How To Calculate Off-Grid/Backup Needs
Those of you who’ve realized the energy savings involved in using solar technology may be wondering how you can use your system to keep the lights on when the power goes out, or to go completely off-grid. To accomplish this, you will need to store solar energy in a battery.
How do you determine the amount power you need to store? While it may seem easy to make an educated guess, it’s best to do a proper load calculation because there’s nothing worse than running out of power during a blackout when you thought you had plenty.
The suggestions below can help you measure your needs properly, including how you can determine wattage requirements for your appliances. If the information below feels overwhelming don’t worry – mtvSolar consultants will help you every step of the way and you won’t have to do any math, we promise!
How many kilowatt-hours do you need to store?
The energy stored in a battery is referred to as kilowatt-hours (kWh). One kWh is 1000 watts for one hour.
If you run a 2kW (2000W) space heater at full power for 5 hours, that’s 10kWh of energy used. If you run the same heater for 10 hours, that’s 20kWh.
Conversely, a bunch of LED lights totaling 50w could run 24/7 for many days on that same battery, even without solar gain. To determine how much power that is, 50w = 0.05kW x 24 hours = 1.2kWh per day. This simple formula applies to all loads. Take the amount of wattage listed on the label multiplied by time to arrive at the energy needed for that load.
For example let’s say you have a critical load which is a CPAP machine. It draws 300 watts and needs to run for 10 hours every night. 300w = 0.3kW x 10 hours = 3kWh need to be available from your battery to ensure that machine can operate for 10 hours at night.
In reality the battery will need more than 3kWh stored to account for energy losses in conversion and power factor, amongst other things. However, for the purposes of this blog we’ll keep it simple.
Determining Consumption of your Appliances
Sometimes you can use the label of an appliance along with how long you need it to run to determine how much energy you need in your battery, like our CPAP example above. However, this can be difficult for intermittent devices such as a refrigerator. Just because your refrigerator is plugged in 24/7 does not mean it is running 24/7 (and if it is, you should consider an Energy-Star rated upgrade!).
For plug-in appliances this can be accomplished with an inexpensive plug-in watt meter. Most home appliance stores carry them, and Amazon has them for reasonable prices. The most popular option is the P3 “Kill-A-Watt” unit and they run from $20 to $30.
Many of these meters have special features to help make the calculation automatic. Some of them calculate your electrical expenses by the day, week, month, and year. For a generally accurate number for a device like a refrigerator, a few days of averaged data can be acceptable. In general, we always slightly over-size the battery to account for consumption variation, power loss in conversion, and battery aging.
When using an in-line plug-in watt meter, be careful in hooking it up. It is possible to exceed the power limit of the meter you purchased (such as with a space heater). As a result, it could lead to damage to the meter or worse. Be mindful of the max load of your meter and the appliance. When in doubt, consult a professional for advice.
EXAMPLE: Calculating Refrigerator Consumption
Considering your refrigerator is a mysterious black box from an energy perspective, it may be using more energy than you’ve guesstimated.
Method 1 is to read the label of your refrigerator and make a rough calculation. This will not be as accurate as the reading from a watt-meter, but can get you in the ballpark.
Usually inside one of the doors, or on the back, is a label listing volts and amps. In the USA, a refrigerator is almost always 120v. The amps can vary depending on size, but lets assume yours says 5 amps. That means your refrigerator draws 600 watts while operating (120×5=600). But when it’s not operating, it may only be using a few watts for electronics.
You can guesstimate that your refrigerator operates for 8 hours out of 24, which is reasonable average for a modern appliance. Take the 600w (0.6kW) and multiply that by 8, to get daily usage of 4.8kWh.
Method 2 is to plug a watt meter inline with the refrigerator to see exactly what it’s using daily. This is more accurate and avoids having to do calculations and guesswork.
Other Appliances to Consider
You’ll want to consider other appliances, even if you don’t use them every day. Things like coffee pots, garbage disposal, well pump, garage door openers, lights, televisions, computers, and that old chest freezer in your basement need to be considered to determine battery storage and the solar needed to fully recharge it each day.
Thanks to Energy Star appliances being the norm now, power consumption in today’s models is much lower compared to even a model 10 years old. Regardless, many households still use older appliances they have yet to update. All of these (and many more) can add up to a significant figure. Batteries are not exactly cheap, so it is more cost effective to upgrade an old appliance rather than throw a larger battery at it.
Once you determine how much energy your critical loads require, it’s time to move on to the next steps in determining your backup needs with a battery bank.
How Much Backup Power Do You Need?
Can you estimate how many days you may need to use your backup system in case of cloudy or rainy days?
Those of you who live in more rainy climates may have to estimate a larger amount of days to make this figure realistic. For instance, here in the mid-Atlantic region of the U.S., this summer brought substantial summer rains, with rainfall totals breaking records in a few states.
During the winter, when days are shorter, we also have ice and snow, making the time you may need battery power much longer because solar gain just isn’t there.
In this case, you’re going to need to make another calculation. This time, it relates to an important term in battery backup design: autonomy. While autonomy is important when applied to grid-tied backup systems, it’s critical for off-grid systems.
What is Autonomy?
To determine autonomy requirements, you’ll determine how many days your battery can realistically give you power on days when sunlight can’t reach your solar array.
If your battery has two days of autonomy, then you’ll have 48 hours of battery power for use on non-sun days.
Designed autonomy can become a problem if you experience more cloudy days than you planned for and your battery isn’t large enough, or if you’ve added loads that weren’t in the original sizing calculations. Most battery backup systems in this area are designed for day-to-day autonomy, because batteries are expensive and outages rarely last more than a few hours.
When you want more days of autonomy, you’ll have to invest in a larger battery or a fossil fuel generator as extended backup. Unlike a generator-only backup without solar and batteries, the generator would only run as needed, and not 24/7. Additionally, with a solar+battery+generator system, the solar will reduce the amount of fuel needed, whereas with a purely grid-tied system the solar will be disabled while the generator is operating to avoid back-feeding the generator and causing damage. In other words, if you want to use a generator AND solar during an outage, you still need a battery.
How much solar do you need?
Your solar array is going to produce the electricity needed for both the loads and battery recharging. To determine the size of the array to get you through winter, you need to know the average number of sun-hours per day in your region. Keep in mind this needs figuring by the least sunniest days in a month rather than an entire year. In the mid-Atlantic area, we typically assume 2.38 sun-hours per day to account for the winter months. If there is a generator, this is less critical.
With the sun-hours and kWh needed per day in hand, you can determine the total wattage of solar panels that you need. This somewhat complex calculation takes into account the sun-hours, desired autonomy, battery size, inverters and more. There are numerous web based calculators that can figure this out for you based on your determined daily kWh and desired autonomy.
However, with Mountain View Solar we do most of this for you! We serve the mid-Atlantic region of the U.S. including West Virginia, Virginia, Maryland and Pennsylvania. mtvSolar has been in the solar and backup business since 2009 and we’re here to make the process easy. We can even help you determine how many kWh you need per day and will provide a completely custom quote based on those numbers.