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Solar Home Power Supply with Batteries
combined with Mains Power-Supply


Having a Solar-System helps you to reduce the energy-costs and is also a backup-system in case of Power-failure. But to increase efficiency it is good to use as much energy as possible direct without loading the batteries, because this produces some losses.

This is one concept how to combine and manage both energy-sources, Solar-Power and Mains Power-Supply.



  • Automatic switch over Mains Power-Supply to Solar-Power depending from the Battery-Status and Inverter-Status (overload).
  • Interlocking switch over Inverter and Mains by two Switches.
  • Charging of the Batteries by Solar-Power with a Charge-Controller
  • Charge of the Batteries by a external Battery-Charger from Mains in Case of Solar-Energy is available (using a LDR-Sensor). This is a option if you have a grid-connected system.
  • Switch off Inverter if not in use to save Power.
  • Fridge, Deep-freeze, Washing-machine and Dish-Washer can also be controlled by Metamorphosis, that they start only from 11:00 till 15:00 and not all at the same time.

Details of the functions for autonomous power supply

1. Switch to grid if the battery is empty (not completely empty).

Low battery voltage delayed switches to grid. With a hysteresis and a delay switch back to battery mode. The Metamorphosis has to be powered by the Batteries, to measure the Battery voltage. This function requires a Battery-powered Backup-System.

2. Switch to Battery-Power if the inverter is ok (no fault and or over current).

Delayed switch to inverter mode if he runs again.

If the inverter does not have an output voltage (over-current or battery empty or manually off), then it will be switched to the Mains. It needs a 24 V transformer as feedback from Inverter and for the Switch.

3. Turn on Refrigerator / freezer / washing machine / dishwasher at sunshine time only.
Timer triggered from 11:00 to 15:00. First refrigerator, freezer, washing machine then then dishwasher f.e.

Switching the fridge on only at daytime for some hours is possible, because the cooling energy stays for several hours if it is good isolated. But you have the problem that if you open the fridge at the evening, the light will not be on. May be a small battery-driven light will help. How to keep cold energy a long time in the fridge?

Use latent heat (Wikipedia:

Latent heat storage works through the use of solid-liquid phase transition (freezing-melting). When charging the latent heat storage frozen salt mixed with water are melted (storage medium) and reverse. They absorb and release a lot of energy by that.

Calculating the amount of the salt:
If you multiply -15.71 g / with the desired freezing point you get the amount of salt per kg of water.
Example freezing point -18 C
m = T x (-15.71 g / )
m = -18 x (-15.71 g / )
m = 280g
Use about 280 g Salt per kg of water. The freezing point would be at about -18 degrees.

According to some Internet-sources, temperature of 10 C in the deep-freeze is pretty enough (official is -18 C for restaurants etc).

4. Switch to battery power during power failure (grid power monitor).

In this case the voltage of the battery is discharged a little bit lower than in the normal mode. Voltage sensor is a 24 V / 2 A power adapter which also is necessary for the Power-Switch. The battery voltage is monitored and turned off if the battery is fully empty.

5. Additional Device on if the battery is full, e.g. a Dryer.

6. Refrigerator / freezer turn on if the temperature is > X C.

One (Radio-) temperature measurement has to be put in the refrigerator. It turns the fridge on at critical temperatures.

7. Charging the Batteries by grid and a external Battery-charger if there is sunshine and the batteries are not full. The sunshine is detected by a light sensitive Sensor (LDR). Setpoint for the LDR has to be Setup by testing. (See channel 12)

8. Inverter on only if Battery-Power needed, to save Power. For example can be switched off in the night.


Switching from Solar-Power to Mains and reverse is proceeded in Milliseconds. Switching by Power-Failure-Mains takes a bit longer.

See the Excel-Sheet for the according functions.

Fridge, Deep-freeze, Washing-machine and Dish-Washer can also be managed better by an external Timer, because wiring is more easy. These Units can get activated every day independent from sunshine, because they need to be on at least once a day. May be washing machine can be shifted to a sunny day.

Proposal how to start the Devices:

  • For example: 11:00-15:00 Fridge
  • 11:01-15:00 Deep-freeze
  • 11:02-13:00 Washing-machine
    13:00-15:00 Dishwashing-machine

external wiring of the System; Here as pdf

Excel-Sheet for the different functions

Manual for this Application

This is one application for the Metamorphosis. Here you find more information

Here is another concept for this task


Here my Board with up Current-counter, two 24 V Power-Supply, Metamorphosis with 30 A Solar charger.
right Mains-Switch and Inverter-Switch, blue is the 100 mF Condensator, then a second 30 A Power-Unit for Solar Charging.