This is a guest post by Jerry Noel. This article will address surviving without the grid, i.e., how to keep thing going when the power goes out – in Jerry words, “Preparing for When the Doo Doo Hits the Oscillating Appliance!”.
During the aftermath of super storm Sandy, many were without power for weeks. In my previous article, How Much Energy Will a Solar Electric System Produce?, I designed a system for my old house using the limited space available. I was planning on designing a grid connected system for our new house, but decided to switch due to numerous questions on the topic of back-up power. I also didn’t want to do a redundant article just yet. In my second article, Emergency Power Options for Your Home, I gave a brief overview of a battery backup system.
This article will focus on sizing the bank of batteries to help you ride out the storm until power is restored. Part Two will show the sizing of the solar array and wiring to keep the battery bank powered. In electrical terminology, this is considered to be a “Utility interactive, battery backup system”. A utility interactive, battery backup system is a system that under normal conditions is connected via your residence to the electrical utility but will act as an emergency power supply during a power outage.
What Batteries Do You Need for a Solar Backup System?
Whenever you need emergency backup power or at night, you need a bank of batteries. Instead of burning up hours explaining them, you can simply look at Battery Basics: A Layman’s Guide to Batteries. There are plenty of articles written by people more knowledgeable than me, so I’m not going to try and B.S. my way through it. What I must say is NO – you CAN’T use a car battery. It must be a deep cycle style. Lithium Ion battery technology hasn’t progressed far enough to be a factor yet.
What Other Design Factors Need to Be Considered?
When designing a grid connected system the limiting factors are:
- Available funds
- Location size
- Energy consumption
(Usually in that specific order.)
When designing an off-grid system – or in this case a grid connected system with battery backup for an extended time – the primary factor is consumption. We need to plan to have the amount of energy necessary to survive an extended time without power. Since we are not going for a complete off-grid system, we can make the battery bank smaller. We can live without games for a few days to save energy.
Before we even think of designing a system we must first make sure that all available efficiency steps have been taken. That means LED lights and Energy Star rated appliances. $1 in efficiency spending typically equates to $3 less in solar panel system savings. That amount magnifies dramatically when batteries get introduced to the system.
I will be using the historical data for our previous residence since it has 27 months (image above). 2010 shows a sizable amount over 2011. That was due to older electric appliances (range & clothes dryer) that were changed to more efficient natural gas ones. The summer of 2010 was also warm and we had an infant (Jack) at home so the a/c was on more often. In 2011, Jack was in day care so we turned the a/c off during the day. We also upgraded the central a/c to a more efficient one. During the winter of 2012, we had already moved to an apartment and were selling the house, so the heat was low so those three months can be disregarded.
In a true off grid system we would have to account for worst case which is the winter months. Since this is only for short duration emergency survival, we will assume that people will make needed sacrifices until the power is restored. We will plan for 3 days of autonomy (cloudy days with minimal sunlight to recharge the system) with a demand factor of 200KWh for the month. This is less than the maximum demand of 400 KWh. This gives us the daily usage of 6 ⅔ KWh that we need x 3 days = 20 KWh of capacity. We never want to discharge our batteries below 50% so now we are up to 40 KWh.
Why so low? Simply put, during an emergency situation you will be surprised at what you don’t need and how you can adapt. Cooking outside on a grill instead of using the oven is one of many options. Reading instead of watching TV for hours is another. Try drying your clothes on a line instead of the dryer sometime. This even works in winter if you have lines in the basement. To get a really good idea of what is consumed, get a Kill-a-Watt meter. They cost roughly $30. Some public libraries stock them for checkout.
What Battery Voltage Should You Choose?
For a small system 24 volts is adequate. For a whole house system we need to increase that to 48 volts. Why not higher like 96? Because the National Electrical Code requirements change (and costs escalate) once you exceed 50 volts nominal. Increasing voltage decreases amperage flowing in the wires. Wires are sized based on the amps they must carry, so lower amperage means smaller wires which saves money. Using Ohm’s Law, we know that Power in Watts divided by voltage yields the current. 40,000 watt-hours ÷ 24 volts results in a current of 1666 amp-hours. Since current is added when sources are connected in parallel, we need to add the amp-hour capacity of the batteries to exceed 1666.
The Trojan L16P battery has an amp-hour rating of 420 Ah. (See image at top of post.) Four of these connected in parallel will suffice and offset any inverter efficiency losses. Larger batteries typically have a lower voltage. This one is only 6 volts, so we will need 4 connected in series to build our voltage to 24 volts. Yes, we will need 16 of these big boys (at $300 each!) to keep us up and running. LED lights sound cheaper now, don’t they?
Here is a basic line diagram for what the system will be like. The critical loads will be transferred from the main panel to the emergency one. The inverter will act as the transfer switch during the power outage. Not shown is the solar array and the battery charge controller, which is the next article.
Scaling Down the System
This seems excessively expensive, and it is. Is there a way to make it smaller and more affordable? Probably. We need to decide what is critical and how much power it draws. We also need to hire an electrician to install a smaller “Critical Load” Electrical Panel. We pull the circuits out of the main panel and power up only those that are crucial to survival.
Where you live is important to what you need. In the north, we need heat in winter. Electric heat is a power hog. An efficient forced air furnace doesn’t draw much, just for the blower. In the south, you don’t always need the central A/C. A small window unit to cool a bedroom might be possible, but beware; they also draw lots of juice. A gas range only needs enough to fire the electric ignition. Microwaves can be useful as long as they are smaller units. Anything with a heating element will draw lots of power, so the coffee maker is out; find a way to brew on the stove. Same goes for the toaster and hotplate, etc.
Lights are useful and as long as you install LED or CFL lamps, they don’t draw much, and turn them off when not in use. My kids like to turn on every light and leave them on. Communication is also vital so have a circuit for radio, television, internet, and cell phone charging. You don’t need the home theater with the 72” LCD TV with video games during this time. DVRs draw full power even when they are “off”, so make sure they are not connected. Suck it up and do without for a while; discover books again. Your neighbors are in the dark so don’t get greedy. Otherwise they will all invade your house and drain your power so that all of you are screwed.
Next up: I will go into the design and sizing of the solar array feeding into the system to keep the batteries charged. This will also include the inverter, charge controller, and other components that will chew up your checkbook.
Disclaimer: This is not a “Do it yourself” article. It is a basic explanation of how a utility interactive, battery backup system works and what goes into the design of one. Designing a system to function 100% off-grid or as an emergency backup system takes in depth analysis of the site and the occupants’ habits and needs. I do not receive compensation from any manufacturer. Anything I show or endorse is based on my opinion.
In Part 1 (Solar Electric Basics), Jerry discussed the three predominant questions of solar power: cost, batteries, and “DIY”. Part two provided Emergency Power Options. Article 3 discussed three types of home solar power projects and how to mount your solar panels. Article 4, Solar Site Check, demonstrated how to analyze your location for best panel placement. Article 5 focused on designing your solar electric system, specifically, getting the most out of your available space. How Much Energy Will a Solar Electric System Produce? provides projected output and anticipated short-term financial savings and long term return on investment (ROI).
View all our Green Home articles here.