Backup Power For My Cabin

Quote of the Day

Today a man owns a jackass worth 50 dollars and he is entitled to vote; but before the next election the jackass dies. The man in the mean time has become more experienced, his knowledge of the principles of government, and his acquaintance with mankind, are more extensive, and he is therefore better qualified to make a proper selection of rulers—but the jackass is dead and the man cannot vote. Now gentlemen, pray inform me, in whom is the right of suffrage? In the man or in the jackass?

— Benjamin Franklin on the practice of allowing only property owners to vote. Ben Franklin provided support for the voting rights of every citizen and not just of property owners.


Introduction

FIgure 1: Tesla Powerwall Battery Pack (Source).

Figure 1: Tesla Powerwall Battery Pack (Source).

My wife and I are in the process of designing our northern Minnesota retirement cabin – the current structure is too primitive for any extended stay. Because power is unreliable in the northern woods, I am researching whole-home battery backup options. One possible option is the Tesla Powerwall, which provides 6.5 kW-hr of energy per battery pack. You can increase capacity by adding battery packs as you need.

Backup power systems are important for remote residences. For example, a friend of mine recently put a whole-home backup system in his cabin, and he almost immediately put it to use after a major storm knocked out power for many hours. The storm unleashed a tremendous amount of rain during the power outage, and my friend needed to run his sump pump continuously during the storm to keep his basement clear of water. His backup system allowed him to enjoy his home and to preserve its value.

I want a backup system that can meet the same scenario he faced. As with all my requirement determination efforts, I begin by defining my use cases – in this situation, there is just one use case.

  • Power outage lasts for 8 hours.
  • Rain is causing me to run a sump power continuously.
  • I need to keep a small number of lights on – all are LED-based.
  • I need to keep a refrigerator running – I want to ensure that I have access to food.
  • I need to keep a gas-heated furnace with electric blower motor running during the outage.
  • I need to keep an well-water pump running – fortunately the well pump only needs to operate intermittently.
  • I do plan on having a powered generator, but I would like to avoid running that for short power outages (i.e. less than 8 hour).

In this post, I will estimate my energy needs and determine if one Powerwall would meet my needs or if I would need multiple units. A single Powerwall and inverter is estimated to cost $7340– ouch! I will need to compare this cost to that of an equivalent lead-acid backup system. That will be the subject of another post.

Background

Limitations of My Analysis

I am ignoring a number of important concerns:

  • Electric motor surge currents

    The usual rule of thumb is that a motor's starting current draw is 4x its nominal current draw. This surge can be accommodated a number of ways that do not require adding more batteries. However, surge current is a real problem that must be dealt with.

  • Temperature-dependent battery capacity

    Battery capacity decreases with temperature. I need to decide how I am going to deal with keeping the batteries from getting cold. I could bury them in a Controlled Environment Vault (CEV). However, they are very expensive.

  • Battery aging

    All batteries age because of various corrosion processes that occur within the cells. This aging is accelerated by high temperature.

This post documents the quick analysis I performed to scope my energy needs relative to what the Powerwall delivers.

Refrigerator Average Power Estimate

The US Energy Information Administration (EIA) requires the listing of the average annual energy usage of appliances, including refrigerators. I grabbed the estimated annual energy usage for a Samsung refrigerator and converted the annual energy usage into a power as shown in Figure 2.

Figure 2: Computing the Average Power of a Refrigerator.

Figure 2: Computing the Average Power of a Samsung Refrigerator.

Analysis

Figure 3 shows how I computed my emergency power requirements and determined how long one and two Powerwalls would be able to provide that level of power. These calculations show that I need two Powerwalls to meet my 8 hour requirement. Since Powerwalls are very expensive, I will need to repeat this calculation for a lead-acid alternative – I want to use the most cost effective solution.

Figure 3: My Calculations for the Backup Time Available Using One or Two Powerwalls.

Figure 3: My Calculations for the Backup Time Available Using One and Two Powerwalls.

Conclusion

This was just a quick calculation to estimate the kind of backup time I could expect from one and two Powerwalls. I need to compare this cost with the equivalent cost from a lead-acid battery pack. I assume that the lead acid-based system will be much larger and significantly cheaper. A little math will reveal that answer shortly.

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