Measuring your consumption values - meters

A question posted on another forum asked how to measure peak energy amounts of his house so that he knows what size inverter to get.
As I pondered this I realized that if one’s objective is simply to reduce your electricity bill this might not be the place to start:
My response was:
‘There are many options to reduce one’s electricity bill and installing a solar PV inverter is one.
Your geyser will be your biggest user of power so look at the (ROI) of a dedicated solar water heating systems as well…
The reason I advise this is because it’s difficult to power your geyser with non-essential PV power. This forces you to have to install a big PV system (and reduce the kW of the geyser element) to cope with the available PV power.’
Kommentaar asb!

Hi Richard

I had the same question this week plenty of times.

Yes, the geyser is usimg quite alot of energy,and the saving will be great, but in the winter, they are almost useless. (Well mine is). I only notice a difference in September that it is start working.

I think a better bet would be:

  1. heatpump, as the saving would be the whole year.


  1. Get a Solis/grid-tie system.
    Yes, it will be abit more than heatpump. But the nice thing is that you have 2 mppt normally on them. You can add 2.5kw about 5 panels, to one string, and then when you have more funds available, add to the second string.
    The saving will be the whole year, and it will not just be the geyser that is a saving, but all the units in the house that uses an element. Geyser, dishwasher, basicly, if you schedule everyhing during the day, you will save!

Ok, power will be off during LS… But your bill will be less!!

My 50c.

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If you only have the cost of an inverter then I find that by reducing your usage vs upgrading your inverter size in most cases have a 1 to 1 ratio. So the cost will be very similar.
But when you want backup and you need to add a battery (that also only last a fixed amount of cycles) it starts to get closer to 2 to 1 ratio. So it will be cheaper to reduce consumption.
You also need to factor in the people. If they are willing to change how they think about power then you can save a lot by reducing your peaks. For some husbands it might be safer to just make sure that the pretty ones in his home can put on every appliance at the same time :construction_worker_man:

My Heat pump has been giving me 3.5kW of heat from 0.9kW of consumption for the last 5 years. It has been a great investment and I can run it from batteries if I need to.


In order to keep the comparison within strict boundaries I think it’s best to omit storage.
The comparison then is between a standalone geyser solar heater and a grid tie inverter.
The geyser heater can be a ETC, PV, flat panel or heat exchanger.

My thinking and I stand to be corrected.

Running a “tight ship”, assuming no one is home, 2kw geyser element, or two, on timers, a pool pump, installing a Solis grid-tied inverter with ±2.5kw panels, would heat the geyser most days of the year, run the pool pump.

Having a geyser on its own solar panels will only heat the geyser, rest of the day will just sit there and look cool.

EDIT: Also want to add, heat pumps, EV tubes have a certain lifespan. Solis too, but I suspect much longer.

I have my geyser automated to turn off within 1s of the total inverter load exceeding 7kW (of 8kW). So in practice, the geyser doesn’t impact my inverter sizing. As long as I know there will be enough window during the day when I don’t have a number of other big appliances on at the same time, all is good.

I have not even bothered to put a smaller element in. And this way all energy produced is available where it is needed.

Yes and no, I would think. I think that “measuring” is exactly where you should start. But I agree that if your objective is to save money (and not necessarily to have backup), then measuring peak power is not where you should start. Measuring the average daily energy use is where you should start.

And that is actually fairly simple. You read your municipal energy meter every day at the same time, for a week, and you put it in a spreadsheet. Assuming you have access to it of course… otherwise look below for another option.

Then, as @Louisvdw says, if you’re just going to plonk down a PV-inverter (where the extra cost of the capacity is 1:1 with what it would cost to reduce consumption), just install a PV-inverter that covers 80% to 100% of the measured daily average. Or whatever you can afford if that is too much.

But if you intend installing backup, then it becomes more complex. Then you want to know what appliances are the big consumers, hot water being the easy and obvious largest one, but pool pumps often a close second, and in my experience, older refrigerators and freezers can use an impressive amount of energy in 24 hours simply by running half the time (due to being old and inefficient, or poorly insulated). Then you turn “hunt the energy hog” into a game for like 6-9 months…

If you’re going to play that game, I would invest in a cheap energy meter (one of those where you program a static power factor and voltage, and you get 90%-ish accurate numbers based on current readings). Efergy sells one.

What I like about this device is you can move the CT around and measure each circuit individually.

And then, if you need to go down to appliance level, a plug in energy meter like this one from GeeWiz.

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How is this control done?

Arduino toys:

I bought a bunch of smart sockets, (not the exact model I am linking to, but I have no reason to think it will work differently). Along with remote control it reports back power and energy usage.


Smart sockets are cool. They are around R300 a pop I think, typically, so it would add up very quickly to the cost of a movable meter. But it is certainly an option now that wasn’t (readily) available ten years ago when I first played the wack-a-energy-hog game :slight_smile:

Ok. With these peak power meters it’s possible to switch off the geyser for a period which will make more (PV) power available.
This begs the question: Isn’t it possible for the inverter to provide this control signal?

A MultiPlus can use assistants to toggle some of its relays, including AC-Out-2. I’m using that to load-shed almost-essentials when inverter load goes over 8000W for 2mins. And also low SOC.

The signal is easy to get out using the assistants if you want “wire control”, and it’s of course available via Modbus / MQTT / NodeRed if you want to get it via an API.

Google CBI Energy control unit.
They can be bought with various current thresholds before they switch.
Basically, it performs maximum demand management, when the load is too high the geyser can’t come on or two geysers can’t be on simultaneously.
You get the drift.
They can be cascaded as well.

I must confess that this RE (solar) journey doesn’t unravel at any point.
So I guess that forums that discuss this subject have a bright future…
But I do sympathise with the regular home owner who wants to take advantage of the power of the fireball in the sky. :thinking:

Absolutely true. If people ask me, and they are 100% newbies, no interest to learn, then it is X + Y + Z, get DEF to install, then you can expect ABC, and if you do as you are told during LS times, you will be fine.

And IF things need to expand later, and you bought X, no problem.

So now, go forth, and do as you are told.

So far, not one comeback. :wink:

Ps. IF they follow the rules given …

What size of system is this for a bare bones application? (assuming 1.5 kW geyser etc.)
The scheduling is still the clunky bit but at least we can do the sums…