A battery is needed because the appliances use electricity at different times and at different rates than the panels produce. For the system to work properly, the battery should be of the deep-discharge type and be large enough to store enough energy to operate the appliances at night and on cloudy days. Also, for the battery to last a long time, it should not be discharged too much or too often. In sizing a battery, it is important to install one large enough to operate the appliances for at least five days without recharging. In climates that have long periods of cloudy weather, a larger battery may be needed.
Remember that battery life depends on how much discharge takes place before a recharge. So another way of sizing a battery is that the battery should be large enough so that one day’s use of the appliances will discharge it no more than one-fifth of its full charge. This limited discharge before recharging will help the battery to last a long time.
When buying a battery, the voltage and the ampere-hour rating must be known. For a solar PV system in a home the voltage will usually be either 12 V or 24 V. The size in ampere-hours will depend on the energy requirements of the appliances. As the battery should store five times the energy that the appliances use in one day, the watt-hour capacity needed in the battery is the total appliance watt-hours per day times 5. Because manufacturers rate their batteries in ampere-hours, not watt-hours, you need to convert the calculated watt-hours to ampere-hours. As watts equals volts times amperes, dividing watt-hours by the battery voltage gives ampere-hours.
When you know the total watt-hour capacity of the battery, you can calculate the ampere-hour capacity by dividing watt-hours by the battery voltage: 1,300 Wh ÷ 12 V = 108.33 Ah. For this example, the battery chosen should be a 12 V deep-discharge battery with at least a 108.33 Ah rating. For these calculations, we are assuming a battery rated at C10 discharge rate, not C100.
If the battery manufacturer rates batteries at C100, you will need to increase the size of the battery you buy by multiplying the C10 calculated value by 1.3 to get the C100 capacity battery to install.Remember that if a deep-discharge battery cannot be found and you have to use a vehicle starting battery, it is best to choose one with at least twice the ampere-hour capacity than would be correct for the deep-discharge battery.
Even then, it will probably not last as long as a deep-discharge battery. A ‘maintenance-free’ battery that does not allow access to the cells for water replacement should be about 1.5 times larger than would be correct for a deep-discharge battery.
Summary of battery size calculations
- Step 1. Calculate the watt-hours per day used by each appliance.
- Step 2. Total the watt-hours per day used by all appliances.
- Step 3. Multiply the total appliance watt-hours per day by 5 for a deep-discharge battery, multiply by 7.5 for a maintenance-free battery or multiply by 10 for a vehicle battery.
- Step 4. Divide the result of Step 3 by the battery voltage. The result will be the required ampere-hour capacity of a deep-discharge battery at a C10 discharge rate. If the battery you want to use has the ampere-hour capacity rated at the C100 discharge rate, you need to multiply the calculated ampere-hour size by about 1.3. So if you calculate the C10 rate as 100 Ah, you need to buy a battery with a rating of at least 130 Ah at C100.
A house with a 12 V solar system has the following appliances in use:
One 18 W light used 4 hours per day
One 60 W fan used 2 hours per day
One 75 W refrigerator that runs 12 hours per day
What battery capacity will be needed?
Light: 18 4 = 72 Wh/day, Fan: 60 2 = 120 Wh/day, Refrigerator: 75 12 = 900 Wh/day
Total appliance use: 72 + 120 + 900 = 1,092 Wh/day.
Total appliance watt-hours 5 = 1,092 5 = 5,460 Wh
Divide watt-hours by battery voltage 5,460 ÷ 12 = 455 Ah
So, for the house in the example, a deep-discharge battery of at least 455 Ah should be used
Too small a system will run out of power when the weather is cloudy and will cause batteries to fail more often. Whenever a PV system is not working properly, always check to make sure that the panels and battery are large enough to provide the watt-hours needed to operate the appliances each day. If the system is too small, you must either increase the number of panels or reduce the energy needed by the appliances, by using fewer appliances or using them for a shorter time. A larger battery may also be needed, but installing a larger battery without first installing more panels will not help. If you do not increase the number of panels, the system will continue to work poorly and the battery life will remain short.
SYSTEM MODIFICATIONS AND SIZING
The size of the panels and the battery are both determined by the watt-hours used by the appliances. The number of watt-hours changes when appliances are added or removed from the system and when appliances are used more or less each day. If you have more panels and a larger battery than you need, it is not a problem. It is a problem, however, when the panels or battery are too small. People often want to add appliances to an existing PV system. If they do, the system will not work properly unless the panels and battery are large enough to provide the extra watt-hours. Whenever a new appliance is added, or an old appliance replaced by a new one, it is important to recalculate the correct panel and battery sizes and to increase the system capacity to handle any increased load. It is also common for people to underestimate the amount of time that lights and other appliances will be used. If the PV system size is calculated using estimates of appliance use that are too low, then the system will not be powerful enough and will not work well.
BATTERY LIFE AND PANEL SIZE
It has been shown that increasing the panel size increases battery life, particularly in a climate with frequent cloudy conditions. With the cost of solar panel capacity fallingbut the cost of batteries slowly increasing, it makes good economic sense to increase the panel size by 20% to 30% over the minimum. This can dramatically improve the reliability of the system during cloudy weather and can greatly extend the life of the battery. This reduces the cost over time as battery replacements are now the most expensive component in a home PV system.
The accuracy of any PV system design depends on the accuracy of estimates of appliance use. Estimates are rarely very good in the long term. People often underestimate the amount of time that appliances will be used. This results in more energy being used than the system was designed for. People may also add more appliances to an existing PV system. This increases the amount of energy needed and the system is then too small.
The best way to make sure that sufficient panel and battery capacity is available is to keep checking the system and if the panels consistently supply too little energy, as shown by frequent power cuts or a low average battery charge, then more panels and possibly a larger battery should be installed.