In normal day to day use, not discharging below 50%, it is likely to use less than 1 unit ( Kwh) to recharge the batteries. At 10p/unit this will cost less than 10p. (From your electricity bill you will find the price /unit you pay, probably between 8p-20p/unit, divide this by 10 and then multiply by the 10p cost to arrive at your cost.)
Once the batteries are recharged the cost to leave the charger on maintaining the batteries will be equivalent to between one and two watts and over a 24 Hr period that will amount to less than 0.048Kwh so at 10p/unit it will cost 0.48p. (£1.75 / Year!)
To achieve optimum battery life it is beneficial not to discharge below 50% routinely. If the batteries are fully discharged it will take approximately 1.7 units (Kw/hr) to fully recharge them.
Standard Tramper batteries are rated at 73Ah C20 rating which means they can release 3.65A for 20Hrs at 24 volts. This is 3.65 x 24 = 87.6 watts for 20Hrs or 1.750Kw/hrs (Note this is the absolute max batteries can produce, at 20 °C, However if the power is used more quickly the batteries will release much less power. This is a fundamental property of batteries and is defined by “Peukert’s exponent”. E.G. if the batteries are discharged at 20A their capacity is reduced to 54.33Ah! The capacity of a battery reduces the faster power is used, and if the temperature is below the rating temperature (approx. 1% per degree below 20°C)
Typically a Tramper will use about 15amps on flat smooth ground and at a current of 15A the batteries will produce 56.4 ah (56.4/15a = 3.76hrs which at 8 mph is 30.08miles.) generally, unless driving on a smooth flat surface this will not be achieved because of gradients and increased rolling resistance of unsealed surfaces.So taking 56.4AH as the most that can be practically taken out of the batteries and as batteries are about 90% efficient it will require 56.4 x 1.1 =62.04 Ah to be put back into the batteries coupled to this battery chargers are, typically , around 86% efficient it will therefore take 1.497 x 1.16 = 1.7 Kw/h from the mains.
At a cost of 10p/unit it will cost 17.3p to fully recharge the batteries fully. Due to th fluctuating energy prices, you will need to work this out using the current rates. Please note it is very unlikely that the batteries will be discharged to this level in normal use. Typical users seldom use more than 50% of the battery capacity per day unless climbing steep hills or on long rambles.