How Many Solar Panels Do I Need to Power My House?
Installing Solar Panels on My House: The Basics of On-Grid vs Off-Grid
From DIY’ers to door to door salesmen, home solar power has exploded in popularity in recent years. There are many reasons one might install solar energy, from environmental to economic to emergency preparation. There are two main methods with which we can install solar power into our homes. On-Grid, and Off-Grid.
You should consider what you want out of solar when selecting either approach. We’ll be giving a brief description of each before diving deeper into each method.
On-grid solar involves connecting your home solar system to the power grid of your city. On-grid solar setups will feed excess energy generated by your panels back into the electrical grid of your utility company. Depending on your local regulations, number of panels, and hours of sunlight, this could result in an electricity bill that is reduced, free, or in a bill that actually pays you for your electrical contributions to the grid. Door-to-door salesmen typically offer on-grid solar setups.
Off-grid solar, as the name implies, involves a home solar setup that is independent of any power grid. This can be appealing for cabins or other homes that are separated from the electrical grid, or as an emergency backup in case of grid failure. Off-grid solar setups typically incorporate battery banks to capture the power generated during the day for use during non-sunlight hours. It’s also common to use off-grid solar in coordination with a diesel generator for emergencies.
On-Grid Solar Energy
As summarized above, on-grid solar setups are connected to the electrical grid of your city. The main advantage of on-grid setups is that you can still use electricity during the night or on cloudy days without a battery bank. Another advantage of on-grid setups is that you can get paid back for your contributions to the electrical grid through something called Net Metering.
Net Metering VS Net Billing
Let’s say you have an array of solar panels rated at 5 kilowatts. In an idealized scenario, this will generate 5 kWh(kilowatt-hours) per hour or sunlight. If your drain is only 1 kWh(kilowatt-hour) per hour, your solar setup would be pumping 4 kWh back into the grid every hour.
How much you’re credited for this extra energy depends on your state. Each state has its own laws and regulations. The two main ways you can be credited are known as net metering and net billing.
If your state uses net metering, that means your utility company credits your bill for the exact amount that they’d have charged you if you’d consumed that energy instead of supplying it. If your utility company would have charged you 12 cents per kWh used, they’ll cut your bill by 12 cents per kWh supplied. This can get slightly complicated because some utility companies charge (and therefore pay) more for energy used during peak hours. In our example above, you’d earn 4 X .12, or 48 cents per sunlight hour credited to your account.
If your state uses net billing, that means your utility company pays you less for the energy you supply than they’d have charged you for the energy you’d have consumed. This varies by utility company, but you can probably expect to be credited at around half of what they’d have charged you. In our example above with net billing, you’d earn 4 X .06, or 24 cents per sunlight hour credited to your account.
An important note: some states have neither net billing nor net metering. In Alabama, for example, utility companies actually charge you for having a solar setup.
Other On-Grid Considerations
In order to know how much energy you’re supplying to the grid, you may need a second meter. One checks how much energy you’re taking from the grid, and the other checks how much you’re supplying. There are also certain two-way meters that can do both in the same device.
When you connect your on-grid system to the electrical grid, you’ll need to sign what’s known as an interconnection agreement with the utility company. Different companies have different agreements. The complexity between these interconnection agreements varies wildly, with some being little more than a piece of paper and others being far longer and complex. Liability can greatly vary from company to company as well. Be sure to research your state’s regulations.
Your utility company will typically send someone out to inspect your setup before connecting you back to the grid. You may find it worth your investment to pay a qualified electrician to set up the connections between your solar setup and the grid.
Another important consideration in your on-grid setup is the addition of a battery bank. Tesla, for example, uses power walls. This is basically a lithium battery bank. A battery bank will allow your house to maintain power in the case of a power outage, and allow you to drain less from the grid. If you live in a state with net billing or no net metering at all, it may make financial sense to invest in a battery bank. You can read more about the different kinds of batteries and equipment for those batteries in the off-grid section below.
On the other hand, if your state has full net metering, it may be useful to think of the power grid as a massive battery bank. You’re “charging” the power grid with excess energy during sunlight hours and only have to pay when you’ve used up all the excess energy.
Off-Grid Solar Energy
But what if you want to install solar energy into a cabin that’s far from the power lines? Perhaps you’re concerned about the stability of the power in your region, or you’re prepping for a potential collapse of society?
If any of those fit your situation, you may want to set up an off-grid solar setup. Before we talk about the different parts of an off-grid solar setup, we’ll want to know how much energy we can expect to use.
Calculating Your Energy Needs: Watts and Watt/Hours
Our first step in calculating the amount of energy that our solar panels will have to provide is to look up the watts of our appliances and then multiplying them by how long we expect them to run for. You can find the watts drained by most common appliances through google. Alternatively, you can use SanTan Solar’s Solar Energy Calculator to calculate the watt drainage from multiple appliances at the same time.
Once we know our wattage drain each day, we need to buy enough solar panels to cover both our projected use plus projected cloudy days and inefficiencies in our setups. Solar panels come rated with a wattage rating. A 300-watt solar panel, for example, will provide around 300 watt/hours of energy for each hour it receives sunlight.
Each application is different. If you’re hoping to power all the appliances in a remote cabin, you’ll need far more than if you’re using just one to recharge your phone and laptop during power outages.
Solar power is by nature unreliable. You never know when you’ll have a few cloudy days in a row or when your power consumption will spike due to an unforeseen use by an appliance. Due to this unreliability, many off-grid solar users will purchase a generator to accompany their solar powers. A single gas generator will provide a great deal of peace-of-mind to your solar setup.
Now let’s talk about the different parts that make up our solar setups:
However you get your energy, every off-grid solar setup will need a battery bank to store energy. Without batteries, you’d only be able to power your appliances during sunlight hours or while your generator is running. Batteries will allow you the freedom to run your appliances at any time.
There are two main kinds of batteries, lead deep-cycle batteries and lithium batteries. Lead deep-cycle batteries are cheap, but can be damaged easily if you’re unfamiliar with their limitations. For example, letting a lead deep-cycle battery get below half charge will cause permanent damage.
Lithium-ion batteries are superior in almost every way, but are far more expensive. For example, Tesla uses lithium batteries in its solar wall installations. However, lithium batteries can be up to ten times as expensive as lead deep-cycle batteries.
We want to make sure our battery bank has enough energy storage for our drain, which we can calculate using SanTan Solar’s Calculator
You can wire your batteries in series or parallel to increase their storage capacity, or to increase their voltage rating. You can read more about batteries in our other article here:
Solar Charge Controller
Solar charge controllers are an integral part of every solar setup, on-grid or off-grid. They perform several important jobs.
They limit the amount of voltage coming from the panels into your battery bank. Too high of a voltage can damage your batteries. They also disconnect the current when your solar panels are out of sunlight to reduce draining batteries.
There are two main types of solar charge controllers, PWM and MPPT. PWM performs all the functions that are described above, but are “dumb.” They simply cap the voltage coming into your battery so as to not damage them.
MPPT solar charge controllers are similar in the above-listed functions, but they are “smart,” in that they convert excess voltage into current which results in a faster charge time. This allows you to wire your solar panels in an easy high-voltage formation without wasting energy.
MPPT controllers are more expensive than PWM controllers, but many people find them worth the investment.
Most appliances use electricity in the form of AC also known as alternating current. Solar panels and batteries both provide energy in the form of DC or direct current. In order to power most appliances inside your home (like a refrigerator, TV, or toaster) you’ll need to use a device to convert the DC energy coming from your panels and battery into the useable AC form. A device that performs this conversion is called an inverter.
Not all inverters are created equal! Some can provide a huge flow of power all at once, but some are cheaper and smaller. Be sure to check the rating for whichever inverter you buy and compare it to your projected load.
Some users attach their inverters directly to their Solar Charge Controllers. Running your inverter directly to your battery bank runs the risk of over-discharging your batteries. Lead deep cycle batteries will be damaged if they ever run below 50% charge. However, running your inverter off your solar charge controller typically limits the amount of current you’ll be able to draw from your batteries.
IMPORTANT NOTE: Many appliances have a larger “startup” wattage. A refrigerator for example, will draw a few hundred watts while running but can draw more than a thousand watts when it’s first activated. Be sure your inverter is designed to handle your appliance’s larger startup load!
Weatherproofing and Safety
When working with electricity, use common sense safety precautions. Be sure to check your cables periodically for corrosion, and be sure to waterproof any holes you drill while installing your wiring in the roof of your home. Electrical shorts (the visible arc of electricity) can reach temperatures of 35,000 degrees Fahrenheit and ionize the air.
Always include disconnects like circuit breakers or fuses between the different parts of your system to avoid melting wires or damage to different components.
Installing solar energy into your home can be a complicated yet financially attractive project. On-grid solar setups can lower your electricity bill while at the same time helping you to feel better about your carbon footprint.
Off-grid solar setups can give you a nice backup in the case of a power outage in your city, or the ability to run a hair dryer in an otherwise rural cabin.