Inverter capacity is a critical part of the offgrid design. Their size will limit what appliances you can run and/or how many you can run simultaneously, and how much energy you can harvest from your solar panels.
Size
During the day, you need to be generating sufficient power to run your house and charge your batteries, but on bad solar days, you might need to draw power both from your solar panels and your batteries.
The number of appliances you can run at the same time, without starting up the generator, will be determined by the inverter capacity – at night, just the battery inverter. During the day, if your batteries are charged, upto the sum of the two (depends on how much solar you’re generating). So you can typically run more things simultaneously during the day.
If you have small solar and battery inverters, and you want to use your coffee machine, microwave, iron and TV, as well as all your normally on appliances like fridge, freezer, lights, wifi, water pump etc, at the same time, you won’t be able to do it at night without the generator kicking in, but you might get away with it during the day.
You might need to modify your behavior to avoid using the generator, if your inverters are not sized to handle all the things you normally do at once. eg don’t iron that shirt for work while you’re making your breakfast. Don’t run the dishwasher, clothes washer, dryer while vacuuming the floor… Stagger task sequentially, and do them during the day if possible, to preserve the battery for when you’re not generating power.
If you don’t want to do this, get a bigger system/bigger inverters!
Make sure you think about what compromises you are prepared to make, and what large current draw appliances/tools you want to run, and tell your solar designer to make sure your system meets your needs. Question the final design to make sure it fits what you want it to do – ask how much it costs to get that extra bit of capacity.
Solar Inverters
The solar inverter converts DC power from the solar panels into AC power that you can use in your house. It also supplies power to the battery inverter, that converts the AC back to DC power to charge the batteries, or converts battery DC power into AC power to supply your house.
Inverters come in discrete sizes, and do not need to match your solar panel capacity.
If your solar inverter capacity is smaller than your panel capacity, you will only be able to harvest electricity up to the inverter capacity. But since your system should be sized for bad weather, shaving off peak electricity production should not be a problem off grid.
Excess panel capacity is usually cost effective. On an overcast or rainy day, your panels may only produce at say 10% of their rated capacity. So if you have 10kW panels and a 10kW inverter you would harvest around 1kW. If you have 10 kW panels and 8 kW inverter, you would still harvest 1kW. So you save money on the cost of the inverter, with no loss of performance for your system sizing case.
If you then spent your savings on a couple of extra panels, say an extra 1kW panels, you would then harvest 1.1kW with the smaller inverter. To get the government rebates, you can only have panels 33% higher capacity than your inverter – 6.6kW panels for 5kW inverter.
We’ve chosen to install the maximum amount of panels allowed for our inverters and still get the rebate.
There is no reason to have a larger inverter than your panel capacity, unless you want to add panels in the near future. In this case, your solar panel production will limit your electricity production.
Battery Inverters
These inverters act as the traffic police for the system – passing generated electricity to charge the battery, if required, or supplying electricity from the batteries to the house, converting to and from 240V to your battery voltage. They should be able to automatically start your generator (if it’s equipped with autostart) to make sure you don’t run out of power.
Their size will determine how much power you can draw overnight. They must be compatible with your batteries.
If you want to charge an electric car, or another large load that you don’t want to prioritise, you need to make sure you can configure the battery inverter to stop the car draining the house batteries. eg if the inverter sees the car as just another load, then it will continue to charge the car until the house batteries are depleted. This would effectively charge your car using your generator. (Car batteries are typically much larger than house batteries). We’ve chosen an SMA Sunny Island – we can wire a car charger to it to be turned off when the battery is at 90% charge. eg it won’t direct more than 10% to the car. Not sure when we’ll get one, and but we’ll need to fully charge it away from the house, particularly in winter.
Inverter Performance
Inverters can also handle much less power when its hot – most of the capacities are quoted at 20oC, but if they’re in a shed like ours will be, they’ll get much hotter in summer, and not handle as much power. Altitude can also negatively impact performance.
We’re insulating the equipment room in the shed, and installing rodent proof ventilation in the walls and a whirly bird on the roof to keep the room to draw hot air out of the room and the equipment as cool as possible.
Some appliances draw a lot more power when they first start up, like pumps, motors etc. Most inverters have the capacity to cope with some initial surge – rated for higher power for a short period of time – but it can vary widely based on manufacturer, so its worth checking surge capacity before you select your inverter.
This power surge may limit what you can have running when you start something up, or may not allow you to run the equipment at all. Once it’s up an running, you can then run other things… Equipment with soft starts limit this current draw, so if you’re looking for a new electric power tools or pump, for example, look for one with a soft start. For an air conditioner, go for one with an inverter.
Inverter Selection
There isn’t the equivalent to the lithium battery test site, as far as I can see, and there is a really wide range of inverters available at wildly different costs. As the heart of your solar system, you are in serious trouble if the inverters fail, particularly your battery inverter. Even if the warranty for your inverter is good, if it takes a month to repair or replace, there’s a good chance you’ll spend some serious money on fuel, and have serious hassle while waiting for it to be fixed. You need to have inverters which have local servicing/good turn around time for repair/replacement.
Even assuming we can run the house off solar for a couple of hours a day (winter), we’re looking at $40 to $100/day, if the battery inverter died, and we couldn’t access the batteries (lower cost is if we shut it down when we go to bed etc and stop running the fridge and hot water etc overnight). Not to mention the hassle factor of buying diesel (we only have a 20l diesel can) and keeping the generator filled (42 hours @ 75% load, 100l tank). And you really don’t want to run out of diesel because it can cause serious problems for the engine and may need to be bled or repaired before it will restart – and then you’ve got no power at all!)
If it’s the solar inverter, then not it’s not quite so bad – we can just run the generator every couple of days for three hours to charge the batteries, around $10/day, and then run the house off the batteries.
There are some really cheap inverters out there, but they may ending costing you a lot and causing a lot of pain. I don’t think the risk is worth the potential pain.
The best brands by reputation appear to be a Fronius/Selectronic inverter combination, but they are really expensive. We’ve had a Selectronic inverter in the shed for 20+ years – still going strong, although only lightly used.
We’ve gone with the slightly cheaper, but still well reputed, SMA Sunny Boy/Sunny Island inverters, primarily due to cost. We’ve had a Sunny Boy on our on grid solar system with no problems.
But we’ve also gone with a 2 x 50% configuration for both the solar and battery inverters. So if one goes down, we’re still OK.
There are lots of inverters to choose from but I haven’t found good data to compare performance and reliability….
The Clean energy reviews website has some reviews and additional information.
Off Grid Living - Building the Dream 