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Battery Bank Sizing
One of the first items to take care of when designing an alternate energy system is correct sizing. Sizing takes into account all the ac and dc loads that are required to be powered. Properly estimating capacity needed, will help stop batteries from wearing out before their time and ensure proper charging. You should be ruthless in your energy usage and practice energy efficiency and energy management.
To figure how many amp-hours storage you need, take your average daily usage in watts and divide by the battery voltage. For example, if you use 5 kwh (kilowatt-hours) per day, and have a 48 volt system, then dividing 5000 by 48 gives you 105 AH. Since you do not want to discharge the battery more than 50% in most cases, you would need 210 AH. If you want to keep running for 4 days of bad weather with no sun, multiply that times 4, which gives you about 850 AH total capacity. UPS and backup power systems are the same, except that the times are often in hours instead of days.
How many Watt/hrs in a battery?: Watts are pretty simple - it is just battery voltage times amp-hours. A 12 volt 105 AH battery can supply (under perfect conditions and to 100% discharge) 12 x 105, or 1260 Watt/hrs. Keep in mind this is an approximation of capacity, there is more than one way to measure AH rating of a battery, the 20hr is most common, ratings drop significantly with faster discharge.
After the appropriate amp hour capacity has been calculated, it is then easy to find the correct charge rate for the battery bank. To rapidly recharge the system, a generator should be able to supply a C5 charge rate. This is equal to 20% percent of the battery banks rated amp-hour capacity at the 20hr rate. Solar systems may not deliver anymore than a 10% rate of charge. Having a generator, as part of the system is essential as it is needed for heavy loads, to top up the battery bank and every few weeks for Equalization Charging.
There are phantom loads from some appliances that remain on all the time. These loads come from standby modes and instant on features that some items need. Motors and some other heavy duty appliances can draw many times their average use when starting up. Calculate all the average on times of your various appliances and do the figures again, this will give you an accurate idea of what your true consumption really is. Divide by .7 to the final figure as a safety factor if you have not done so on you’re A/C watt calculation.
Inversion is 10:1 if you draw 5amps of A/C you will need to supply the inverter with 50 amps of D/C. Determine your average A/C amperage, multiply that by 10 to get D/C amperage supply size that will be needed. Inverters are only 85% efficient this means you loose 15% through the inversion process in heat.
It is best not to discharge more then 50% from the batteries at one time to get maximum life from your investment.
Examples for 12 volt DC calculations:
If you use 700 amp hours per week, 700 amp-hrs divided by 7 (days) = 100 amp-hours per day. If you have 100 amp-hr batteries, at 50% discharge, 50 amp-hrs would be available from your 100 amp-hr battery.
Example 1. You would need 2 100 amp-hr batteries for enough storage per day. You would also need more than 100 amp-hrs of charging each day to have a balanced system so you would have your 100 amp-hrs for the next day.
Example 2. You would need 14 100 amp-hr batteries to give you enough storage to take 700 amp-hrs for the week, you would then be at 50% discharge by the end of the week. You would then need more than 700 amp-hrs of charging to replenish the battery bank.
Amp-hrs=Watt-hrs divided by voltage. If you use 1200 watt-hrs per day [A/C side] you would need 100 amp-hrs of charging. [D/C side] 1200whr/12v=100ahr
System storage requirements:
How much storage, in amp-hours, do you need? This will vary with the application and what part of the world you are in. As a rough rule for solar systems, the total battery capacity (in amp-hours) should be three to five times your daily usage, 3 is usually sufficient in most of the sunbelt states, 4 in most of the Midwest, in the East and Northwest, 4 to 5 days is better. If you are in a good wind area and have a wind generator, you can probably cut this down a day. For backup power systems, the total capacity should be enough to cover about twice the longest anticipated outage.
Solar Charging
Generally a rough estimate of charging capacity is around 1-1.5 watts of solar panel per battery bank amp-hours @ 12 volts (double that for 24 volt, 4x that for 48 volt systems)
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