier use about as much power "off" as when we run the wash a couple times a week).

I can start with the numbers you have provided (3.3 kWH per day or 100 kWH per month is about the minimum for an "off grid home" with refrigerator/well pump/clothes washer, lights, laptop computer, etc... So your 5.4 kWH per day is certainly in the right area.

"Mechanical Loads" (fans) and devices that are used for many hours per days (computers, TV, refrigerator, etc.) are usually the "killers" for off grid power--So, understanding your present needs, and possibly replacing with more efficient appliances (if available) is usually a better 'investment" for your money. Insulation/shading of south/west walls+windows/etc. can help reduce your use of fans too (as well as using less electricity in your home which creates "waste" heat in summer).

And, it would be nice to know how much actual sun per day you get, by month or season (you get less sun in "winter" but may need to use the fans either).

So--the basics (and my first guesses). Assume 48 volt battery bank (flooded cell), typical efficiencies, and 2 days of "no sun" storage ("balanced" system design). Assume most of your power is used late afternoons and into night (charge during day, use power during night)--Note the numbers/prediction are "not that accurate"--Using all digits so you can repeat my math and follow the numbers as I use them later:

• 5,400 Watt*Hours * 1/48 volts * 1/0.85 inverter efficiency * 2 days of storage * 1/0.50 maximum battery discharge = 529 AH battery bank

To recharge a battery bank, 5% to 13% is a good range for a solar array. Normally, I would highly recommend 10% to 13% (roughly) rate of charge. 5% is a small amount of charge current and you should not use much power at all during the day. Notice, if you "oversize" the battery bank, this pushes the size of the solar array larger too:

• 529 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 2,027 Watt array minimum
• 529 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 4,053 Watt array nominal
• 529 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 5,269 Watt array "cost effective" maximum

And we should confirm how much power (solar array) you need to meet your minimum loads... Normally, in the US/North America, around 4 hours of noon time equivalent sun per day (or 4,000 WH per square meter per day) will be the minimum for ~9+ months of the year. In Pakistan, you might get 5+ hours per day for much of the year (higher elevation, drier climate). Anyway, the math:

• 5,4000 WH * 1/0.52 end to end system efficiency (AC power) * 1/4 hours sun minimum per day = 2,596 Watt array minimum

Note, on days where you get 4 hours of sun or less, you will need to cut back on power usage, or use a generator/grid power to make up for the loss of sun.

So, for your system, my guess is that you would need around a 2,596 to 5,269 Watt solar array--With around 4,053 watt being a nice sized array and will give you some extra power during during good and poor weather (and should keep your battery bank happy):

• 4,053 Watt array * 0.52 system efficiency * 4 hours minimum average sun = 8,430 WH = 8.4 kWH per day available power (on average, minimum) per day

Anyway--A starting point and some guesses... Your thoughts and requirements?

-Bill