Batteries for Stand Alone Solar Power Systems: Sizing Battery Banks

Part 1 of 4

Previously we here at Solar Choice wrote a bit about some of the things you would need to take into consideration when thinking about installing an off-grid (or stand-alone) power system in your home.  There are a number of reasons you might want to install an off-grid system, the most obvious but most important of which is that you live in a remote area away from the power grid and want a reliable, cost-effective source of power to run your home.

The key difference between a grid-connected system and an off-grid system is the need for a battery bank.  Since sunlight is not available at night, and since you’re most likely to require electricity at night for lights, heating, and so on, batteries are indispensable to any off-grid system, and some people may want them even in a grid-connected system in case of a power outage.  Unfortunately, batteries are also often the most problematic and costly part of a system.  Whereas a typical photovoltaic (PV/solar power) array itself requires relatively little maintenance (besides the occasional cleaning!) and contains no moving parts, the utmost care is required through the process of battery selection, installation, and maintenance; one false move can severely impair the performance of your batteries and result in costly repair bills.

This entry, a rough tutorial about how to size a battery bank, is the first in a series of four entries about batteries.  The following entries will cover:  types of batteries available and typically used, lead-acid battery maintenance, and the different types of stand-alone system configurations and their special considerations.

How do I know what size battery bank I will need?

Solar Choice can put you in touch with an accredited installer who can give you more detailed advice particular to your circumstances, but your required battery bank capacity will depend firstly on your typical daily peak energy demand and the size of your PV array; how much energy do you use a day and does your PV array meet that demand on the shortest day of the year, when the least amount of sunlight is available?  If the answer is no, then you are going to need to run a generator or some other power source daily to make up for the difference.

Even if the answer is yes, though, you’ve got to plan for the worst by oversizing your battery bank so that you have enough battery capacity to last you through an extended patch of inclement weather–up to 10 days if you’re really paranoid! The real benefit of having such a system is that it will be a fairly simple system wiring-wise, and you won’t have to worry about equalising battery bank by running a generator (see the upcoming entry on battery maintenance).  You’d also get to brag to your greenie friends and neighbours about how your home being no slave to fossil fuels.  The downside, however, is that if something goes wrong with your system, like you run out of juice after two straight weeks of rain, or there’s a short circuit that drains your batteries without you noticing, you’re going to have to wait for the sun to come out and charge your batteries up again.  In the meantime, you’ll be powerless.

The benefit of having a generator connected to your off-grid system is therefore fairly obvious–when the sun lets you down, you’ll have a backup plan for charging your batteries.  We’ll cover this topic in the upcoming entry about battery maintenance.

The following is a back-of-envelope way to estimate the size of the PV array and battery bank you’ll need for a stand-alone system.  It is a bit technical, but the concepts are not too complicated, and reading through it will give you some idea of what you need to consider when trying to size your battery bank.

1.  Roughly divide your energy use patterns into two seasons–you might do this by looking at old electricity bills from summer and winter.  It doesn’t have to be exact.  Let’s say your daily average power use for summer (Oct-Apr) is 3kWh/day and winter (May-Sept), when you run a heater or electric blanket or something, is 3.5kWh/day.

2.  Find out the average number peak sun hours (PSH, a measure of the available solar energy, which you can find out more about here) your region gets on a typical day for each month of the year.  Let’s say you’re in a rural area at approximately the same latitude as Brisbane, where the month with the least amount of daily solar irradiance is June, at 4.1PSH (=4.1kWh/m2).  (This ˜worst month’ tends to be in winter, but could be in summer if you need the air conditioning in a tropical climate.)

3.  Using this data you can calculate which month has the worst irradiance (sunlight per day) relative to your winter load.  It’s necessary to set design your system around this to ensure you don’t end up running out of juice unexpectedly.

4.  The next bit (sizing the PV array) is outside the scope of this blog and involves derating for temperature and dirt, amongst other things–someone can help you with the technical aspects of this.  In any case, in our scenario your typical daily load for the month is 3.5kWh, and on an average day in June at your location you get around 4.1PSH.  Let’s say you’ve got a 4kW PV array.  As a rule of thumb, the voltage of an array is typically 12V for any array under 1kW, 24V for arrays between 1 and 3.5 or 4kW, and 48 for arrays over 4kW.  Let’s choose a 48V system.

5.  Now the battery fun begins.  How many days of autonomy would you like to plan for; i.e. how many days do you think you may end up going without sunlight?  Let’s select 7 days of autonomy for our scenario.  3.5kWh * 7 days = 24.5kWh

6.  Batteries of course are not perfectly efficient in storing and dispensing energy, and you’ll have to derate them by about 10% to ˜oversize’ them, plus any deratings the manufacturer recommends for differences in temperature from standard operating conditions (assumed to be insignificant here.)  24.5kWh  * 1.10 = 27.2kWh.

7.  As over-discharging the batteries shortens their life span, it’s necessary to set a maximum depth of discharge (DOD) for the batteries, typically 30%.  This is basically oversizing your battery bank again.  27.2kWh / 0.7 = 38.8kWh.  If we divide this by the system voltage we get 3880Wh / 48V = 808Ah.  This is roughly the capacity we want our battery bank to have.

8.  But we’re not done yet!  Battery capacity is measured in Amp-hours (Ah), since voltage (V) for battery cells is typically 2V, 4V or 6V. (Amps * Volts = Watts, and Amp-hours * Volts = Watt-hours, for reference).  Battery capacity varies depending on the rate (amperage) at which you discharge them.  If you use multiple appliances simultaneously (i.e. at a higher amperage) you will actually consume more stored energy (i.e. at a higher discharge rate) than if you used them each separately, one after another for the exact same amount of time (i.e. at a lower discharge rate).  This is why you see batteries rated with not one Ah rating, but typically 4 or 5, denoted by a C followed by a number in sub-scripts: C5, C10, C20, C50, C100, etc.  This number indicates how long the battery will run at a particular discharge amperage, usually indicated on a chart like the one below.

9.  For the next step you’ll need to imagine what appliances might actually be on simultaneously in your home, in Watts, then divide by your system voltage.  Let’s assume some lights, your freezer, your fridge, and the television are often on at the same time, at a total of 867W.  Since the array is 48V, this means we have 867W / 48 V = 18.2A.  18.2 Amps is our typical discharge rate, just to be on the safe side.

10.  Voltage adds up in sequence, so if we choose 24 2V cells, we’ll have 48V, but we still have to choose a battery based on the discharge rate.  Since we determined in step 7 that we need a battery bank of 808Ah, and in step 10 that our typical discharge rate is 18.2A, we can now select a battery that will give us what we want.  808Ah / 18.2A = 44.39h.  We want a battery that will give us around 800Ah at its C45 or C50 rating. Luckily, the model highlighted above is just about what we want.

The next installment in this series of battery entries will be about different battery types.

Written by James Martin

Solar Energy Analyst

Solar Choice

© 2010 Solar Choice Pty Ltd

Source: http://www.yourhome.gov.au/technical/fs67.html

Image taken from a Sungel Batteries catalogue:

http://www.batteryenergy.com.au/02_sungel.htm