Resilience (n): an ability to recover from or adjust easily to misfortune or change
As we enter the colder months of the year, preserving heating during potential power outages becomes a top priority for homeowners.
Unlike a heat pump or resistive heating, gas or oil burners use relatively little power. They usually use mains voltage, however, so they must be backed up with an Uninterruptible Power Supply (UPS) or an even more capable Battery Energy Storage System (BESS) that can be supplemented by supply from local solar or wind resources, or a small fossil fuel generator.
UPS for Backup Heating
Since our focus is on immediate, rapidly deployable backup, we explore a UPS solution. Because the UPS will be operating the boiler and associated pumps, it is important to use a sinewave-capable or a “pure sinewave” UPS and not one that provides a modified sine wave. Motors and transformers can be sensitive to the distortions of a modified sine wave supply. The boiler is a critical piece of equipment and expensive to repair, so why take chances? Also, a modified sine wave supply will create more losses in transformers and motors, resulting in a reduced runtime from the UPS.
The optimal UPS size depends on cost, length of minimum run time needed, and maximum energy drawn by the boiler appliance.
Measuring Energy Draw in Your Home
There are several useful devices on the market that can be used to measure energy draw. For instance, you could use a small remote-controllable switching device that is managed by an app from your smartphone or tablet. These plug in between the wall socket and the appliance, and most provide energy monitoring capabilities so you can easily read appliance draw directly from the app.
There are two important readings from the app. One is the average daily energy drawn, measured in kiloWatt-hours (kWh). The other is the maximum draw, usually seen when the unit is operating in full heating mode rather than in standby mode. The maximum draw is measured in kiloWatts (kW).
Let’s compare this to the speed and distance of a car. There is an average speed and a peak speed, both measured in miles (or kilometers) per hour. To get the distance traveled, the average speed would be multiplied by the hours driven (or mph times hours), leaving a resulting distance in miles. In this analogy, kiloWatts are analogous to speed and kiltoWatt-hours are analogous to distance.
Determining Which UPS to Use
My gas boiler uses 4 Watts (W) on standby and 70 Watts peak, which means I need a UPS that has a maximum supply capability of more than 70 Watts.
When I was preparing for cold weather earlier this year, my monitoring device told me that the average consumption per day was about 100 Watt-hours (Wh) or 0.1 kiloWatt-hours (kWh), which was low, but more or less expected during the warmer months.
Assuming the unit will be loaded fully about 50% of the time in the winter, I estimated that my boiler’s daily energy consumption will be about 840 Wh or 0.84 kWh by multiplying 12 hours (e.g., half a day) times 70 Watts (e.g., the maximum supply capacity). Therefore, I need a unit that can comfortably supply about 1000 Wh or 1 kWh before the battery is discharged if I wish to have about 24 hours of backup capability.
Working Through a Real-World UPS Example
This is the CyberPower Sinewave UPS model CP1500PFCLCD PFC, 1500VA/1000W, 12 Outlets, AVR, Mini Tower that we can analyze as an example. The picture and specifications are borrowed from amazon.com.
From the Spec Sheet, the relevant information is:
- Maximum load: 1000W
- Runtime at full load: 2.5 minutes
- Runtime at half load: 10 minutes
The 1000W maximum load easily meets my maximum load of 70W, but we need to know how long the unit will be able to keep the boiler operating during an outage. To get that we have to interpolate the runtime numbers, which means we need to do some more math.
When we take a casual look at the numbers, we can determine that the curve relating the power supplied to operating duration is not linear. The exact curve is not important since backup duration can be affected by variables other than load alone. For instance, it can vary with battery age, as it will degrade over time. Instead, we are looking for a close fit.
For this device, it seems that as the load diminishes, the runtime goes up disproportionately. Since we know the runtime of the battery approaches a longer duration when the load draw is lower, we can estimate this using the mathematical function:
y =a/(x + c)
The x-axis represents runtime, while the y-axis represents the load.
Solving for (1000, 2.5) and (500, 10), which represent maximum and half loads, respectively, gives a = 7500 and c = 5. This is an approximation, but it should get us close enough to estimate a runtime for my boiler.
The maximum load for my boiler is 70 Watts (y), so we can plug that into the equation to solve for runtime (x). This gives us about 100 minutes (70=7500x+5), which is less than two hours. However, we estimated that the unit will be loaded fully about 50% of the time and in standby mode for the other half. That means it will be drawing 70W about half the time and about 4W the other half, so the backup time is closer to 200 minutes or about 3 ½ hours.
The UPS in this example is relatively inexpensive at well under $300 or €300, but it is still not ideal for the purpose of providing backup power for gas or oil heating sources. These types of devices are designed to buy just enough time for a computer system or network-attached storage (NAS) to be safely shut down after all pending work-save operations have been completed. As such, the inverter’s maximum power capacity is higher than needed and thus more expensive than needed.
Also, the duration of supply is relatively short because the battery capacity is only sufficient to power a computing system for a few minutes. Some UPS devices can be attached to supplemental batteries, but that adds substantially to the cost.
Battery Systems for Backup Heating
Another option is the camping battery system. Here is an example priced at roughly $230, which is comparable to the cost of the UPS above. It is a 240 Wh device, meaning it has an average power flow of 240W over a 60-minute period. If we use our earlier estimate, it should provide backup power to the boiler for about 6 hours, roughly double that of the UPS.
While the camping battery has a larger capacity than the UPS, its maximum supply wattage (or output) is much lower at 200W. This is still more than sufficient for our 70W boiler. It is also less efficient, but not markedly so, because it does not include switch-over capability. In other words, power is always flowing through the camping battery system rather than switching on only when needed, like the UPS. This is one reason the cost remains affordable despite being a higher Watt-hour (e.g., longer-lasting) system.
Camping batteries can be charged from a vehicle, mains power supply (e.g., via a wall outlet) or portable solar panels, as you would expect for a device aimed at the camping and recreational market. It also can be charged while powering the heating system, and if power is lost, the appliance will continue to operate as long as the battery charge lasts.
Home Energy Resilience Assessment
Parts 1-4 of our energy resilience series have discussed relatively quick and easy ways to implement independent backup systems for internet, lighting and gas or oil heating. We have come to the end of what can realistically be made resilient in a piecemeal manner.
If we go back to the Table of Loads in the first blog in this series, all other home or SoHo loads have a relatively high current draw. Fitting each with a dedicated battery backup quickly becomes very cost prohibitive compared to a single large Battery Energy Storage (BESS) device that integrates with the building electric supply panel, shares the backup capability intelligently between multiple loads and rations backup power according to custom priority settings. This will be the subject of our next blog series.
Part 1 | Part 2 | Part 3 | Part 4
