I'm looking to design an off-grid power system for my house. I'm a complete novice at this point but I'm willing to do a lot of reading to complete the project. There is a lot I still don't yet know and I have questions.

I want a system that will allow me to connect solar, wind, water and a gas generator. I'd like to have both the A/C and the D/C power sources go into the charge controller to charge my battery bank and then the battery bank connect to an inverter. This way, I have many types of input into my battery bank and failing everything else, I can power up the gas generator and that will charge my battery bank too.

I understand that the charge controller can push any excess electricity into a bypass.

  • Am I seeking to do anything that is not possible with this?
  • Is there a way for me to combine the A/C output from the inverter and the excess electricity from the charge controller as input to my house?
  • If so, what is the device that would allow me to do this?
  • Are there charge controllers that will receive both A/C and D/C input so that I can hook all my power sources into a single controller or do I need something more complicated to handle this scenario?
  • Am I going about this the right way?

A little more background: As I commented in the notes, at the end of the year I am moving completely off-grid, so I have a year to figure out everything this entails and set it up - chickens, pigs, cows, water provisions, sewage and the whole 9 yards. I'm used to learning complex systems, so I know this is not beyond me. I've also lived an outdoor life for quite a lot of my life, so I'm used to being without the grid, so this won't come as a complete shock to me, but at the same time, I'm not looking to forego all modern conveniences. It's hard to throw out numbers as I'm not yet sure how exactly to calculate my needs.

Our hydro bill for last month has us pegged at averaging 29.84kW/h per day in January which thus far has been the coldest month of the year, December was 19.23kW/h per day.

Our heating/cooling and hot water are presently electric and the place we're presently living in has a criminal underuse of insulation between the units; this is not something I will suffer from with the next place. Not only will the insulation be better, but we're looking to move to geothermal to maintain a more temperate environment that will be supplemented by wood stove and propane for hot water, so I'm quite sure our energy requirements will drop significantly from our present requirements. We have high efficiency appliances, washer, dryer, fridge, freezer, dishwasher - I'd like to be able to continue using them - though I'm uncertain what the water situation will be yet, so the jury's still out on the feasibility/need for the dishwasher.

Our cooking stove is presently natural gas and I'll transition that to propane until I've mastered cooking on a wood burning stove.

I've put together a list of every piece of electrical kit we own, along with Wattage and Amperage requirements for each of them and put them into a spreadsheet, there's nothing particularly suprising on the list. I'm an avid cook/baker and I'd also like to be able to use all the tools in my workshop, most of which are presently electric, though I'm not averse to trading as many of those as I can to air drive.

I've been given advice that "peak" usage is what I should be gauging my electrical needs on - which means adding up all the appliances that will be in use concurrently, though a qualified(?) electrician we talked to suggested that that's not how you do it at all - can anyone advise if this is the case?

If I add up every electrical appliance we have, it totals 29kW & 234 amps. Near as I can tell, our peak load (max concurrent devices) would be around 15kW / 182 amps - and this assumes that washer, dryer, dishwasher, microwave, toaster, kettle, coffee machine and stand mixer, the tv, dvd player and sound system are all running, every light in the house is on and the fridge/freezer kick on - which rarely (if ever) happens. More typically, the washer, dryer and dishwasher don't get put on until after the kids are in bed, which means that most of the lights in the house are off and there's no cooking going on - though it's quite usual for the washer, dryer and dishwasher to all be running at the same time - whilst using TV (100W)/Sound (30W)/DVD (11W) and a couple of lights (perhaps 60-120W).


closed as too broad by Ricardo, PeterJ, Daniel Grillo, markrages Feb 17 '15 at 0:18

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    \$\begingroup\$ You should split this problem into modules, you want to much at once without having experience in it. Start maybe with thinking about the usual solar+battery+inverter. You can plan DC lighting for your house which is directly supplied by the battery pack as the next step. When you have thought this out, think about the other modules like wind and diesel. When you have every module well thought out, try to rationalize, reduce the number of components. Don't think about too much at once! \$\endgroup\$ – WalyKu Feb 14 '15 at 23:23
  • \$\begingroup\$ Hahaha that's never been my style :P I'm a software engineer by trade, so configuring multiple complex systems at once is quite within my realm of capability - as long as I can figure out what pieces I need. I have a year to get all these pieces together, at the end of the year, I am moving to a location that is entirely off-grid, so I have that time to figure out which pieces I need, figure out how to configure them and acquire them all ready for then. \$\endgroup\$ – BenAlabaster Feb 14 '15 at 23:27
  • \$\begingroup\$ Nice project that you have there. You should give some numbers. Say what kind of consumption you are expecting to meet. What kind of appliances are you going to use(load)? Can these appliances be supplied by DC directly? How do you want to control all your equipment? Add these to the question, and maybe make it more "list-like". Just suggestions, explaining a problem to others can help you solve it even if you don't get input from the others! \$\endgroup\$ – WalyKu Feb 14 '15 at 23:38

You gave enough information now. I'll just write point for point what comes to my mind.

Things to think about:

  • Starting current for motors can be 5 or more times the rated.
  • You can buy DC appliances, these can feed directly from the batteries DC. You should be able to find computer PSU-s that take in DC
  • All of your lighting can be DC fed
  • Adding supercapactors in parallel to your batteries lowers the loading of the batteries in transient processes. The caps don't store much, but can supply it really fast. This is of more use for e-mobility(regen. braking), as you will have enough battery capacity that it won't matter.
  • Look at inverter modules that can (explicitly) be used in parallel, that way you can buy another module if you see that it won't fit your needs.
  • Your wind turbine will produce AC of a specific frequency(I guess you won't buy a brushed generator). This AC will have a frequency and voltage that won't match the declared values of the appliances, therefore it is converted to DC and back to AC. Sometimes a rectifier and inverter are integrated in the turbine, so it supplies usable AC directly.
  • Regarding your diesel/gas turbine, look for one that gives you single-phase AC directly. It will give you the right voltage and frequency to power your appliances (after spin up).
  • Appliances can be susceptible to low quality of the supplied electrical energy. For your case the most important factors will be frequency, harmonic (non 50/60Hz) components, voltage, reactive power(power factor).
  • Your inverter has to be sized to the apparent power. This is composed of the real(usable), and reactive(out of phase) power. The reactive power is mainly consumed by motors for magnetization(for example). A lot of other appliances also have a non unity power factor. You can compensate it though by using capacitors at the load point, but this can again cause higher voltages, when not loaded. You can use relays to remove them from the circuit when not needed. I guess you won't have to go that far.
  • You will need DC/DC converter modules for your solar panels, I assume you knew that.
  • If you get(have) an electric car, you can use the battery of it as backup source for your home. Where you get all the energy for charging is another question(if it's full electric).
  • Match your equipment (rated in/out)
  • You can use your diesel in parallel with the inverter, but you must synchronize it. That means that both need the same frequency, voltage, and voltage phase-angle. If you don't do that everything can go bad very quickly (circulating currents, resonances, instability, overloading/heating and finally blackout).
  • You need to get some device that will allow you to do this synchronisation, instead you can feed the batteries from it and accept the losses in the rectifier and inverter.
  • Another option is to create islands, meaning that you use the inverter and generator on different circuits that have no connection. You can switch between regimes (all inverter, part inverter part generator, all generator), this can be done with mechanical relays easily. You have to figure what configuration would make sense.
  • if you want to use wood for heating think about wood pellets too. They can be fed automatically to your heating stove, meaning less work.
  • One important thing. There are inverters that need to be connected to the grid to work (they can't create their own AC system). Obviously you shouldn't buy those.
  • If you have a well and get a lot of excess power, you can use it to pump the water to a high reservoir. Kind of pumped hydro storage. But that will be expensive and complicated.
  • Geothermal uses some power for pumping, I'm not sure though how much. Don't forget to factor this in.

Look at the inrush currents of your big appliances, that is the thing that I'm most concerned about. Without looking at the overload capacities of the inverters you shouldn't make decisions on sizing. A thing about sizing, what is the expected growth of the consumption? I'm hesitant to give you recommendations on the inverter ratings, but I wouldn't go below 1.5 times the worst case(the sensible combination of appliances working at once). I don't feel good about making recommendations, as it depends on a lot of parameters.

You must keep in mind that my knowledge regarding this is mainly based on grid level problems. You should get some more input from someone who did this. Try off-grid forums maybe.

  • \$\begingroup\$ Thanks for that very detailed response Kurtovic, you've given me much food for thought. I had already considered some parts, but not others. I didn't know you needed a DC/DC converter for the solar panels, I thought you ran them right into the charge controller and on to the DC batteries. I already have a gas powered 8kW/10kW generator, not sure how suitable it is yet amazon.ca/Briggs-Stratton-30471-Portable-Generator/dp/… (scored it at an auction). I wasn't aware there were inverters that required the grid either. Great to know! \$\endgroup\$ – BenAlabaster Feb 15 '15 at 14:42
  • \$\begingroup\$ With the DC/DC I actually thought more in the direction of power optimizers, you are right if you match everything(with the same irradiation too) you will be fine without a DC/DC. Your generator should do, just find a way to hack the control. The synchronisation could be tricky though, but your inverter could be able to do the sync automatically. The inverters that require the grid have thyristors, but I think that they aren't used in the \$\mathrm{W}\$ and \$\mathrm{kW}\$ scales. \$\endgroup\$ – WalyKu Feb 15 '15 at 21:29
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    \$\begingroup\$ There are many inverters, some with integrated charge controllers, which can take the series DC of a "string" of PV panels. But if you're going to do PV, you want maximum power-point tracking (MPPT). The best way to do that is with power converters (DC - DC) which will do the MPPT per panel and provide a steady, regulated DC output which you can connect in parallel. That will simplify the inputs to your change controller / inverter. Get a few issues of HomePower magazine. Each issue usually shows a couple installed systems like what you're describing, what equipment, power calc, etc. \$\endgroup\$ – Meower68 Mar 3 '17 at 22:05

I am not sure what is available on the market but you will need an inverter that is synchronized with the power grid. So that switching between systems or feeding back into the grid your output will match the grid frequency and timing. Any system you build Unless it is extremely powerful will probably not run your electric stove or heating system but supplement what is coming from the grid. That is where synchronizing would allow you to feed some current into the system while still running off the grid.

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    \$\begingroup\$ There will be no feeding back into the grid, this is a totally off-grid system. I'm not interested in paying the hookup to the grid, nor am I interested in paying the delivery charges that come with being connected to the grid. I'm done with grid living. \$\endgroup\$ – BenAlabaster Feb 15 '15 at 1:21

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