METHODS: Off-Grid Feasibility & Sustainable Load Shifting

Intermediate
Shifting loads
Shifting loads renewably can be difficult
Shifting loads

With no load shifting and no generator, the author tackles the economics of taking her almost all-electric on-grid home off the grid.

When people visit our rural house for the first time and see the roof-mounted PV array—52 modules covering most of the south-facing rooftop—they often assume that we’re off-grid. Nothing could be further from reality—in fact, all but one of our household loads are supplied by electricity, which would be highly unusual for an off-grid house. The exception is space heating, which is normally provided by direct solar gain and a small, efficient wood heater.

I often find myself explaining why we chose a grid-tied PV system—how, even though we have a multitude of electrical loads, our system capitalizes on the utility’s net metering program and offsets nearly 100% of our electricity use annually. Then I explain how much an off-grid system would cost to meet our home’s loads—especially in winter, when it may be necessary to supplement our passive solar gain with electric heat, but sun-hours dwindle. Without any load shifting (using a non-electric energy source for a task, such as propane refrigeration) or generator use, an off-grid PV system would be ridiculously large, complex, and come with a similarly ridiculous price tag.

Nerding on the Numbers

Just how big? My son and I took an afternoon to record, measure, and estimate every wintertime household load—down to the LED nightlight in the master bath. Even though we rarely use it, we also included the estimated energy use of our backup hydronic heating system, which is served by a 30-gallon electric water heater. (We almost exclusively use our wood heater for space heating, but I wanted this exercise to reflect taking an all-electric, “efficient” home off the grid.) We calculated an average daily winter electrical load of approximately 48 kilowatt-hours (kWh).

Generating that detailed list was time-consuming enough, so I turned to AltE Store’s online off-grid system calculator (bit.ly/altEcalculator) to estimate how much the system might cost. I didn’t get into the complexity of sizing for surge loads of various appliances, which was beyond the scope of this exercise, but those need to be considered to size the inverters.

I sized the system for only two days of autonomy (i.e., relying strictly on the battery bank), which would mean extreme load shedding measures or dependence on a backup generator during longer periods of cloudy weather. According to their calculator, we’d need:

  • 32.84 kW of PV modules. Cost: $32,550.

If we used 315-watt SolarWorld modules, that means we’d need about 105 of them. At 39 by 78 inches each, we’d need 2,218 square feet to accommodate them. The south-facing roof on our house is about half that size, so we’d also need to consider a ground-mounted system or build a very big barn with a very big roof. It’s important to note that location plays a prime role in the system’s size. For example, if we lived in Boulder, Colorado, we could get by with an array that is 54% smaller—48 modules instead of 105.

  • Depending on the system’s configuration, we’d also need five OutBack Power Radian 8,000 W inverters. Cost: $22,587
  • The 48 V battery bank would consist of about 72 Surrette flooded lead-acid batteries; 2 V, 1,765 Ah at the 20-hour rate. Cost: $67,800.

We’d need to build a power shed to house this equipment or dedicate a room in the new barn we’d need to build—adding more cost to the project.

  • We’d also need nine charge controllers for the huge battery system. Cost: $4,990
  • Total for modules, inverters, controllers, and batteries: About $128,000—without shipping.

To fund just the basic equipment cost, we’d need very deep pockets indeed. Keep in mind that this is the bare-bones price; I haven’t included mounts, combiner boxes, circuit breakers, wiring, mains panel, nor labor costs, which would add thousands more.

Revisiting Load Shifting

Unless we win the lottery, an off-grid system would definitely be out of reach for us without some serious load shifting (see “Shifting Loads Renewably” table). Some loads are obvious and easier to shift in a more sustainable way (i.e., without resorting to fossil fuels). For instance, on sunny winter days, we can use our portable solar oven to bake instead of an electric oven, and hang clothes on a line to dry outside. When the rain hits, though, we’d be out of luck on both counts, and would need to shift loads away from electric. For example, we could use a wood cookstove for baking and line-dry our clothes indoors.

The most difficult load for us to meet and still be “sustainable” would be water heating, since cloud cover can be long-lasting here during the winter and solar water heating collectors will work only moderately well. Marrying the solar collector piping to a hydronic loop(s) in a wood heater could be a renewable solution, albeit somewhat complicated, as we’d have to retrofit our existing wood heater or replace it.

The consequences of this load shifting would be substantial: a reduction in our electrical loads to about 14 kWh, which would be a daily savings of about 34 kWh.

The lesson: When you’re off-grid, it’s usually far less expensive to load-shift than to try to serve heating loads with PV-made electricity. An additional investment in appliances and systems for load-shifting ($14,700) reduces the off-grid system equipment costs to about $45,000—together, only about one-third of the cost of the originally designed system (9.58 kW: 31 modules vs. 105; 24 instead of 72 batteries).

Comments (14)

Mike Swift's picture

Another way to dry clothes without having to hang them around the house would be to use one of the new heat pump clothes dryers from Whirlpool. These units draw about 1 kW vs 5.3 kW for a conventional dryer.

Mike Swift's picture

I believe a far less costly system to get hot water would be to add another one or two kW of solar voltaic panels and use a heat pump hot water system like the one from Nyle. When I installed one on my electric water heater it reduced my electric use by two-thirds. Adding two kW of panels should produce much more energy than the heat pump uses, and would cost about $ 3,000 total plus labor.

Colin McCubbin's picture

Hi Mike.. I agree that solar PV is the best 'bang for your buck' as far as water heating goes, no pumps, no glycol, no valves, no leaks etc.. However since I live in a heating zone (Canada) I do have a problem with air source heat pumps that use heat from within the house. I spend $$$ to heat the house for approximately half the year, and yes, a heat pump like the Nyle-R (a neat unit BTW, thanks for the heads up) would suck that heat in and give me hot water, but it would also cool the house in the process, costing me more to heat the house.

Thinking about conservation of energy, it seems to me that for every KW of heat put into the water, a KW of heat must have been taken from the house.

So for me it is a conundrum, and makes me ask " Does the saving in water heating bills exceed the extra cost to heat the air in the house"? If anyone here has an answer to that I would appreciate a reply. Thanks!

Michael Welch's picture
Maybe it can go in the garage, and be a three-season (or 2.5 or whatever) water heater.
Jenz Yoder's picture

Getting your house as efficient as possible is key to being off grid. I think it would be very hard to to retro fit a house to be off grid without extreme expense as stated in the article above. Also depending on where you live can make it very hard to make the numbers work for off grid. It would have cost me 20,000.00 to run electricity to my new house so I went off grid. I am starting with a passive house (r60 walls and r100 ceiling) with stained concrete floors that have solar hot water heat in them. An air exchanger system keeps the air fresh. Since it consistently cools off at night here we will pump in the cool air at night and close up for the day. The only energy required for this system is 12 volt pump and 12 volt fan. We currently live in the barn as we build the house and have a 3000 watt pv array with 400 amp hour Iron Edison batteries. I plan on adding 2 1000 watt wind turbines since our average wind speed here is 12-13 mph. We live in in a semi arid area that in the last year we have had a total of 4 days in the last year that we did not have full sun or winds over 15 mph. I have friends in other states who want to be off grid but when we crunch the numbers it just does not make sense financially to do it. Since i can do the install all myself my cost for my system should come in under the 20,000.00 it would have cost to run power to my house.

kenbell48's picture

Colin. I got a TEEL 1P 760A 0.5amp 115V water pump from Grainger and about 50' of 1/4" copper tubing along with the necessary fittings to connect and run the tubing to my wood stove. A very large pot sits on top of the wood stove filled with water from a toilet valve and T in the copper tubing. There are 3 coils of copper tubing in the pot as a heat exchanger. The pump then circulates water to my gas water heater(to the in side and from the out side in the regular water line to and from the water heater). You can control the temperature in the pot by covering/uncovering it, but it will never do more than boil. If you have thermal solar panels you can hook it up to those in the summer. If your wood stove top has two levels the higher one runs about 100 F degrees warmer than the lower surface. You can use an on/off valve or a float valve for the water level. I tried using sand as a heat exchange medium originally and it was worthless. You also have to be patient. It takes time to heat water. I hope this helps.

Colin McCubbin's picture

Thanks Ken, very neat! I'm a recent arrival in Canada from Europe (the UK) where there are literally dozens of manufacturers of wood stoves with built in water jackets of various sizes and combinations, so was really amazed that Canada (and I suspect the USA) has no authorized manufacturers. I realize that N America in general has no great history of water based hydronic heating systems, everything here seems to be based on moving air 'bout the place, usually noisily too ! ;-)

Mike Coldren's picture

Have you considered a geothermal HVAC system? The new and better ones heat water at the same time and dramatically reduce power consumption and they are environmentally attractive. A local contractor says I can convert an existing (forced air) system for about $6,500 a ton.
Would save a ton off your numbers.

hans harder's picture

I like your article but did you consider incorporating your existing Pv system in this exercise? It would be a more real life example and more realistic for most people who already have Pv including yourself. AC coupling your existing system would allow you to keep your existing grid tied inverter and infrastructure. It would also work better if you are putting Pv on two different structures in the future.
Maybe a follow up article would be AC coupling your existing system, load shifting some appliances(don't do the propane fridge) and finally getting a generator to reduce the size of the battery bank you would need. Gen could also be your source of hotwater in winter and it wouldn't need to run very often.
Regardless I appreciate the lesson, offgrid is a lifestyle change and it is typically very hard to take an existing grid tied house totally offgrid without a ton of money.

kenbell48's picture

Your numbers look impressive but are totally unrelated to reality. I live off grid and use a propane stove that cost $800, not $2600, and uses about $10/month worth of propane. I have 3 indoor drying racks that cost $15 each. I have a heat exchange system on my wood stove that heats water in the winter and cost less than $200. Propane backup tank cost $300, rarely used. My solar panels and excess electricity heat water in the summer, so I have never had to use the copper and glass heat exchange panels I bought 25 years ago. With a wood stove and adequate insulation you don't need hydronic space heating. The massive $128,000 you cite for the total system cost is if you do no load management and continue to use power with no regard to cost or efficiency. Even your last line about cutting the cost to $45,000 is extravagant. I have 24 six volt batteries, two load controllers and a 3.6KW Outback inverter for my system. Total cost: Under $15,000. Location: northern California, 2500' elevation.

Colin McCubbin's picture

Can you tell us more about your " heat exchange system on my wood stove that heats water in the winter and cost less than $200." As far as I know, (and I've looked) there are no 'legal' CSA approved water heating wood stoves for sale in Canada, (I realize you are in the USA) so I'd be interested in your solution/system. Thanks!

Claire Anderson's picture
Hi Ken ~ Your points are well taken. That Methods was meant to explore the feasibility of taking an existing on-grid, almost all-electric home off-the-grid— but without load-shifting to fossil fuels (like propane), which come with other costs. Thus the pricing for a wood cookstove ($2,600), which uses a renewable source of energy, instead of one that relies on fossil fuel. It's true that through conservation efforts (such as using indoor drying racks instead of a clothes dryer) and investing in good insulation (and wearing a sweater!) can go a long way to reducing the need for generating energy in the first place, and thus will shrink the PV system size accordingly. As a solar-electric installer, however, my husband routinely encounters people who think they want to take their homes (which have far more extravagant loads than the loads specified in the article) off the grid, but they're not interested in efficiency or conservation measures. The article was intended to demonstrate, then, the excessive system size that would result if the practical measures you point out (load management, load shifting, and conservation) are not implemented.
Claire Anderson's picture
Hi David ~ Sorry for that oversight. My house is in southern Oregon (a far cloudier climate than Boulder, Colorado). December peak sun-hours, for example, are 1.66 for this location versus 3.88 for Boulder.
David Kingsbury's picture

Claire,
Great article. However, the article does not tell us your location (at least what state and perhaps microclimate). It just mentions that if we lived in Boulder, CO we would need an array 54% smaller.
Thanks, David

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