Off-Grid Systems Design

Designing an off-grid system doesn’t need to be complicated but it needs to be done. Not doing it can be expensive. Things continue to change (mostly, things get cheaper) so this is new 2017 information.

Right-sizing your system is the most important part of the design. But what does right-sizing mean?

  1. You need a system which can deal with both your peak loads.
  2. You need sufficient storage to cover most of your usage.
  3. Charge capacity.

The first two items are very different. And the word most in the second is important.

Peak Load Capacity

This means how much total power consumption your system can handle at one time. When calculating this you will likely need to make some decisions. For example, you may accept that you cannot run your coffee maker, toaster and power saw at the same time. That is unlikely to cramp your lifestyle as each item is something you use for a minimal amount of time.

To calculate what you need in terms of peak load capacity, it is easiest to start with your long-term loads. For example, lights which will be operating for many hours per day. Assuming you elect to use efficient lighting (e.g., LED) the total usage is not going to be particularly significant. Then toss in the things that may be operating for long times (e.g., the refrigerator) that you don’t have control over. Add up all these demands and treat that as a base load. Then start looking at high-demand units such as:

  • Well pump
  • Coffee maker
  • Toaster
  • Hot water heater
  • Any power tools you may have

With these, you can make a conscious decision of what can be operated at the same time. For example, if your well pump fills an elevated tank you could schedule times when the well pump would operate. Namely, times when other high-load devices will not be in use.

Here is a made-up example:

  • Base load consisting of lights, stereo, refrigerator: 500W
  • Well pump (programmed to only run between midnight and 5AM): 700W
  • Toaster or coffee maker (maximum of either one): 1500W
  • Power tools (one at a time): 1400W

This would give us a peak load of 3400 watts. Thus, a 3500 watt inverter would be sufficient. Note that moving most of the base load to battery operation rather than off an inverter would both decrease the size of the inverter needed and be more efficient by avoiding inverter loses.

I didn’t include water heating as it is generally better to do with a propane heater. Water heaters are high energy demand units. A propane flash heater is likely the best choice.

Sufficient Storage

Storage means the ability to run all these things when the PV panels are not producing energy. While not the only approach, this typically means batteries. (I will talk about other storage methods and the option to do a synchronous grid-tie system in other articles.)

To figure out how much storage you need, you need to look at the total energy (measured in watt hours) that you will consume. Generally, what you want to work with is typical consumption per day. You calculate this by multipling the wattage of a device times the number of hours per day it will operate. Here are some made-up numbers to better explain the concept:

  • Lights: 30 watts times 5 hours = 600 Wh
  • Refrigerator: 120 watts times 6 hours = 720 Wh
  • Well pump: 700 watts times 1 hour = 700 Wh
  • Coffee Maker: 1500 watts times 1/4 hour = 375 Wh
  • Toaster: 1300 watts times 1/6 hour = 217 Wh
  • Power tools: 1400 watts times 1/4 hour: 350 Wh

That gives us a total of 2962 Wh. Let’s call that 3000 Wh or 3 kWh (killowat hours). Now, the inverter is less than 100% efficient so let’s add 10% or 300 Wh for inverter losses. Finally, battery charging and discharging is not 100% efficient so let’s add another 20% or 600 Wh to cover those losses. That gets us to 3900 Wh. Again, rounding, lets call that 4 kWh.

Does the sun shine every day, all day? Probably not. A rule of thumb is to have storage to cover three days of no sun. The right number will depend on your local weather — three months rather than three days would be more like it in Seattle — but three days is a good starting point. That would mean you need 12 kWh of energy storage.

The assumption is that you will use batteries for this storage but what type of batteries must be considered. In particular, if you are using lead-acid batteries (still the cheapest alternative) you need design your storage such that the batteries are not discharged below 50%. That would mean you need 24 kWh of energy storage. With Lithium batteries, they can be fully discharged without damage. This means that as soon at Lithium batteries only cost twice as much per kWh, their initial cost would be the same and their life will be longer.

Let’s work with 24 kWh of lead-acid batteries. If you elect to use a 24 volt (nominal) system, that would mean you need 1000 ampere hours of storage capacity (24,000 / 24). You would probably make up that capacity with an array of 6 or 12 volt batteries connected in series-parallel. (All this will be detailed in another article.)

Charge Capacity

If you use 4 kWh per day, you need a charge capacity of 4 kWh to cover that but you also need more capacity to make up for the days when you didn’t have enough solar energy to fully charge the batteries. As the price of PV panels has dropped sufficiently in the last few years, adding more charge capacity is relatively cheap. So, let’s pick 6 kWh per day as your charge capacity.

How many solar panels is that? Panels are rated in peak watts — the power produced in full sun. A reasonable estimate of total energy produced from a panel is six times the peak wattage. Thus, a 300 watt panel would produce 1800 watt hours (1.8 kWh) per day.

Using the 6 kWh per day requirement, four 250 watt panels should meet your energy need. If, however, you can add some panels without having to upgrade the PV controller, it is probably worth it.

Explaining “Most”

Back in the beginning I said that you need sufficient storage to meet most of your needs. If you have a system that always meets all needs, you probably over-sized it. Demand can vary and so can weather. For that reason, you should always have some type of backup such as a gas or diesel generator. You don’t want to have to use them regularly but it is desirable to run them at least once a month.

In addition, if you very much over-sized your (lead-acid) battery bank, the batteries can sulfate from lack of activity. So, again, not having a system too big is a plus.

That’s the basics which should help get you started. Note that the system size I have proposed will work for a small system but if you use a computer all the time, a TV, … make sure you get the usage estimates in the energy budget.

More coming soon.