Adding on from the title, what are the restrictions to ensure that the various components of the PV system are compatible with each other? Are there rules around how many batteries, total power of panels, and power of the inverter?
This more general question comes from a very specific real-world problem that I have right now.
We've got a basic off-grid set-up, but it looks like we need to upgrade.
We're currently going with:
- 2 × 295 W panels
- 4 × 12 V, 102 Ah batteries
- 3kW 24 V pure-sine inverter (with built-in controller)
Things are generally fine But with just one or two overcast days we often start to struggle with our freezer (90 W when the compressor is on, time avg. about 25 W) and we'd really like to be able to run an automatic washing machine (cold wash only, booster pump also needed as we don't quite get enough water pressure from our header-tank - only about 0.5 bar). With the current setup we could run a washing machine on sunny days, but we'd like to be able to run it fairly consistently (every 2-3 days as we have a baby and are doing cloth nappies) and since the system is already taking strain, it feels like we need an upgrade (even without adding in a washing machine). We're generally happy to be careful with our usage - our main priorities are as above, but also do increase the lifespan of our batteries. We work for an NGO in the rural Eastern Cape of South Africa, so cost is a concern, but we want to prioritise long-term savings over any short-term concerns - so we aiming to get the minimum amount of quality components that meet our needs, keeping the long-term in mind.
Which of the following would be best, and in what order?
- More panels
- More batteries
- A stronger inverter
- All of the above
- Some of the above
Our two panels are north-facing. I'm thinking that some more panels would capture some more afternoon sun (the next space on our roof is north-west facing) as well as just more photons when it's overcast. But do we need a stronger inverter for this? Although our inverter is 3kW, the manual says its rated power is 600 W of solar energy (is this for the built-in controller?). Does this mean that if we have panels that added up to more than this threshold and it was super sunny that we'd just "lose out on the extra energy" or would it actually be a problem for the inverter and we definitely shouldn't add in panels whose total wattage exceeds the power rating?
Updates and further information on the specific context
- The inverteris a Mecer - as far as I can tell, it's this one.
- The inverter has a built-in charge controller.
- The inverter has a warning alarm that goes off at a voltage of about 23 V. (I think, it's actually been really hard to figure out when it goes off and the manual isn't particularly clear.)
- We get an estimate of the state of charge on the batteries on a 4-bar scale and we can also read the voltage across the batteries.
- The batteries are all deep-cycle batteries (SMF100) made by First National Battery in South Africa. The battery specs (see image below) from right at the bottom of this page
- The whole setup is about 17 months old. We started with two batteries and then got another two after about 2-3 months. So two of the batteries are 17 months old and the other two are 14-15 months old.
- We try really really hard not to let the batteries go lower than 75% which is part of the struggle we're having.
- The freezer is the Bosch 220L Inox Full Freezer GSN33VI30Z. It's rated A++, 90 W when the compressor is running, and an average of 225 kWh per year (I haven't been able to measure what the consumption is in our specific case), and food stays frozen for about 24 hours or so if we've had to put it off. When I say we're "struggling with our freezer" we're struggling in the sense that some days we can only put it on for a couple of hours and it just needs very careful management to try and keep our battery levels up. We haven't lost any food, it's not thawing out or anything like that (so mainly just added stress on very cloudy days.)
- We don't have a fridge. We're using a cooler box as our "fridge" and we just swap ice bricks between the cooler box and freezer daily. Since we put the freezer off at night, we do this ice-brick swapping at about lunch time so that the freezer has had some time (sunlight) to recover without giving it an extra load to freeze and so that there is some daylight left and time to freeze the new ice bricks.
- We're at a latitude of 32° South in a summer-rainfall region. Winter is mostly fine despite the shorter days and summer is generally fine except for several overcast days in a row. We get about 10 hours of sunlight in mid-winter and about 14 hours of sunlight in mid-summer.
- The panels provide DC voltage which is then managed by our controller. The brand is Canadian Solar, but other than knowing that they're 295 W each (connected in series) I can't say more - I don't have a manual or anything.