The Limits to Emergency Solar Power

by Scott Wetenkamp and Bob Buddemeier

Loss of electricity is the most common and most probable of the various emergencies that we try to prepare for. Outages of days to a week or more can result from storms (more likely in winter), wildfires (summer and fall), earthquakes (any time), and war, terrorism, or vandalism (any time).  In the case of a major earthquake, an effective hostile attack, or long lines downed by fire, the electrical grid could be down for weeks.

Access to adequate electrical power can be a life-or-death question for some (those dependent on oxygen or home dialysis), and a major health issue for others (those who need air purifiers, CPAP, etc.).  For all of us, it is critical for comfort, convenience and safety (communication devices, light, etc.). We take our power for granted. How many times have you flipped the light switch when you know the power is out?

At RVM, possible sources of emergency power include emergency generators for the high-rise buildings.  For long-term use these require refueling, which may also be interrupted in a disaster.  Fuel use depends on demand, so estimates are approximate, but without refueling, the Manor can generate 1-2 weeks of electricity, the Terrace several days, and the Plaza, a few days.  If it is not clear that refueling will be possible, or if demand is unsustainably high, the Incident Commander might ration use.  There is no backup power for the cottage areas other than provided by the residents.

This article addresses options for residents who need or want access to more, or more reliable, power than provided by the very basic personal  emergency supplies to provide light and communication (for RVM recommendations, log into MyRVM, then click https://files.mwapp.net/FILES/92680409.pdf/  — Residents Preparedness Group advice is at https://files.mwapp.net/downloads/83481020/cottage_electricity1.PDF on MyRVM).  After reviewing some access issues, we’ll address possible needs and uses for battery packs, and the potential for solar generation by cottage dwellers.

Apartment access:  Only the Terrace has emergency electrical outlets in the apartments; in the Manor and the Plaza there are a limited number of outlets in the residential hallways. In the Plaza and the Terrace the emergency outlets are red; in the Manor all outlets are white, and the emergency outlets will need to be identified by testing.  Use will have to be shared as there are far fewer outlets than apartments, and for things that can’t be charged or used in the hallways, extension cords or portable power supplies will be required (some Plaza apartment doors are more than 60 feet from the nearest emergency outlet).  Anyone who might rely on a common area outlet should add a plug-in outlet multiplier to their emergency supplies.  Battery packs may be a useful way for apartment residents to access emergency power, since they can be charged when convenient and used when needed.

Cottage access:  People could relocate to a high-rise, but in a serious emergency or disaster it could take many hours to days before staff or volunteers could assist.  In addition to having a plan for transportation, those who need reliable power should consider a battery supply that could provide at least a day’s worth of electricity, to ride out short outages or provide time to arrange for relocation.

DEMAND AND SUPPLY

We will start by identifying the electrical requirements of some common home medical equipment and basic appliances. Then we will review some battery storage units, and provide estimates of how long they might support specific equipment if fully charged.  Finally we will consider power generation, concentrating on solar power for cottage residents.  We will provide estimates of the equipment that can be supported by a solar generator under various conditions.

Electrical calculations can be confusing.  A watt is a measurement of power, describing the rate at which energy is being used.  Watt-hours are a measure of energy, describing the total amount of energy used over time.  Although we use watts (W; kW = kilowatts = 1,000 watts) for comparison of different conditions or pieces of equipment, our summaries of potential uses require no technical knowledge.  For those interested in understanding how the results were arrived at, we provide Appendix I.

Equipment requirements:  Table 1 shows the electrical power requirements (W) of a variety of appliances, and the energy use (W-hr) based on estimated usage time per day.

The numbers will be used later for comparisons, and don’t require immediate study.

The tabulated numbers are approximate, and will vary depending on the specific equipment and the pattern of use.

Table 1 includes some items that are not strictly essential for survival or safety – kitchen appliances, computer, and TV (assuming the individual’s emergency supplies include a suitable food supply and an emergency radio).  Other items are omitted because their power requirements are unrealistically high.  Space heaters are in this category, but an electric blanket is an effective and relatively low wattage way to keep warm.

Battery power:  There are many small battery packs on the market; some include or can be attached to small solar panels.  These are suitable and useful for recharging cell phones or laptops, and for low-wattage uses such as LED lights.  However, they lack the electrical capacity for high wattage or long term use.

For comparing possible battery and generator scenarios we will use the Jackery line of products.  There are many alternatives, but Jackery is a well-regarded company with a line of power stations and solar panels that provide a good baseline for comparisons.

The Jackery power stations that we will consider are the ones rated for 1000, 1500, 2000, and 3000 W-hr (they also sell smaller ones).  These units have 120  volt AC, 12 volt DC and USB 5 volt DC outputs, and they take approximately 2 hours to charge from a 120 v AC outlet.

Their effectiveness as temporary backup power supplies is easy to assess – divide the power station rating in W-hr by the appliance’s wattage.  Examples:  a 1000 W-hr unit will keep a 5 W LED lit for 200 hours.  A 1500 W-hr unit will power a 100 W CPAP for 15 hours, or about 2 nights.  A 3000 W-hr unit will power a large air purifier on high speed for 50 hours, a refrigerator for 24 hours, a portable O₂ concentrator in constant use for 10 hours, and a static concentrator about 6 hours.  If more than one appliance is powered at the same time, simply add the two individual wattages together before dividing.

An additional energy source that many people have is a car battery, which typically contains about 800 W-hrs of 12 volt direct current (DC).  To access this you need a connecting cable with an appropriate plug or battery clamps on one end. For small items like a cell phone, a cigarette lighter-compatible USB port can be used (Figure 2). These are small and quite inexpensive.   Virtually all modern cars already have USB ports. Powering AC devices from a car battery requires an inverter.

Solar (and other) generators:  For long-term power some sort of generator is required, either to provide continuous power or to recharge a battery system.  Since RVM does not permit resident storage of gasoline or diesel fuel, we will focus on the possible use of solar power, in the form of systems that can be deployed and managed by the resident.

Small solar panels, either separate or built into equipment, can be placed on a windowsill for charging a small battery used to power LED lights, cell phones, radios, and other low wattage appliances.  Unlike the large panels, these can be use in apartments as well cottages.  Figures 3 and 4 show some examples.  In Figure 4 the light is collapsible and water proof, with multiple brightness levels; the radio also has built in and removable batteries and a hand-crank charger, as well as a USB port for phone charging.

For sustained use of higher wattage equipment, much larger panels are required.  At present, RVM does not allow roof or wall installation of solar panels, so we will base our examples on the Jackery 3000 W-hr power station with 6 200-watt solar panels (Figure 5).

Before getting into examples, we need to review some discouraging aspects of solar power.  It seems that 6 200-watt panels should deliver 1200 watts of power, but that rating assumes unrealistically ideal conditions; actual output is probably closer to 160 watts. That assumes that the panels are tracking the sun on a bright sunny day in summer, and even then, there is maximum output only for about 3 hours around noon.

Continually adjusting 6 panels to track the sun is not a realistic option, so we assume that the panels are in the optimum fixed position (facing south and tilted at about the same angle as our degrees of latitude (42^o).  Going from tracking to fixed position reduces the energy collected, as does going from mid summer to mid winter, and clouds or smoke. Table 2 presents estimates of daily energy collection (kwh) in different seasons under various conditions.

Table 2: Seasonal and environmental effects on solar energy collection

A solar generator can provide for full-time operation of an appliance only if it collects at least as much energy as the appliance uses.  If we compare Tables 1 and 2, it’s clear that in the worst case (protracted cloudy days in winter), the system can provide continuous power only for the CPAP, the air purifier (which is probably not needed at that time of year), and the smaller household appliances.

The numbers suggest that a portable oxygen concentrator might be supported under good conditions (full sun, spring or fall), but that encounters another problem – storage capacity.  The totals are similar but the energy collection occurs over 12 hours, and the use over 24 hours.  In order to build up enough reserve in the day to operate the concentrator overnight, the power station would have to contain more than 3700 watt-hrs, but its capacity is no more than 3000.  In practice, that would mean there was not enough electricity available for at least 3 night-time hours.

When there are multiple low-wattage devices being used, and especially when there are other battery packs that can be charged, the “excess” electricity can be captured by concentrating use in the daylight hours.  However, battery capacity is a practical limit to year-around support of high-wattage equipment by solar power.

Other considerations:  In the example given above (portable concentrator in good spring or fall weather), the power deficit over 24 hours was only about 1000 watt-hrs, starting from the first daylight hour.  If the battery were fully charged at the beginning, it could discharge for 3-4 days before going to zero, and in midsummer the time would be even longer.

The panels weigh 18 pounds each and the main unit 64 pounds (but it does have wheels and a handle).  Fewer panels and smaller powerstations are more manageable, but have fewer potential uses.

Panels are $700 and the 3000 power station is $2800.  Add in a few cords and accessories and you are well over $7000.  A 1500 watt-hr station with 2 panels is less than $3500, but with considerably less capacity.

Usability at RVM – in case of a major emergency a resident would need to have unshaded outdoor space where the panels could be set up in a suitable orientation.  This would be possible around the lake, but difficult in areas like Horizon, Peach Tree, and much of the Quail Pont Circle area.

See Appendix I for additional considerations.

Overview: This review addresses options for individuals and individual households.  At the corporate or community level there are many other options available.  Solar units of the sort described may be suitable for people who: (1) want or need substantial reserve power even though it might not be fully available under all conditions; (2) can afford the cost; and (3) can perform or arrange for setup and operation.  Such individuals may want to consider this if their residential setting is appropriate.

For most people, settling on one or more smaller battery packs after evaluating potential needs and uses is probably more appropriate.