The Dream Setup: Free Fuel From Your Roof

If you have solar panels — or you are planning to install them — your EV charger can use surplus solar energy to charge your car for free. No electricity bill, no petrol station, no fuel cost at all. It sounds too good to be true, but the technology works, it is available now, and homeowners across Rotherham and South Yorkshire are already using it.

This guide explains exactly how solar and EV charging work together, what equipment you need, and what to realistically expect in terms of free miles.

How Solar Divert Charging Works

Solar divert — also called excess solar charging or solar surplus charging — is a system that monitors how much electricity your solar panels are generating and how much the house is using. Any surplus energy that would otherwise be exported to the grid is diverted to charge your EV instead.

The basic principle

A typical 4kW solar panel system in South Yorkshire generates electricity throughout the day, peaking around midday. Your house uses some of this for lighting, heating, appliances, and other loads. Whatever is left over — the surplus — normally gets exported to the National Grid. You may receive a small payment for this through the Smart Export Guarantee (SEG), typically around 4p to 15p per kWh depending on your tariff.

With solar divert, instead of exporting that surplus for a few pence, you send it to your EV charger. Given that charging from the grid costs 24p to 30p per kWh on a standard tariff (or 7p to 10p on an overnight EV tariff), every kWh diverted to your car saves you significantly more than the SEG payment.

How the charger knows when to charge

The system uses a CT (current transformer) clamp on your solar generation cable to measure how much power the panels are producing. A second CT clamp on the grid supply cable monitors how much power is being imported or exported. When the system detects surplus solar generation — meaning the panels are producing more than the house needs — it sends that surplus to the EV charger.

The charger dynamically adjusts its output to match the available surplus. If there is 2kW of surplus, the charger runs at 2kW. If clouds pass over and the surplus drops to 1kW, the charger reduces to 1kW. When the sun comes back out and surplus rises to 3.5kW, the charger increases to match. This happens automatically and continuously throughout the day.

What Equipment Do You Need?

A compatible smart EV charger

Not all EV chargers support solar divert. You need a smart charger that can communicate with the solar monitoring system and dynamically adjust its output. Popular chargers with built-in or compatible solar divert include the Zappi by Myenergi — specifically designed for solar integration, with built-in CT clamp support and three charging modes (Eco, Eco+, and Fast). Ohme and some other smart chargers can also integrate with solar systems through their apps and APIs.

The Zappi is the most popular choice for solar divert in the UK because it was designed specifically for this purpose. Its Eco+ mode uses only surplus solar energy, while Eco mode tops up from the grid if the solar surplus is not enough to maintain the minimum charge rate.

CT clamps

You need CT clamps to monitor energy flows. The solar generation CT clamp goes on the cable between the solar inverter and the consumer unit. The grid supply CT clamp goes on the main incoming supply cable. Your installer will fit these during the EV charger installation. If you already have solar panels installed, the CT clamps are simply added to the existing cables — the solar system itself does not need modifying.

Solar panels

Any grid-connected solar panel system works with solar divert. The larger the system, the more surplus energy available for charging. However, even a modest 3kW to 4kW system can provide meaningful free charging, particularly in spring and summer.

How Many Free Miles Can You Expect?

This is the question everyone asks, and the answer depends on several factors: the size of your solar system, your household electricity consumption, the time of year, the weather, and how efficient your car is.

A realistic South Yorkshire example

A typical 4kW solar panel system in South Yorkshire generates around 3,400 kWh per year. After powering the house (average consumption 2,900 kWh for a three-bedroom semi), there is typically 1,200 to 1,800 kWh of surplus energy per year, depending on how much you use during daylight hours.

If all of that surplus goes to the EV charger, and assuming your car averages 3.5 miles per kWh (a reasonable figure for most modern EVs), that gives you approximately 4,200 to 6,300 free miles per year. For a typical commuter doing 8,000 miles per year, solar surplus could cover 50 to 75 percent of your annual driving — for free.

Seasonal variation

The surplus is not evenly distributed through the year. In June and July, a 4kW system in our region can generate 15 to 20 kWh per day, with 8 to 14 kWh of surplus available for the car. That is 28 to 49 miles of free driving per day — more than enough for most commuters.

In December and January, daily generation drops to 3 to 5 kWh, and most or all of it is used by the house. Very little surplus is available for the car. During winter months, you will rely on grid charging (ideally on a cheap overnight tariff) for most of your driving.

Spring and autumn fall between these extremes, with useful but variable surplus depending on weather conditions.

The car needs to be at home

Solar divert only works when the car is plugged in at home during the day. If you drive to work every day and the car is away from 8am to 6pm, you will miss the peak solar generation period. Solutions include working from home (even one or two days a week helps), charging a second car that stays at home, using a home battery to store surplus during the day and charge the car in the evening, and scheduling the charger to use cheap overnight electricity when solar is not available.

Adding a Home Battery to the Mix

A home battery storage system takes the solar-EV combination to the next level. Instead of losing surplus solar energy when the car is not at home, the battery stores it for later use.

How it works with EV charging

During the day, surplus solar energy charges the home battery. In the evening, when you return home and plug in the car, the battery can discharge to the EV charger. You are still using free solar energy, just time-shifted to when the car is actually available.

The economics depend on the battery size. A typical 5kWh home battery might store enough energy for 17 to 18 miles of driving — useful but modest. A larger 10kWh to 13kWh battery could store enough for 35 to 45 miles, which covers most daily commutes.

Important safety considerations

If you are considering home battery storage, be aware of the new regulations. PAS 63100:2024 sets fire safety requirements for domestic battery installations, including restrictions on where batteries can be located (not under staircases, not blocking escape routes, minimum clearances from windows and doors). See my full guide on battery storage rules for 2026.

The Economics: Is It Worth It?

Charging costs compared

Grid charging (standard tariff) — at 24p to 30p per kWh, charging a 60kWh battery from empty costs £14 to £18, giving approximately 200 miles of range.

Grid charging (overnight EV tariff) — at 7p to 10p per kWh, the same charge costs £4 to £6. This is already dramatically cheaper than petrol (an equivalent petrol car would cost £25 to £35 in fuel for 200 miles).

Solar surplus charging — free. The electricity has already been generated by your panels and would otherwise be exported for 4p to 15p per kWh. Every kWh diverted to the car instead of exported saves you the difference between your grid rate and your export rate.

Annual savings

If you divert 1,500 kWh of solar surplus to your EV per year (a realistic figure for a 4kW system), your savings depend on what you would otherwise pay for that electricity. Compared to a standard grid tariff at 27p per kWh, you save approximately £405 per year. Compared to an overnight EV tariff at 8p per kWh, you save approximately £120 per year (but you are already saving significantly by using the EV tariff versus petrol). Compared to the SEG export payment at 10p per kWh, you are better off by approximately £255 per year (the difference between not exporting at 10p and not importing at 27p).

Over the 25-year lifespan of a solar panel system, those savings compound significantly, especially as electricity prices tend to rise over time.

Charging Modes Explained

Most solar-compatible chargers offer multiple charging modes to suit different needs.

Solar only (Eco+ on Zappi) — the charger only uses surplus solar energy. If there is no surplus, it stops. This maximises free charging but means the car may not fully charge on cloudy days or in winter. Best for second cars or when you have a large solar system.

Solar priority with grid top-up (Eco on Zappi) — the charger prioritises solar surplus but tops up from the grid when surplus is insufficient to maintain the minimum charge rate. This ensures the car always charges but uses as much free solar as possible. The most popular mode for everyday use.

Fast (grid only) — charges at full speed from the grid, ignoring solar. Use this when you need a quick charge and cannot wait for solar generation.

Scheduled — charges at a set time (usually overnight on a cheap tariff). No solar involvement. Use this to take advantage of overnight EV tariffs like Octopus Go or Intelligent Octopus.

The best strategy for most people is solar priority during the day and scheduled overnight charging as backup. This maximises free miles while ensuring the car is always ready.

Can You Add Solar Divert to an Existing Charger?

If you already have an EV charger installed and want to add solar divert, your options depend on the charger model. Some smart chargers can be retrofitted with solar monitoring by adding CT clamps and updating the charger’s firmware or app settings. Others do not support solar divert and would need replacing.

If you are buying a new charger and have solar panels (or plan to add them), choosing a solar-compatible charger from the start avoids the cost and hassle of replacing it later. The additional cost for a solar-compatible charger is modest compared to the long-term savings.

Do You Need to Upgrade Your Electrical Supply?

Adding an EV charger to a home that already has solar panels does not usually require a supply upgrade, but it does increase the load on your consumer unit. The charger needs its own dedicated circuit with appropriate RCD protection, and the consumer unit must have a spare way for the additional circuit breaker.

If your consumer unit is already full (common in older properties where solar, a new shower circuit, and other additions have used all available ways), it may need upgrading. Load management — where the charger monitors total demand and reduces its output during peak usage — is particularly important in homes with solar, battery storage, and an EV charger, because the combination can push the total connected load well beyond the main fuse capacity.

For more detail on supply capacity and load management, see my guide on whether your home electrics can handle an EV charger.

Frequently Asked Questions

Can I charge my EV for free with solar panels?

Yes. A solar-compatible EV charger with solar divert functionality can use surplus solar energy to charge your car at zero cost. A typical 4kW solar system in South Yorkshire can provide 4,200 to 6,300 free miles per year, depending on household consumption and weather.

What is the best EV charger for solar panels?

The Zappi by Myenergi is the most popular choice in the UK for solar divert. It has three charging modes (solar only, solar priority with grid backup, and fast grid charging) and monitors solar generation via CT clamps. Other smart chargers including Ohme also offer solar integration.

Do I need a battery to charge my EV from solar?

No. Solar divert works without a battery — surplus solar energy goes directly to the car. However, a home battery allows you to store surplus during the day and charge the car in the evening when you get home, which is useful if the car is away during peak solar hours.

How many solar panels do I need to charge an EV?

A 4kW system (around 10 panels) provides meaningful surplus for EV charging. Larger systems (6kW to 8kW) generate more surplus and can cover a higher proportion of your annual driving. Even a smaller 3kW system provides some free charging, particularly in summer.

Can I add solar divert to my existing EV charger?

It depends on the charger. Some smart chargers can be upgraded with CT clamps and firmware updates to enable solar divert. Others are not compatible and would need replacing. If you are buying a new charger and have solar panels, choose a solar-compatible model from the start.

If you want to set up solar divert charging at your home in Rotherham or South Yorkshire — or if you want advice on the best charger and solar combination for your property — call me on 07817 171954 or get in touch here. I am Mat from MP Electrical, a NAPIT-registered electrician, and I install EV chargers with full solar integration across the region.