General question

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?

Specific Context

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 enter image description here
  • 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.

Panel setup

  • 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.
  • I think this question would better suited for Eletronics.SE.
    – Erik
    Aug 29, 2019 at 13:36
  • "We often start to struggle with our freezer." Can you be a bit more specific? Does it stop running? Or is it running a lot but not keeping things frozen? Something else?
    – LShaver
    Aug 29, 2019 at 14:22
  • 1
    @Erik there's a bit of a gray area on whether a PV question fits better here or there. However, since this is for an off-grid home, and the topic is a basic understanding of such systems, I think it's best to see what sort of answers we get here. Electronics.SE is generally a better fit for more specific questions.
    – LShaver
    Aug 29, 2019 at 14:24
  • 1
    Can you post the brand and model of your inverter or include more complete specs in the question? In particular, I wonder if it is a combined inverter / charge controller. This is important for a good answer. I also wonder if your panels are AC or DC output (DC seems more likely but I'd rather not guess). Also, in what condition are the existing batteries? Are they lead acid? If so, can you measure the specific gravity? If not, can you give a sense of how they've been used? How long have they been in service and to what depth do you typically discharge them? Does the system give you SoC info? Aug 29, 2019 at 15:43
  • 1
    @Jean-PaulCalderone I've added a lot more information into the post. Thanks for asking the clarifying questions! Aug 30, 2019 at 4:17

3 Answers 3


I agree with LShaver's number-crunch, although I reach a slightly different conclusion.

I agree you have plenty of inverter. Your are covered not only for routine loads, but also for startup surges.

The inverter has a charge controller built into it. It's perfectly fine, but it is merely there for convenience, so on a small system you don't have to buy a separate $150+ (R2500+) MPPT charge controller for a small system. Surely they allow you to use an external charge controller, either "also" or "instead". My point is, don't be distracted by this. Don't go thinking you need a new inverter if you need a bigger charge controller.

The Dump factor

When the battery is full, and there aren't enough household loads to use the power being generated, better charge controllers will route the energy into dump. For instance, "dump" could run heaters to warm a thermal storage tank... Top up irrigation storage tanks... Or in a micro-hydro system, pumped storage (backpumping).

Of course, simpler solar systems don't dump, or have any way to measure their dumpage. They just increase impedance and prevent current from flowing, which makes the solar panels slightly warmer. I for one would rather actually "dump", because then, you can measure it.

"Dump" tells you a lot about the sizing of your system. It makes hard problems obvious.

The numbers on your system

I agree with LShaver's de-rating of your freezer from 220 to 365 KwH/year (1 KWH/day). That's because you're opening the door far more often than expected, making ice, and it's a vertical unit.

Converting water to ice (both at 0C) involves a phase transition. Those, themselves, store heat energy. That's why it takes a long time on the stove for water to go from 99C (liquid) to 101C (vapor). The ice-water transition stores 92.7 watt-hours (334kJ) of heat energy per 1kg of water. The liquid water to vapor transition stores a whopping 627 watt-hours (2260kJ) of heat energy per 1kg. Vapor to ice takes (add 'em up) 720 watt-hours (2594kJ). It's more efficient than that, because freezers pump heat, not make it. But it's still costly.

So if you make 5kg of ice a day, you are moving 464 watt-hours. Let's wild armwave that your heat pump efficiency is 2:1, so the cost is 232 watt-hours of energy use. I don't like vertical units because every time you open the door, all the dense, cold air "falls straight down" and is replaced by humid warm air. Nevermind re-chilling the air; the air is full of water vapor, and chilling the air obliges pumping away that vapor-ice transition energy. Ouch. I prefer chest freezers because you don't change air.

All that to say, LShaver's dramatic de-rating of your freezer to 1000Wh/day feels right to me.

There's another load we haven't discussed, which worries me a lot. Inverters have standby "vampire" load. Just like a mechanical "rotary converter" has the energy cost of being physically "spun up", so does an electronic inverter. Like any vampire load, it is insidious because it's 24x7. I don't know your unit, but my armwave-rule-of-thumb is 1% of capacity. That‘s 30 watts X 24 hours that means 720 watt-hours per day. Holy smoke, that's nearly as much as the freezer! It would be good to actually measure the inverter, rather than wild-guessing such an important number.

But regardless, an important facet of system design is spin up inverters as rarely as possible. This is one reason I do not like "all-in-one" devices like that inverter+CC. (This, and they tend to be jacks of all trades rather than masters of one). Charge controllers don't have vampire loads and should be left on 24x7. However, inverters should be turned off whenever they are not urgently needed. So I would want to shut that thing off except a few times a day when needed for the freezer.

For all I know, you are already doing this, so I will totally exclude inverter numbers from the below calcs. If your inverter is on 24x7, things will be much worse.

Putting it together

So your freezer takes 1000Wh/day, and for now let's ignore the difficult-to-quantify inverter losses. Together, you two have concluded that you have prudent battery capacity of 1200Wh/day, and I agree.


In other words, if the freezer were your only load, you have 1.2 days of prudent reserve.


So, if you have a sunny June day (think December, northern hemisphere folks) with wel-aimed panels and the equivalent of 6 hours of full solarization at 600W,

  • you only use 2 hours of it and then you dump, dump, dump.

  • And then, 20% of the way into your second cloudy day, you cross the "prudent" line of discharge.

That means exactly one thing. Your panels are fine; your battery is far too small.

And I would call that good news, becuase batteries are the cheap part.

The rule of thumb I learned in eastern USA weather systems is have battery range for 3-5 days of no useful solarization. You have enough for 1.2 ignoring inverter losses, and just for the freezer. If you triple the size of your battery pack, you now have 3.6 days in the prudent range.

I know you were reticent to even double it, but the numbers are plainly showing your pack is the choke point.

Of course, with lead-acid batteries, it is fine to dip into the "imprudent" range, as long as you don't do it very often. So you need to run the numbers, based on your weather patterns, how your weather patterns might change in the batteries' life, and availability of alternate power. Segue:


Here I must agree with juhist's advice. But I am partial to taking a very hard left turn into carbon-neutral fuels, and the top of that list is Waste Vegetable Oil and classical engines like a Lister (being a Commonwealth nation). This does several things for you: It reduces the theft risk, because it's incomprehensible. It reduces cost, while increasing the "eco" appeal of your community. And the Coolness Factor increases the acceptance of the engine and its noise and smells. Nobody wants to be around the GRRRRR and diesel stench of a Kubota. But the Tubba Tubba Tubba and french-fry smell of the Lister makes it a point of pride when people know what it means.

Waste vegetable oil (as opposed to #2 diesel) brings two big benefits: vastly improved lubricity of the fuel, reducing wear on diesels' critical fuel injection pumps, and far-less-toxic exhaust. But it creates four challenges:

  • Particulate (french fry bits) - the cure is filtering. Not so bad.
  • Acidity - this is not in the hydrocarbon/carbohydrate fuel itself, but is in contaminants. The cure is "washing" the fuel with water, which is ionic and the also-ionic acids will attach to the water and leave with it. Ignoring it means more corrosion in fuel injection systems, depending on if the fuel is left to stand in the injection system while the engine is off.

  • Water content - the cure is settling the fuel. Water settles to the bottom of the tank given enough time. This too can be ignored, at the expense of more fuel injection system maintenance.

  • Viscosity at cool temperatures - the cure is either transesterification (making biodiesel, which includes all of the above) or simply warming the pipes and fuel injection system to 40-50C. Engine heat will do this, the issue is preheating on startup. This is where a simple, open engine like the classics makes the job a lot easier.

An engine optimized for SVO/WVO will work directly on biodiesel or plain diesel; the preheating will simply be unnecessary.

  • Welcome to Sustainability.SE! This is a great answer -- dumping and having a parallel charge controller are both great suggestions.
    – LShaver
    Sep 5, 2019 at 21:24

To give you specific recommendations to address your issue, you'll likely need to have a qualified electrician out to look at your system, or talk to the system vendor. However, here are a few general rules of thumb that may help you understand the issues, and some general suggestions for your system.

1. The inverter output should be sized to supply the peak power demand.

In your case, you have a 90 W freezer, and want to add a washing machine. According to this Australian power company which looked at several washer models, you will need a minimum of 400 W capacity for the washer. Thus at a minimum you will need about 500 W of inverter power capacity. Generally speaking you will want to have some overage allowance -- with 20% safety factor you'd need about 600 W.

Given that you have a 3 kW inverter, this is not your issue.

2. The battery bank should be sized to meet the total energy requirement.

Your batteries are the "fuel tank" for the system, so you'll need enough of them to last you throughout the day. The freezer spec estimates 225 kWh per year -- to give a bit of cushion, let's say you need 1 kWh (1,000 Wh) per day. The washing machines in the previous link require at least 50 Wh per wash load, which is essentially a rounding error compared to the freezer requirements.

Your battery bank is roughly 400 Ah, at 12 V, giving a total capacity of 4,800 Wh (4.8 kWh). As you've recognized, it's important not to let the batteries discharge too low. You've stated that you're trying to keep them at 75% or above (which seems like a good goal, but I'd recommend confirming with the battery vendor). This gives you a functional capacity of 1,200 Wh -- which is just slightly more than you need on an average day.

With just enough battery capacity to cover the freezer and batteries, you will need to add to your battery bank. Two more of the same batteries (a 50% increase) would give you 1,800 Wh of capacity per day -- but this may not be sufficient for other reasons discussed below.

3. The solar array should be sized to supply enough energy every day to fully charge the batteries.

You could do all the math, looking at solar insolation, angle of the sun, hours of daylight, etc... but an easier way is to do some guess-and-check with PVWatts Calculator.

I set the location to Johannesburg, South Africa, left all other defaults in place, and adjusted the array size until the system generated 1,800 Wh per day in the worst month (June). I came up with 1.2 kWdc.

To supply sufficient power in the worst month (June), you'll need to increase your array from 600 W to 1,200 W.

4. Go back to your inverter and make sure it can handle the power output from your array.

The inverter you have is specified to a max input of 600 W, which is insufficient. To increase in size, you will likely also need to increase the voltage of your system from 24 V to 48 V -- this refers both to the input from the panels, and the voltage across your battery system. By adding two more panels in series, your output voltage will double and should work with a 48V system (since it works for 24 V now). This will also meet your daily energy input requirement of 1,800 Wh.

For your batteries, right now you have two "strings" of two batteries. Each string measures 24 V. You could arrange these as one string of four batteries to give 48 V. Then, to increase your energy storage capacity and properly balance the system, you'd need to add four additional batteries in parallel (keeping the system voltage at 48 V, but increasing storage to 800 Ah). This would supply 2,400 Wh per day (800 Ah * 12 V per battery * 25% depth of discharge), which is more than your required energy (from step 2).

While it might seem unfortunate to spend extra money on batteries you don't "need," it will actually save you money since it will give you the capacity to store most of the "excess" energy produced in months that aren't as bad as June.


Your current system does not store enough energy to meet your needs. To remedy this, you'll need more batteries to store energy, and more panels to supply enough energy to charge those batteries each day. But, your inverter is at the limit of the input power it can handle.

Since off-the-shelf PV systems go together a bit like Legos, you need to make a few changes to get the whole thing working in harmony.

  • You will need to replace your inverter with one rated for 1200 W of input power, which probably means going to a 48 V system.
  • To continue using your four 12 V batteries, you'd need to put them in series (making 48 V) and add four more in parallel to keep the system balanced.
  • To keep using the two panels you have, you'd need to add two more in series (doubling the voltage and keeping the current the same).
  • This is a remarkably well-written, informative, and detailed answer to my specific context. Thank you! I see that it's impossible to answer the general question of which to upgrade first since the components are so interdependent. Based on 1 and 2, it really does seem that getting two more batteries than required (i.e. 4 extra instead of just 2 more) and a stronger inverter is a little bit of overkill? Is it possible to get a separate charge controller to handle the extra panels so that the requirement 'drops' to two panels, an external charge controller, and just two batteries? Aug 30, 2019 at 19:54
  • @AdamReynolds you might be able to find a 24V inverter with a higher input power rating. That would be the easiest way to avoid needing four more batteries. Having a parallel charge controller could work but you'd likely need to talk to the vendor of your current inverter to see how to do that.
    – LShaver
    Aug 31, 2019 at 22:20

I know I will be downvoted for this, but I'll add one more suggestion:

A generator

With a generator, you can provide yourself backup power during the times the sun doesn't shine enough.

It generally doesn't make sense to size your batteries for the highest demand you expect from them. Occasionally, it will be cheaper (and perhaps even more environmentally friendly, as battery mineral mining has bad consequences) to supply the missing 5% by using fossil fuels (or renewable fuels, if you want) and the 95% by solar power.

I wouldn't operate an off-grid PV system without a generator. It gives you peace of mind, even though most of the time you won't be using it.

Apart from that, I would say upgrading your batteries may be a good idea. Lithium ion would be better than lead-acid, but you probably aren't looking forward to completely replacing every single battery (and perhaps the charge controller).

For the downvoters-to-be: it would save this planet if everything was electric and even only 95% of electricity was generated by renewables and the rest 5% by fossil fuels (or even biofuels!). No need to size the renewable capacity to 100%. And, there are plenty of companies (e.g. Neste) that make renewable fuels that are fully compatible with fossil fuels in engines. If only 5% of generation was based on fuels, then all of the fuels could probably be made in a renewable manner.

  • Thanks - you make some very good points, and a generator is a possible option. I think I'd like to add more panels first to see if we can do without a generator though (I have no idea where we'd keep it!). But your arguments to get a generator before getting new batteries are definitely worth considering. Aug 30, 2019 at 4:21

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