This page gives an overview of the generation, storage battery and EV charging equipment we have installed and explains how we use it to save energy and reduce costs. We also discuss charging an EV away from home.

Solar Panels & Associated Equipment

We have purchased 32 solar panels and currently have 24 installed: 12 on the house, 4 on the office and 8 on the garage. The picture below shows the set up. The garage roof is angled (pitched) at 25 degrees, the other two at 45 degrees.

The photo was taken in the late morning, in early-October, with the early sun roughly behind the photographer’s back. As you can see, the two arrays on the house and office roofs are in full sun: the last remaining shadows of tree branches are just moving off the bottom right of the four panels on the office roof.

At this time of day, in autumn, the office shades five of the eight panels on the garage roof. As the sun rises higher in the sky, all three of the arrays come into full, unshaded, direct line-of-sight to the sun. In the late afternoon, the garage roof catches the sun full on, but the office increasingly shades the main house roof, mostly affecting the panels closest to the chimney.

The remaining 8 panels will be embedded in the roof of an art studio we are building for Anne’s ceramics business. These not yet installed. Construction of the studio was intended to start in March 2020, but coronavirus lockdown prevented work. As a result, the data reported on this website for the year 01 October 2019 to 30 September 2020 is based on only approximately 7.5 kWp solar panels generating electricity, not the full 10 kWp which will be achieved once the 8 studio roof panels are commissioned into service.

Please note that the capital cost we use to calculate the economic payback of the system takes the entire cost of all 32 panels into account. Therefore the payback calculations are somewhat conservative: we’re accounting 100% of the cost of the whole system but only 75% of the solar generation capacity is currently installed. Bear in mind that solar generation accounts for only part of the economic benefit: other major contributors are ‘peak-shifting’ (avoiding peak hours grid electricity by storing off-peak grid energy in the Powerwall battery) and EV use.

The studio roof will be at 90 degrees to the other three pitches, and will face roughly South East. It will stand in front of the wooden fence, with the solar array facing the camera. It will catch more of the morning sun, compensating for the garage roof being partially shaded by the office during this early part of the day. We will do some blog posts about the studio installation when it happens, especially focussing on how to make solar panels an integral part of a new-build roof, as opposed to mounting them on an existing roof, as with our other 24 units.

Solar panels are measured in Watt peak generation capacity (Wp). In our case, we opted for relatively high-efficiency 330Wp panels, as shown in the schematic below.

The panels convert solar energy into Direct Current (DC). The panels are then connected in ‘strings’ to an inverter which transforms DC into Alternating Current (AC), which can be used in the house and exported to the National Grid, if it’s not consumed on site.


As the schematic shows, our set up involves String 1 connecting 12 panels on the main house roof to Inverter 1, which is installed in the house loft. String 2 connects the 4 panels on the office roof also to that same Inverter.

Inverter 2 collects the generation from two strings: String 3 connecting the 8 panels on the garage roof and String 4, which will add the 8 panels on the ceramics studio roof when the building work is completed over the next few months.

So each inverter handles 16 panels. 16 x 330 Wp means that all 16 panels could in total produce 5,280 Watts at peak generation, or 5.28 kilowatts at peak (kWp). Given that there will only be a few hours each summer when the system is at its absolute peak, we specified each inverter for a maximum capacity of 5kW. In theory, this would mean that on a perfectly sunny afternoon all 32 panels could be producing a total of 10.56 kW for an hour or so, but the two inverters would be ‘maxxed out’ at a total of 10kW, therefore we’d be ‘wasting’ half a kilowatt. But the inverters have a margin of about 10% built in, so they’ll each happily go to 5.5 kWh, meaning that our inverters will easily handle the theoretical maximum production of our panels.

Solar Panel Optimisers

When configuring the panels and inverters, we discovered one exceptionally important fact. If your installer simply connects each string directly to the inverter, the whole string will only produce the power of the weakest panel on that string. For example, if only one of the main roof panels’ output were reduced by 90% – say by shading from a passing cloud – then all the remaining eleven panels would also be reduced by 90%, even if they were in full sunlight, not shaded by the cloud. The solution is to connect each panel to the string using a Solar Panel Optimiser, which allows every panel to produce at its maximum efficiency, regardless of what any other panel on the string is doing. The optimisers are represented by the 32 small green squares next to each panel in the schematic.

Grid Export Limiter

Another vital issue is that your local Distribution Network Organisation (DNO), the companies which run the local electricity grid networks – not the utility companies which bill you for electricity – impose a limit on power that you can export to the grid. This is for safety reasons, so that anybody working on the transmission lines isn’t electrocuted by energy coming into the system from microgeneration sites the DNO doesn’t know about. Your solar installer will submit an application on your behalf when you order your system. There’s a ‘fast track’ procedure, which allows small systems up to 3.89 kWp (roughly 12 panels) to be connected without any particular hassle.

If, as in our case, you’re building a bigger system, you have basically two choices. Either follow the full application process to agree a higher limit with your local DNO, who may insist on upgrading your supply to handle the additional load. Or, much simpler, include a Grid Export Limiter in your system design, to prevent more than 3.89 kWp ever being exported to the grid.

We went for the Grid Export Limiter option (shown in the red box on the schematic). This works fine for us, because the objective of our system is to attempt never to export a single electron to the grid, but to use everything our solar panels produce locally for charging the Powerwall and the two EVs. The limiter is there to cover those exceptionally rare occasions we can foresee, most likely in high summer, when all three batteries are full (that’s 144 kWh in total!), or both cars aren’t on site, and the house is consuming only a couple of hundred watts. In this very rare case, the limiter will ‘throttle back’ the inverters, to prevent too much power reaching the grid. It’s an extra piece of kit, but well worth it, to avoid the hassle of a full DNO application.

Back to top of page.

Energy storage – Tesla Powerwall

The Tesla Powerwall is a 14 kWh static battery, using essentially the same technology as Tesla car batteries.  It can charge and discharge at a rate of 5 kW.  Therefore, if the sun is shining brightly, and our 32 solar panels are generating at a healthy 10 kW near-peak capacity, it will take just under 2.5 hours to fill the Powerwall from completely empty to 100% using around half of the solar energy production. The other half of the power coming off the roof remains useable for other purposes, including charging one or both electric cars, operating domestic appliances and any other load from the house or the art studio.

When there is only weak sunshine, the Powerwall reduces its charge rate to use only the surplus amount of solar energy (over and above what the house is consuming), constantly varying its charging rate in realtime to use only whatever solar power is available. So, as clouds drift across the sky, the Powerwall charge rate goes up and down dynamically in response to The Amount Of Solar Generation minus The Amount Of Consumption In The House to ensure that every available Watt of surplus energy is stored in the battery, and none ‘gifted’ outwards to the grid. Conversely, it never uses imported peak-rate grid electricity to charge.

We also use the Tesla app to set the Powerwall to charge between 00:30 and 04:30 every night, to take advantage of the 5p per kW electricity available with Octopus Go tariff

What typically happens is that:

  • The Powerwall will intelligently adjust its charge rate to the lowest possible required for the 4 hour Octopus Go cheap period and fill to 100% by 04:30.  
  • It will then discharge as required to meet the domestic electricity load throughout the day.
  • It will be topped up by solar during the sunny part of the day.  This will then generally ‘see it through’ the evening and night time requirement from the house, until the next Octopus cheapo window starts at 00:30
  • On this basis, our entire daily domestic consumption often costs only 70p, as a result of using the Powerwall to store 14 kWh of the Octopus Go cheap grid electricity, plus the solar ‘top up’, which costs 0p.

On summer days, there will be more solar energy stored in the Powerwall.  This, coupled with the shorter evenings, means that the Powerwall is likely to be still be part full at 00:30, and so require less grid energy to reach 100% full by 04:30. This will reduce, and possibly eliminate, even the 70p cost. On sunny summer days we expect to pay Octopus only the 25p per day standing charge, on the assumption that we will use the zappi intelligent functions to charge the EVs only during the sunshine hours and only with the surplus solar energy, over and above that being used to charge the Powerwall and to meet domestic loads.

The image below shows the equipment installed in our garage, including:-

  • Our Tesla Powerwall (the big white rectangle).
  • Our zappi smart EV charger, which is explained in the next section).
  • Our SolarEdge inverter (Inverter 2). This is the white box to the left of the zappi, which converts the DC power generated by the solar panels on the garage and studio roofs into AC power usable around the house, as explained above.
    Curiously, AC is converted back into DC when it is used for charging the Powerwall or electric car batteries – this is just a fact of life wiring-wise, don’t worry about it, your installer will be perfectly familiar with which bits of your system are DC and which are AC. The conversion happens inside the Powerwall and the cars.
Tesla Powerwall – myenergi zappi – SolarEdge inverter and grid export limiter

The little square grey box above the garden hose reel is the Grid Export Limiter, and the oblong white box next to it is the fuse board protecting all this equipment (and Anne’s pottery kiln, which is on the same sub-main).

The tiny white rectangle under the inverter is the Generation meter, which records how many kWh the 16 solar panels attached to this inverter have produced. There’s another meter attached to Inverter 1, which manages the 16 panels on the house and office roofs.

You can also see how each piece of equipment is protected with its own isolation switch. This is to allow each individual item to be individually switched out if maintenance work is ever required, avoiding the need to shut down the whole system.

Back to top of page.

EV Charging – myenergi zappi

There’s an exceptional UK company, myenergi ltd, who make one of the cleverest bits of kit on the market, and it aligns perfectly with what we are seeking to achieve. This is the ‘zappi’ electric vehicle (EV) charger. Unlike a ‘bog standard’ charger, zappi is smart. In its ECO+ Mode, zappi constantly monitors the amount of energy being produced by your solar panels and then routes only the excess to your EV. So, let’s say:

  • your solar is producing 5.2 kW at a given moment;
  • your house is consuming 0.5 kW at that same moment;
  • zappi will route only the ‘spare’ 4.7 kW to your EV.
  • This means that you can super-efficiently consume every last electron of your locally-produced energy for your own local consumption;
  • and means that you’re not ‘gifting’ free energy to the grid;
  • and means that local consumption means that the distribution grid doesn’t have to take in the energy you have generated, and then store it and/or redistribute it to other consumers. This welcome side-effect will become hugely significant when millions of households make the switch to local generation plus local consumption for the bulk of their energy needs. Depending on the rate of uptake, potentially billions of pounds could be saved if local storage reduces the need for new power stations and grid-scale storage facilities.

Highly intelligently, zappi constantly monitors the varying rates of solar (or any other micro generation) production and constantly varies the rate of charge to the car, to use only the excess being produced at any given moment.

If the available excess power drops below 1.5 kW, zappi will temporarily suspend charging in ECO+ Mode, because 1.5 kW is the minimum rate of charge specified in the EV charging standards used by the car manufacturers. When that [insert expletive here] cloud blows away, zappi will restart the charge.

The zappi unit is also smart enough to ‘play nice’ with the Powerwall (or other domestic storage battery). Using the on-screen controls on the zappi itself, you can set its ECO+ Mode to “avoid drain”, so that it won’t counterproductively use stored domestic battery energy to charge the car battery. In practice, this means that:

  • either zappi will wait until the Powerwall battery is full before starting to charge an EV;
  • or zappi will use any available power above the 5 kW maximum the Powerwall can accept to charge the EV (for example, solar is producing 9 kW, 5kW goes into Powerwall, zappi routes the 4kW surplus goes into the EV);
  • and zappi will not draw stored energy from the Powerwall to charge the EV. So, if your Powerwall is at 100%, your solar is producing 4kW, and your house is using 1kW, zappi will put only the surplus 3kW into the car, thereby saving your Powerwall energy for domestic use after the sun has gone down

Needless to say, beyond the exceptionally useful ECO+ Mode, zappi has several other operating modes, which allow for very flexible use. The full range of modes is as follows:

  • ECO+ Charge power is continuously adjusted in response to changes in generation or power consumption elsewhere in the home. Charging will pause if there is insufficient solar power, continuing only when there is surplus free power available.
  • ECO Charge power is continuously adjusted in response to changes in generation or power consumption elsewhere in the home. Unlike in ECO+, charging will continue until the vehicle is fully charged, even if some power is drawn from the grid, e.g. when solar is not generating enough to cover the minimum 1.5 kW required to charge an EV.
  • FAST In this mode, the vehicle will be charged at maximum power. This is just like an ordinary ‘dumb’ Mode 3 charging point, which can only charge at maximum rate.
  • TIMED BOOST Using the myenergi app, or the menus on the unit itself, you can programme zappi to charge an EV at zappi’s maximum possible rate of charge (just like FAST mode) between a specified start time and a specified stop time. TIMED BOOST is a vital function for us; further detail below.
  • SMART BOOST You specify a certain amount of kWh to be charged to the EV battery by a certain time. The zappi unit will work out the latest possible time it can start charging the car to deliver the specified kWh and ensure they are delivered by the required time.
  • BOOST Regardless of whatever mode zappi happens to be in, BOOST will simply immediately start charging the car at the maximum possible rate (just like Fast mode). This is useful if you need a charge quickly between trips.

The TIMED BOOST function is exceptionally useful. We use it to tell zappi to fill up the Tesla battery at the maximum rate possible on our single phase domestic grid supply (7.4 kW) rate during the 4 hours of dirt-cheap Octopus Go electricity between 00:30 and 04:30 every day. If the Tesla battery requires charging, the off-peak four hours of cheap electricity will charge the car with 7.4 kW x 4 hours = 29.6 kWh of energy.

Contact myenergi ltd

If you are interested in finding out more about zappi, please use the form below to contact myenergi ltd.

  • Please note, a zappi connected to a 3-phase supply is capable of supplying 22 kW. This will put approximately 56 miles every hour into any EV capable of accepting this higher rate of AC charging. Most UK domestic premises do not have 3-phase grid supplies, but many workplaces do. It’s definitely worth considering a 3-phase unit if your grid supply is suitable and your EV can handle higher rates of charging on AC.
  • Please also note, Tesla vehicles are capable of accepting vastly higher rates of charge when connected to Tesla’s own (DC) Superchargers, which are strategically located around the country to facilitate long road trips. Our three year old Model X (and similar Model S cars) will accept charging which peaks at over 400 miles per hour on a 150kW v2 Supercharger. Tesla’s current cars can accept charging which peaks at over 1,000 miles per hour on the latest Supercharger v3 units, which will be incrementally rolled out over time.

The TIMED BOOST rate of charge equates to around 18 miles for every hour. So, in the four cheapo ‘Octopus hours’, the Tesla gets ‘fuel’ for around 72 miles driving, which is more than enough to ‘refuel’ after a typical day’s local use.

On the Octopus Go tariff the total cost of those 72 miles is £1.48 (29.6 kWh x 5p per kWh).

Assuming 35mpg for a very high performance internal combustion car (which the Tesla will still simply pulverise from 0-60) those 72 miles would require 2.057 gallons = 9.339 litres. Assuming fuel at £1.32/litre, a petrol car would cost you £12.33 in fuel for the same 72 miles.

Over a typical 12,000 miles annual mileage, using 5p/kWh off-peak energy to charge a Tesla battery will cost you £246. Petrol in a 35mpg car will cost you £2,055.

Unless you really, really want to stand in the middle of the road setting fire to fifty pound notes, you may find the following links helpful. See the Links page for benefits disclosure.

Back to top of page.

Charging EVs when away from home

When on longer distance business trips, we refill the battery at Tesla Superchargers en route. Because our car came with the brilliant incentive of “free Supercharging for life”, we pay absolutely nothing (£0) for any electricity we charge up in this way.

As an example of just how valuable the Tesla ‘free supercharging for life’ incentive can be, our own data is instructive. To date, we’ve ‘filled up’ roughly 17,100 kWh at Superchargers. This equates to roughly 40,000 miles.

  • In our particular car, with free Supercharging for life, these 40,000 miles cost us £0 in a Tesla.
  • At the Octopus Go off-peak rate (5p/kWh), 40,000 miles would have cost us £855, charging at home.
  • If we’d used UK average cost electricity (15.5p/kWh), 40,000 miles would have cost us £2,650.
  • If we’d been pouring fuel into an internal combustion car at 35 mpg, we’d have paid £6,848 in petrol.

Please note, some other manufacturers’ newer EVs (e.g. Porsche Taycan) are also capable of very fast charing at Tesla-esque rates. However, unlike Tesla, they do not have an ‘own brand’ network of ultra-fast chargepoints already installed, you will have to wait until the 150kW public chargepoint networks (e.g. Ionity, owned by a consortium of car manufacturers) are rolled out. These will be on a pay-to-charge business model.

Because some of these networks are owned by the oil majors, you’ll also probably have to resign yourself to paying petrol-equivalent rates for your electricity at some of the networks. BP Chargemaster is currently trying to extract 40p per kWh plus a £1.50 fee for anybody using their 150kW charge units with a contactless payment card. To fill our Tesla from 20% to 100% on a road trip, that would cost us £33.50 if we were daft enough to use BP chargers.

You have to ask yourself, what does BP think electricity is? Some kind of virtual petrol? Invisible diesel, perhaps? Whatever, they’re clearly not ‘getting it’. If they gave the energy away at cost EV drivers would happily spend money in forecourt shops whilst getting a quick fill up. You would have thought that this business model would be familiar to people who make their real money from the franchised shops on their forecourts rather than from the wafer-thin margins on selling their own (messy, polluting, planet-killing) core product. Cynically, there’s a suspicion here that it might not be exactly the uppermost priority of an oil company to encourage electric-powered motoring. That is rather forcefully reinforced by the observation that BP currently (Autumn 2019) has a total of 6 charging stalls in operation capable of delivering electricity at 120kW or higher. By contrast Tesla has installed over 14,800 globally (as of September 2019).

Tesla takes a diametrically opposed approach to BP’s 40p/kWh + £1.50 The company states that “Tesla is committed to ensuring that Supercharger will never be a profit center.” In the UK, the ompany charges 24p per kWh at its Superchargers, which squirt out energy at ultra-fast rates.

In short, don’t be fooled by oil multinationals greenwashing. You’re better off buying a Tesla. You can fuel them for next to nothing using cheap overnight electricity at home (which is where most charging is actually done) and use Tesla’s not-for-profit Supercharger network when you’re away from home. Anybody using the link below will get 1,000 free supercharger miles as a bonus on top of that.

All Tesla drivers also get to use the company’s Destination Charger network, free of charge. These are located at many hotels, restaurants and other locations where you’ll be parking up. At a hotel, for example, you arrive, plug your Tesla in whilst you have dinner, or sleep overnight. When you leave the next morning, your car will be fully charged. And the charge will have cost you absolutely zilch. £0.

And, if you follow the link, you may get lucky and be able to pick up an Inventory Car from Tesla with the Free Supercharging for Life incentive bundled in. Such cars are easily recognised on the Tesla website, as the screenshots below illustrate, note the blue ‘lightning flash’ and adjacent wording. These images were captured 03 November 2019. Please be aware that Tesla promotional offers change constantly, so please check carefully. You also need to check whether a particular ‘inventory’ car officially counts as “new” when registered to you: certain tax breaks depend on that being the case We’ll be writing more on this subject at a later date.

Tesla Model S – 03 November 2019 Free Supercharging Offer

Back to top of page.