In recent years, solar panels on homes have become so popular that the electricity grid has trouble absorbing the power generated on sunny days. At the time when the solar panels are most productive, their owners may not be able to use them to supply power to the grid, or they receive a very low payment.
A possible way to profit more from the electricity which is generated by the solar panels at times when there is plenty of sunshine and no matching demand, is to store the low cost electricity in batteries. Does this make (financial) sense?
Before putting in the numbers, let us define a few scenario's:
General: household with electric car (20 000 km per year). Electricity consumption 5600 kWh per year (2850 for the car and 2750 for the home).
- A. Without solar panels and a fixed electricity rate € 0.40 per kWh.
- B. With 12 solar panels, energy supplied to the grid € 0.09 per kWh.
- C. With solar panels and a 5 kWh home battery, loading max 2.2 kW, supplying max 1.7kW.
Scenario A, no panels and no battery
The first case is simple, the yearly electricity bill will be € 2240. There are no investments, therefore no pay-back times.
Scenario B, only solar panels
This case is a bit more complicated. The solar panels supply energy when the sun shines, even when it is cloudy, but not at night. So if the car is used during the day and charges at night, electricity from the grid is used. The energy harvested from the sun is, according to several sites, 0.85 - 0.9 kWh/year per Wp installed. When taking the positive value, 12 solar panels with 400 Wp each can produce 4320 kWh per year. The investment for the panels is roughly € 8500. In practice, it will be difficult to adjust energy use to the times when the solar panels are active. Moreover, in winter, much less electricity is generated by the panels than in summer (see Figure 2).
If we assume the car is only charged at night, and half the energy use of the home is at night, then the electricity bill will be € 1434. The cost saving compared to scenario A is then € 806 per year. Not counting interest, this leads to a pay-back time of 8500/806 = 10.5 yr.
Scenario C, both panels and a battery
This scenario is more complicated still. The solar panels produce, on average, 8.8 kWh per day. The battery ensures 5 kWh is stored if it is not used directly. However, in January, February, November, December, the battery will not be fully charged due to the short days. The net electricity bill becomes € 1013. The cost saving compared to case B is € 420. The investment for the battery is roughly € 3250 + installation cost (lets assume € 250). Not counting interest, this leads to a pay-back time of 3500/420= 8.3 yr.
The financial incentive is not very big, but there is also an emission advantage. The production of electricity for the grid causes, at present, roughly 290 kgCO2 per 1000 kWh. Both case B and C buy 4320 kWh/yr less from the grid than case A, which corresponds to a reduction of 1253 kg CO2 per year. To put this in perspective, the EU Emission Trading System values this at 1253 kg * € 85/tonne = € 107 per year.
The effect on the load on the electricity grid varies over the year. With solar cells, the demand is zero during the day for the best months, but the demand at night is the same in december for all scenarios. The batteries do reduce the maximum amount of energy supplied to the grid. This is good news for the electricity companies.
In this example, the solar panels with the battery give a small saving over a 10 year period. After 10 years, the combination gives a yearly saving of € 1227, if the battery still works after ten years. The saving could be more if variable pricing is taken full advantage of, and the outcome depends on many variables which can vary unpredictably. The solar panels only (case B) has a yearly saving of € 806.
The outcome depends, of course, strongly on the scenarios.
One thing is strange, though. A Tesla Model 3 costs around € 44 000 and has a capacity of 57.5 kWh (€ 765 per kWh). The Sessy costs € 3250 (without installation) for 5 kW. (€ 650 per kWh). If you team up with a few neighbours and buy 24 Sessy batteries, you spend € 78 000 for 120 kWh. If you spend € 10 000 more, you can get 115 kWh of storage AND two Tesla model 3 cars. You could use one and keep the other plugged in. At present, however, Teslas do not have 'bidirectional charging', so this is not a serious option, but why is Sessy ('just a battery') so expensive compared to the Tesla (battery, body work, motor, wheels, brakes, seats etc.)?
Conclusion
The home battery leads to a lower supply to the grid from the solar panels, but, in this example, the maximum demand (at night) is the same for all scenarios. A better effect could be achieved by installing more batteries, but this would be more expensive. A cheaper home battery is needed!
