We’re often asked for details and specs on our solar/wind setup, especially when we talk about the abundance of power we have 9 months of the year. That abundance of power (and honestly, lack of battery storage) means that we’ve come up with some pretty extravagant ways to use the free glut of summer power.
I’m not complaining, I love being able to live in modern luxury while off-grid 9 months of the year. The other three power-starved months are another story…
Nonetheless, you asked for it, you got it. Here’s the story of our system.
When we found our off-grid homestead, we were intimidated. We’re agriculturalists, not tinkerers.
Both of us lack the mechanical aptitude to repair a car, regardless of how much time we put into trying to learn. A whole-house solar system was daunting, to say the least…
Beyond that, we were moving in blind, with no spec sheets or system documentation. The builder/owner had died, leaving no notes about the design. Much of the system was unfinished, literally open wires alligator clipped together dangling in the basement.
The house had been vacant for 2 years, and some of the system was in disrepair.
It took us over a year to get it functioning on a consistent basis, and even longer to feel like we even understood the basics of the system. After 5 years on our homestead, we’re still learning every day, and making improvements anywhere we can.
AC and DC Wiring
The entire house is wired in both AC as well as DC wiring. The wall outlet plugs are set up with two different outlets, and all critical systems run on DC (direct current) electricity straight from the batteries.
That includes the refrigerator, chest freezers, lights and heat pumps for radiant floor heat. That means that the inverter can be turned off at night to conserve electricity.
The AC side of the house powers things like our computers, the well pump and other modern electronics that you’d use during daylight hours.
Our setup has a total of 12 solar panels. Four of the panels are on our attached greenhouse, and they provide shade for the plants during the hot summer months.
During the winter months, the solar angle changes and they don’t shade the plants. When the snow flies, the heat in the greenhouse means that the panels melt out quickly.
These 4 panels were enough to supply the house with electricity, and did so without a backup generator for 10 full years from 1997 to 2007.
Later, a shop/office was added and 8 panels are high up on its roof. These panels are more modern, installed in 2007, but they’re still 10 years old and not as efficient as today’s panels.
The roof on the shop is super-insulated, and holds snow for long periods, meaning that these are encased in ice during cold spells.
When the shop roof is iced in during the winter months, we tend to get some power from our wind turbine to help make up for the loss of solar.
To get the specs on the greenhouse panels, we took a picture of the specs sticker on the back and zoomed in. Those things are way up there, but since they’re on the greenhouse at least we can see the back of them. But this spec sheet shows that they’re 300-watt panels.
The panels on the shop are still a mystery. We’re going to assume they have the same output, but we’re not exactly taking them down to check.
If they’re all the same, our total solar generation capacity is 3.6kW.
Take a look at that close-up picture, there’s a lot of information there.
Here’s a translation:
Pp: Maximum nominal power (ie these are 300W panels)
Voc – Open Circuit Voltage. Theoretical maximum voltage with zero resistance
Isc – Amperage with Short Circuit. Again theoretical maximum with zero
resistance, as if you took the two wires from the panel and short-circuited them together
Vp – Voltage at max power ie ideal conditions
Ip – Current (Amperage) at max power
Notice 17.5 volts * 17.1 Amps ~ 300 Watts
For more information on translating solar specs, check out this article.
These are some old panels, circa 1997. Energy efficiency has improved over time. If you’d like to know how much, check out this graph from the National Renewable Energy Laboratory.
Our wind power comes from a Bergey XL1 (1kW) wind turbine on an 80-foot guyed tower. The problem is, we hardly ever see wind.
We’re located in a micro pocket where there’s hardly any wind, even above the tree line. The air is still, and the wind turbine only turns during storms or in foul winter weather.
Why was it installed? Good question.
Our house was built by a solar contractor, and he needed to train and practice installing wind systems. What better way to learn than to install one on your own land?
Even if the wind did blow, a 1kW model is relatively small for wind power. I did a bit of research as to costs and found that about half the cost is in the tower. For ballpark figures, the turbine itself is about $4500 new, and the tower is roughly $3000 plus installation costs.
The manufacturer recommends a 6kW setup to power small homes and energy-efficient farms, but one of those will set you back about $22,000 plus tower and installation.
In our case, the wind turbine actually is really helpful just for the tower, even though we don’t get much wind. We get our internet through a radio internet provider, and the receiver in the picture above only works because the wind turbine gets it above the tree line.
Hooray for high-speed internet, but an 80-foot tower is a pretty expensive way to get it when you’re not getting much wind…
The generator was added after the initial owner died, as a requirement imposed by the insurance company before the sale. The house ran without it, under careful stewardship, for 20 years.
Under our inexperienced stewardship, the generator saw heavy use our first winter, and still a good bit of use every winter since.
It’s a Kohler Model 8.5RES rated for 7.5kW.
It’s wired so that there’s a bypass and the house can run directly from the generator even if the inverter, batteries or the rest of the system isn’t functioning. We can also run it back through the inverter to charge the batteries. Both have come in handy, believe it or not.
When we blew a fuse over a holiday weekend, the batteries weren’t charging from the panels, and the generator couldn’t get power to them. We were still able to run the AC portion of the house directly from the generator.
We have a 24-volt system, with three 8-volt lead-acid batteries (Model: Rolls 8CS25P). Since output depends on how quickly you draw them down, here are the actual specs per battery:
394Ah @2hr, 640Ah @8hr, 853Ah @24hr, 1,156Ah @100hr
If you’re slowly using your batteries at a baseline level, they’ll keep the house powered a lot longer. In the wintertime, unless we’re actively charging, we don’t use anything but the base load of the freezers if we can help it.
I often see others quote their battery capacity in kW. Since battery output varies based on how quickly you draw them down, it’s hard to give a good number in kW.
A reader sent me a note telling me that there is, in fact, a convention for how battery kW hour ratings are developed:
“The Rolls Surrette batteries you have are 820 amp-hours @ the 20 hr standard rate. Since you have three 8 volt batteries in series, the amp-hour capacity of your bank is also 820 amp-hours.
With your 3×8=24 volt system, you would therefore have a total theoretical battery bank capacity of 820×24=19,680 Watts-hrs (or 19.6kWh).
Since you NEVER, ever want to discharge your bank more than a 50% depth of discharge, your total usable bank capacity would be 1/2 of 19680 Watts-hrs = 9.8kWh. That 9.8 kWh value is the only one you want to consider quoting, and that is the real total MAXIMUM energy budget you can use before you absolutely must fully recharge your bank.
In reality, I think that a 20% maximum depth of discharge (I.e., staying at 80% capacity) is the best way to extend your battery bank life. Using just 20% for your bank would mean an energy budget of 3900-4000 Watt-hrs per charge cycle. My guess, based on your house description, is that would be close to your daily power usage, so that would only give you 1-2 days autonomy between complete recharges.”
That seems pretty accurate to me, as we do tend to have 1-2 days of complete autonomy on our batteries if there’s absolutely no sun (or the batteries are encased in ice in the winter).
I would absolutely love a longer period of energy independence between solar re-charges, as often we have no sun for longer than that. Ideally, I’d hope for at least 3-5 days, and then we’d almost never need a generator.
The previous owner was running 16 batteries of a type no one seemed to be familiar with. They were completely dead and wouldn’t hold a charge, and we replaced them when we moved in.
We were told that these three Rolls batteries would be more than enough. In hindsight, I would have gone with double the capacity. We fill them before noon in the summer and are power scarce with a lot of generator usage from December through February.
Inverter & Charge Controllers
Our inverter is a Magnum Energy MSH-RE series inverter model number MSH4024RE. It’s rated for 4000 watts of continuous power output, or up to 5800 for a 5-second surge.
Since our system is a bit cobbled together, we’re running one Outback C40 charge controller and two Outback MX60 controllers.
Full System Specs
Solar: 12 Solar Panels, 300 watts each, for 3.6kW
Wind: Bergey XL1, 1kW wind turbine
Battery Model: Rolls 8CS25P
Battery Specs: 394Ah @2hr, 640Ah @8hr, 853Ah @24hr, 1,156Ah @100hr
Battery Output @100hrs: ~27k watt-hours
Inverter: Magnum Energy MSH4024RE, rated at 4000 watts continuous output
Charge Controllers: Outback MX60 and Xantrex C40