At Sun City Solar, we hear from potential prospects with this question quite often. Customers are currently grid-tied and paying a utility (the power company) for their electricity usage. The motivation to go off-grid is typically 1 of 2 reasons. One, customers are tired of being tethered to the power company and want to “cut-the-cord” as they’ve done with the phone and cable. Two, they hear about what happens when the utility grid goes down for an extended timeframe (like the winter storms in TX, a hurricane along the coast, or wildfires in CA) and they want to be prepared for this type of grid outage.
In either case, you CAN go off-grid if you are currently grid-tied to the power company electricity now. The bad news, though, is that typical residential-constructed houses are not MADE to be off-grid, and the price to do so may not be in your budget.
Going Off-Grid: Considerations
Two considerations when going off-grid are the AMOUNT of power that you need at any one time and the LENGTH of time that energy will need to be provided during periods of no or reduced sun.
First: The amount of energy needed
We’ll start with the AMOUNT of power needed first. Most houses in the US are built with 200A service. That means the power company can provide 200 amps at a given time to power all the loads in your house. I like to start with the most significant load; these are electrically driven motors or heating elements that use the largest share of electricity in your home. They include your:
- Oven
- Range Top
- Electric Central Heat
- HVAC compressor
- Hot Water Heater
- Electric Clothes Dryer
- 220V Pool Pump
Make a list of these for your house. If any of these loads are powered by natural gas and not electricity, they don’t have to be included. The more of these loads on your list, the more inverter power you need to make sure they can be powered when the grid is down.
Second: The type of loads on your system
Second, you typically have appliances on continuously for example a refrigerator (even though the compressor runs periodically), lights, and ceiling fans. Then, of course, you also have loads on occasion, like a microwave, hairdryer, curling iron, phone charger, TV, and other electrical devices.
Now imagine needing to power ALL of these loads at the same time. That’s why you have a 200 amp service to give you the ability to power your house all the time.
Off-grid inverters typically have only a 25-40A output with the ability to surge to 50A for a short period. So you’ll need 4-5 inverters operating in parallel to provide the same power output that the grid can provide your house.
Saving Money on Off-Grid Solar Systems
To keep costs down, we’ll typically design an off-grid system with only 1-2 inverters. We know that you probably won’t be running your pool pump and your clothes dryer during a power outage. You can also limit the large loads, so they don’t run at the same time. Plus, many of the greater loads in your home may be gas-run and not powered by electricity. A system with 12,000 watts of inverting power would be sufficient for you; also, you’ve adjusted your electrical consumption “lifestyle” during the off-grid periods. With a 12 kW system, a customer realizes he can’t run ALL of the loads in his house simultaneously, or he will overload the inverter.
It’s the second consideration, LENGTH of time to run the loads, that is the cost driver. When you are grid-tied, you have access to electricity 24/7. So the power is always available. When you are off-grid, your system only generates electricity when the sun is shining. Otherwise, energy must come from a battery for use at night and when it’s raining or the sun is not shining. We use batteries to store the electricity, and those batteries are the most expensive part of the solar system.
Consider
95%+ of the solar systems installed today are grid-tied. When the solar system produces more energy than what the house consumes, the excess energy is sent to the grid, and the power company gives you credits. These energy credits are later used when you need electricity at night and during periods of no sun. Thus, the grid acts as your energy storage (ESS) device.
For off-grid systems, excess energy production is stored. We use Lithium-Iron-Phosphate (LiFePO) batteries to do this. The batters are the ESS and how much storage you need depends on all those loads you listed earlier and how long they will run.
Example of Off-Grid Energy Usage
A 100W lightbulb running for 10 hr uses 1,000 Wh (or 1 kWh) of energy. You multiply the wattage of the load by the length of time in hours that it runs to determine its energy consumption. That is how you are charged for energy consumption at your house. Take a look at the attached graph from an electrical bill of a typical residential home.
This house uses an average of 26 kWh per day of energy. That’s 26,000 watts which is some combination of the house loads multiplied by hours those loads were running.
If we want this house to be off-grid, we need 26 kWh of energy stored in the battery so the house can use it at night and when the sun isn’t shining. A 26 kWh battery bank provides a single day of autonomy, meaning the batteries can store enough energy to operate the loads for one whole day if no sun is available on the solar modules to recharge the bank. However, you probably want several days of autonomy because there may be no sun for a couple of days when it rains. So with 2-3 days of independence, you need a battery bank storage capacity of 52-78 kWh.
Generator
Typically, we design for a single day of autonomy and recommend a backup Generator be installed to supplement the system during several days without sunshine. In addition, the generator helps keep initial costs down.
However, 26 kWh/day usage doesn’t paint an accurate picture of this house’s daily use. Look at the graph again, and pay particular attention to the summer months. In Aug & Sep, when the HVAC system is constantly running, this house uses a lot more energy than other months. Specifically, Aug uses 3000 kWh for the month or nearly 100 kWh/day. Sep it was over 115 kWh/day on average! We will need a battery bank that holds that much energy each day if we expect to live off-grid in this house during the summer. If you go with several days of autonomy, you will need a 200+ kWh batt bank.
Currently, you can expect to pay roughly $6,000 per 10 kWh of installed energy storage. So if you use 120 kWh of stored energy, you are looking at a battery bank that costs $75,000 for just a single day of autonomy. Not to mention that those typical residential HVAC units are pulling 20-30A while they are running. That means 3-4 inverters to support the loads.
A typical residential house moving off-grid can be done, but not at a cost most people will afford. I’ve provided some tips and information you can consider if you are building an off-grid house. I recommend reading that article if you are looking to go off-grid.
Conclusion: Can I go Off-Grid?
Can you go off-grid? Take a look at your electric bill and pay particular attention to the month with the heaviest usage. Then, determine the daily energy storage you need. If you remove your heaviest loads. Then plan to make lifestyle modifications around the use and availability of electricity from a solar/battery system. Then you CAN make it work within a reasonable budget. Typically, that means reducing or removing your HVAC system. In the southern summers, that is a difficult trade-off to make.
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