I have a three-phase supply and am looking to install solar panels and a Home Energy Storage battery. Would I need a Single-phase or Three-phase inverter?

That’s understandable. Scaffolding is expensive.  Fortunately I have my own!

There will be lots of advice here about the ‘correct’ size for the battery. As a rough guide my 8kWh unit is clearly too small for my house. Tesla have done their homework and concluded that their base-unit should be 13kWh. That’s more like it… and they also score well for the £ per kWh evaluation.

But Tesla only have AC coupled units and their Li-ion battery chemistry is the same as their vehicles (LiMNC). That C stands for Cobalt, not carbon. This raises ethical considerations because it’s mostly mined in the Congo where it helps to buy arms.

Moreover for a static battery you don’t need the high-currents and fast responses which are required by an Electric Vehicle. You can opt for LiFePO4 cells which will also last 5x longer!

As you had expected I absolutely disagree with the installer who suggested you required an AC-coupled battery in order to export and have the opportunity to choose a Time Of Use tariff. These are completely separate issues, unrelated to the technology.

The more relevant questions are:

a: what are your aspirations towards having an EV with its charger?

b: what are the electricity implications of the ‘expansion’ you are contemplating.

Put all the cards on the table before trying to choose battery sizes and connectivity!

Right I’m signing off for the night and opening a beer!

The more relevant answers are:

a: what are your aspirations towards having an EV with its charger?

> not in the short term

b: what are the electricity implications of the ‘expansion’ you are contemplating.

> sauna - 9kW heater (expecting regular short duration usage 1hr max inc. warm up)

> indoor endless pool (heated / powered swim lane) - not sure if diverter can help here

> garage conversion to gym (with air con)

Current usage about 8,000 kWh, very difficult to predict ‘future usage’

Looking at 18 panels initially, so far quoted between 370W to 455W from various manufacturers (still reviewing but advice seems to be ‘invest in inverter rather than panels’)

Also considering water heating via a diverter

Looking at this list I know why this isn’t easy…

Thanks @Mark C - and this topic suddenly looks like it’s going to be huge!

We’ve already got a tutorial and a topic discussing Load Diverters. 3-phase Diverters are available, but haven’t yet been researched/discussed here on the Forum.

Let me also point out that there are significant benefits in running electric heating devices from 3-phase.

Consider a heating element through which we will pass 2-amps:

The heat output for three single-phase devices is lower because there’s only 240v between phase and neutral. But there’s 440v between phases if you don’t require a neutral.

3-phase immersion heaters are available at 6kW, 9kW and 12kW with ‘standard’ 2¼” BSP threaded base. Prices are £120 - £250 and don’t vary much wrt the wattage, but there are large variations depending on the material from which the element casing is constructed. This depends on the characteristics of water in your area.

You obviously need to check that an immersion heater specified by an Installer uses the elements in ‘delta’ pattern (as shown above) and does not require the neutral. There will be some which are effectively three single-phase units in one package.

When choosing a load-diverter, it is important to note that you require one which varies the amount of energy being sent to the immersion heater. You don’t want a cheap one with a relay!

Suppose you have a 9kW immersion heater and your PV panels were generating 5kW more than could be exported to the Grid. If a relay simply switched on the heater, then it would take the 5kW plus another 4kW imported from the Grid !

A Diverter must operate using pulse-width modulation (PWM). This has transistors which are pulsed rapidly to chop the mains frequency on/off many times for each AC cycle.

The higher proportion of time that the transistor is ON, the more power will be sent to your immersion heater.

Ok…. next concept - Thermal Store:

A thermal store looks like a hot water cylinder from the outside, but the main body of water is simply a reservoir of heat.

When you turn on a hot tap it passes cold water through a coil to pick up the heat which is required.

Thermal stores may have a number of different coils and one or two points for immersion heaters. The column of water exhibits thermoclines, so you use the lowest coil to input energy from the coolest source. The diagram above is merely illustrative. In practice all the input coils would be lower in the tank, and the immersion heaters just above them.

The general idea is to select a tank capacity according to the amount of heat you need to store, and then specify as many coils as you envisage having as heat-inputs.

Here’s my 300-litre thermal store which is slightly more complex than most because we use a well rather than mains water(!):

If I was choosing my TS now, I’d want a second port for an immersion heater. The present 24v one is a ‘dump-load’ in case there’s too much energy arriving at my main off-grid battery-stack.

This means I no longer have a 240v mains immersion-heater which could be fed via a load-diverter.

@Mark C- it may help your planning if you consider both your swimming-pool and the sauna as secondary ‘thermal stores’. Either could act as a reservoir from which you draw heat when not being used in its primary role.

From that viewpoint you are absolutely correct to be thinking of a diverter for the swimming pool… especially so if there is insulation between the pool walls and the ground.

@Transparent We live in a village, you missed off the oil boiler and the Oil Aga!!!😜

We do have underfloor heating, but rarely use it due to the heat from the Aga, but hadn’t really considered that as a separate element to our overall system  

The pool will be well insulated, including the cover, hence considering it in a similar way you our hot water system. 
I really appreciate this content as it enables me to understand so much more from a single location rather than trawling through all sorts of websites / you tube content

Ah… thanks for reminding me!

So your Aga can be considered as another possible input to a Thermal Store.

And if it dies in 10-years time and you install a wood-gas boiler (or whatever is the best solution at that time) then it simply takes over the same heat input coil in the cylinder.

May I also pick up your comment about the gym in the converted garage having air conditioning?

Where would the heat go?

What about using a Mechanical Ventilation with Heat Recovery (MVHR) unit such as discussed here?

An MVHR unit would also be able to re-capture heat from the sauna of course. The water in the air would be expelled through the condensate drain, and the heat transferred to fresh incoming air for the house.

@Transparent  Added to my list to review… The gym will be adjacent to the pool.

The plan was to have a dehumidifier in the pool area as it will get quite humid with the powered lane running and air-con in the gym as it can get hit in the summer. 
For the brief reading I may be able to replace this with an MVHR taking the warm moist air from the pool area…

Precisely, @Mark C. When you’re spending money to bring in energy in the first place, you should then try to use it several times within the house.

I don’t know how busy you are, but I would strongly suggest you take a day out to visit one of the Homebuilding and Renovation Exhibitions. There’s one in Harrogate at the end of next week (5-7th Nov).

When I moved to our current home it required extensive renovation and I applied to Devon Building Control to install a new sewer pipe as a matter of some urgency!

The Surveyor/Inspector agreed to visit that very same afternoon and arrived on the doorstep with two free tickets for the next Homebuilding & Renovation Show. It was truly inspiring!

The Harrogate exhibition will be full of supplier-experts in all the technologies we’re discussing here. So in the space of a few hours you can not only get to see the equipment, but also grill them with questions about efficiency, maintenance and failure rates.

@Transparent Great idea, I will head up and have a look 👍

@Mark C  I’ve been taking a look at electricity data for the Retford area.

You have an amazingly low carbon footprint, with almost all power coming from local renewable sources, even tho’ these are actually flagged as low generation for today!

Your Bulk Supply Point is at Checkerhouse, where the Mansfield Road crosses the railway line, west of Retford. That’s where the transformers take an input of 132kV and send 33kV to the 9 Primary substations in the area.

You can get loads of data from your BSP by entering a post-code into the Carbon Tracer website for Western Power Distribution.

Here’s the snapshot showing how much of that renewable generation is local, as opposed to being imported from the National Grid.

Were you aware of such large scale renewable generation in your area?

As the country moves towards customers having variable Time Of Use tariffs, this data will affect how ‘valuable’ are any exports you may wish to send back to the local Distribution Grid.