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.
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.
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!
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 and equipment as cool as possible.
Some appliances/equipment also 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. 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.
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.