Sizing Off-Grid Systems And Using Gen Support

[pleppik]

Scheduling loads is one way to use peak solar power during the day. However, it is not always possible. We are many times out working in the field during the day, or gone doing other things. We designed our system on purpose to be able to do what we want when we want to do it. And we are not afraid to have to use a genset to make it happen. It is about convenience.

Have you explored trying to recover waste heat from the generator to supplement your home heating? I know there are some commercial "Combined Heat and Power" units on the market--not very common in the U.S., but apparently more popular in some European countries. It seems that it shouldn't be too hard to build a simple system to collect at least some of the heat from the generator.

I would guess that if you use propane for heat in the winter, you probably spend more on that than you spend on either batteries or fuel for the generator.

[ChrisOlson]

Robert1234 -
The XW-AGS has four load start triggers. One of them is based on the sum of the current on L1 and L2 and is a VA measurement to determine actual load on the system. I always use VA or kVA instead of watts because to convert from watts to VA on AC requires use of the Power Factor. Watts is amps x volts on DC. VA is amps x volts on AC.

The VA setting (or Load Start Amps) prevents long term inverter overload.

The other three are based on battery voltage and are applied for long term high continuous loads that do not exceed the VA setting. A timer is applied on the three battery voltage based load start triggers. There is a 2hr timer for intermediate long term loads, a 15min timer for higher long term loads, and a 30 second timer for loads that almost approach overload but not quite. All three of the voltage-based triggers are cleared based on battery bank voltage recovery to a level that you set that stops the generator without charging the batteries when the load goes off.

We do not charge batteries with gensets. They are only used for load management. The charger in our XW is set to Disabled so the gensets cannot use it. The only thing that charges our batteries is either solar or wind power. We only resort to using a genset for battery charging in extreme emergency like when we recently came home from sailing in the Caribbean for a couple months and our system is pretty much dead to the world with the solar panels buried under 6 feet of snow and wind turbines froze up. We need to get it back online. So I enabled the charger in the XW, got one generator started, and used it to bring the batteries up and carry loads until I got the rest of the system working again.

pleppik -
Scheduling loads is one way to use peak solar power during the day. However, it is not always possible. We are many times out working in the field during the day, or gone doing other things. We designed our system on purpose to be able to do what we want when we want to do it. And we are not afraid to have to use a genset to make it happen. It is about convenience.

We get home and my wife wants to do some laundry and run her electric clothes dryer, or we want to make a pizza in the oven for supper. Or I want to do some welding in my shop with my Lincoln 225A spark welder at 9:00 at night. So we just do it and if it causes the peaking generator to start and run for a half hour or whatever, so be it. The system was designed to be able to handle the load at any time totally automatic. There are some things we can't do like run the electric clothes dryer and oven at the same time. Or I can't do any welding when either the oven or electric clothes dryer are operating. But we work around that because they are all short period very high draw loads. But that is the only load scheduling we do.

[pleppik]

Thanks for this great introduction. Someday, if and when I get around to building my cabin in the woods, this will be very helpful to me.

One topic that probably should also be addressed is demand. Since storage is the most expensive component of an off-grid system, it makes sense to move as much demand as possible to the times when the sun is shining.

Other than the obvious (run the dishwasher during the day, etc.), do you have any strategies or tools for managing demand?

[Robert1234]

Interesting read. Need to think on it so as to digest it well. I did not realize there were off-grid inverters that actually synched up with a generator. I have a magnum PAE that you reference and essentially the batteries run to a predetermined point then the generator starts. The inverter goes off-line while the generator gives pass through power while charging the batteries back up.

So the main advantage of the synch allows you to increase peak load? Smaller inverter / smaller battery bank / smaller generator? If so, is it the load AND battery status that starts the generator? Do you have a seperate charger for the bank?

[ChrisOlson]

I'd like to try to make a contribution on what seems to be a very hard thing to understand for most folks looking at making a move to off-grid. This is more applicable to medium to larger full-time off-grid homes. Smaller systems like single 120V Outback setups in part-time off-grid cabins, or very small off-grid full time homes where daily energy consumption is under 5 kWh are probably not going to benefit from it.

I am dealing with sizing of the inverter, batteries and generator(s). Sizing battery banks and solar/wind/hydro capacity is a totally different issue. But the basis of my contribution is to save money on batteries and inverters and generator fuel by properly sizing the generator and inverter to meet loads, and using Generator Support to prevent having to buy expensive stacked inverter setups and the additional battery capacity required to power them at full rated load. Stacked inverters might look impressive on the wall. But if you don't have the batteries to power them they're useless. And few off-grid folks can afford the batteries it takes to power even ONE 6.0 kVA inverter at full rated load for more than about one hour.

There's two schools of thought on off-grid. The one assumes off-grid means never running a generator unless in extreme emergency, and using a generator is a "sin". The other (more targeted to larger systems) integrates the generator with the system with auto-start, etc. and it is run on a regular basis to enjoy a more comfortable off-grid lifestyle. If you think making the move to off-grid is "going green" you are dreaming. You only make the move to off-grid because you can't get utility power where you want to live. It has nothing to do with "going green" except for the fact that it takes a lot of green (money) to do it.

Common mistakes I see made all the time:
1.) Buying a generator that is way too big. The result is that you can't afford to run it because it sucks OPEC dry as soon as you turn the key. A combustion engine generator needs to be operated at 80% rated load or better for least cost/kWh. If you buy a 8.0 kVA generator do you have better than 6.0 kVA load for it when it is running? Few do - even with larger systems.

2.) Installing stacked inverters for intermittent heavy loads when a single one with generator support will do the job.

3.) Purchasing excessive battery storage capacity (and not near enough RE capacity to charge them, which is a separate topic), thinking you should be able to go three days without running a generator if there's no sun. This is ridiculous. 24 hours is adequate on straight battery power if you design a system using a tiny prime generator to manage how much you use from batteries during peak draw times (like in the evenings in the winter). The result is that you only need 1/3 of the battery capacity you think you need by managing loads with a little genset to keep the peak draw on the batteries below the 20 hr rate of discharge. Put the pencil to it over the expected life of batteries and you will find that even with fuel costing twice what it is presently in the US, the fuel in a tiny generator to manage battery capacity is cheaper per kWh than the battery replacement cost at 7 years.

Off-grid power is VERY expensive. Much more expensive than utility power. Batteries are the single most expensive component of an off-grid power system. Not inverters, generators, fuel, solar panels or wind turbines. Batteries. I have seen more "hobby" hybrid grid/off-grid systems get abandoned when the batteries failed than for any other reason. Having to spend $3,000+ on a battery bank at replacement time becomes a reminder of the realities of how much off-grid power really costs.

There is a common misconception that you cannot cycle batteries below 50% DoD or it will ruin them. This is totally and blatantly false for real deep-cycle batteries. They can regularly be discharged to 80% DoD and it doesn't hurt them one bit. And in fact, it makes them more efficient on charging.

The rules of thumb I have developed over 14 years of doing this:
1.) Purchase the battery capacity you need to power your average daily consumption with the batteries discharged to 80% DoD over 24 hours. Example: you use 15 kWh/day in your home on average. You need a 19 kWh battery bank. Normal cycle depth will be around 50% on the average day if you have adequate RE generating capacity to recharge them daily (under average solar insolation conditions) plus carry normal loads during the recharge period. On poor RE days you will cycle them deeper and manage how much you use from the battery by using your little prime genset. Minimizing the size of the battery bank up front prevents "sticker shock" when the day comes that you have to replace them.

2.) Determine what your peak load will be and buy an inverter that can carry that peak load for a few minutes, then switch to using Gen Support to power the peak load.

3.) Size your peaking generator so it reduces the load on the inverter during peak load to no more than the 10hr discharge rate of the battery bank. Example: your peak load is 7.5 kVA. You have a 6.0 kVA inverter that can handle the 7.5 kVA peak for 15 minutes before the load on it must be reduced to below its rated continuous output. You have a 1,000ah battery bank at the 20hr rate (40 amp draw on the bank to 80% DoD). The 10hr rate will be approximately 70 amp draw on the bank with Peukert Effect. 70A draw on a 48V system @ 90% inverter efficiency is 3.0 kVA. You need a generator capable of delivering 4.5-5.0 kVA.

And this brings up the topic of Generator Support, which is the only way all the above works. I have talked to many people who think their Magnum PAEs et al have it. They don't. With an inverter that does NOT have generator support in it, you are relegated to purchasing an excessively over-sized generator to be able to carry the peak loads because the inverter does not have the capability to sync with the genset and assist it. This does not give you the ability to carry a wide variety of loads and always keep the genset at peak efficiency (read above about sucking OPEC dry). Nor can you even come CLOSE to efficiently use this big genset for battery charging if you charge batteries with it thru the inverter/charger. There's only a handful of off-grid inverters that have Generator Support. And these inverters are quite expensive compared to the ones that don't have it. Carefully research what you're buying for an inverter before you buy it. While the batteries are the most expensive thing, the inverter is the heart of it all that makes it work.

So how does peak load support using this system work, exactly, and what does it do for you? In summary:
1.) It conserves battery energy during peak load events on your system by reducing the amp draw on your battery so the Peukert Effect doesn't sack them out.
2.) It allows the use of a smaller more efficient generator that is able to be kept at >80% rated load at all times
3.) It saves money on batteries and inverters by not having to buy twice as much for stacked inverter setups.

I made a demonstration video of it with our XW Power System to show you what it does in a real life off-grid home where it is used every single day. And has been used here for many years, previously with our Xantrex SW Plus before we replaced it with the XW system: