diff --git a/cmd/charger.go b/cmd/charger.go
index 9a8594be2..0a86ef50a 100644
--- a/cmd/charger.go
+++ b/cmd/charger.go
@@ -24,7 +24,10 @@ func runCharger(cmd *cobra.Command, args []string) {
 	log.INFO.Printf("evcc %s (%s)", server.Version, server.Commit)
 
 	// load config
-	conf := loadConfigFile(cfgFile)
+	conf, err := loadConfigFile(cfgFile)
+	if err != nil {
+		log.FATAL.Fatal(err)
+	}
 
 	// setup mqtt
 	if conf.Mqtt.Broker != "" {
diff --git a/cmd/config.go b/cmd/config.go
index a6ca22710..560051cb6 100644
--- a/cmd/config.go
+++ b/cmd/config.go
@@ -21,6 +21,7 @@ type config struct {
 	Levels     map[string]string
 	Interval   time.Duration
 	Mqtt       mqttConfig
+	Javascript map[string]interface{}
 	Influx     server.InfluxConfig
 	HEMS       typedConfig
 	Messaging  messagingConfig
diff --git a/cmd/demo.go b/cmd/demo.go
new file mode 100644
index 000000000..3e42b404b
--- /dev/null
+++ b/cmd/demo.go
@@ -0,0 +1,140 @@
+package cmd
+
+import (
+	"github.com/spf13/viper"
+	"gopkg.in/yaml.v3"
+)
+
+const demoYaml = `
+log: info
+
+javascript:
+- vm: shared
+  script: |
+    state = {
+      residualpower: 200,
+      pvpower: -3000,
+      batterypower: 200,
+      gridpower: -2000,
+      chargepower: 0,
+      maxcurrent: 0,
+      battery: 63, // car
+    };
+    function get() {
+      console.log("state:", JSON.stringify(state));
+    }
+    function set() {
+      console.log(param+":", val);
+      console.log("state:", JSON.stringify(state));
+    }
+
+meters:
+- name: grid
+  type: default
+  power:
+    type: js
+    vm: shared
+    script: state.gridpower = state.pvpower + state.chargepower + state.residualpower + state.batterypower;
+
+- name: pv
+  type: default
+  power:
+    type: js
+    vm: shared
+    script: state.pvpower += 100*Math.random();
+
+- name: battery
+  type: default
+  power:
+    type: js
+    vm: shared
+    script: state.batterypower;
+  soc:
+    type: js
+    vm: shared
+    script: "30"
+- name: charge
+  type: default
+  power:
+    type: js
+    vm: shared
+    script: state.chargepower;
+  soc:
+    type: js
+    vm: shared
+    script: "30"
+
+chargers:
+- name: demo
+  type: default
+  enable:
+    type: js
+    vm: shared
+    script: |
+      set();
+      state.enabled = val;
+      if (state.enabled) state.chargepower = state.maxcurrent * 230; else state.chargepower = 0;
+  enabled:
+    type: js
+    vm: shared
+    script: |
+      state.enabled;
+  status:
+    type: js
+    vm: shared
+    script: |
+      if (state.enabled) "C"; else "B";
+  maxcurrent:
+    type: js
+    vm: shared
+    script: |
+      set();
+      state.maxcurrent = val;
+      if (state.enabled) state.chargepower = state.maxcurrent * 230;
+
+vehicles:
+- name: demo
+  title: e-Golf
+  type: default
+  charge:
+    type: js
+    vm: shared
+    script: |
+      if (state.chargepower > 0) state.battery++; else state.battery--;
+      if (state.battery < 15) state.battery = 15;
+      if (state.battery > 100) state.battery = 100;
+      state.battery;
+  cache: 1s
+
+site:
+  title: Demo
+  meters:
+    grid: grid
+    pv: pv
+    battery: battery
+
+loadpoints:
+- title: Carport
+  charger: demo
+  meters:
+    charge: charge
+  vehicle: demo
+`
+
+func demoConfig() (conf config) {
+	demo := map[string]interface{}{}
+
+	if err := yaml.Unmarshal([]byte(demoYaml), &demo); err != nil {
+		log.FATAL.Fatalf("failed decoding demo config: %+v", err)
+	}
+
+	for k, v := range demo {
+		viper.Set(k, v)
+	}
+
+	if err := viper.UnmarshalExact(&conf); err != nil {
+		log.FATAL.Fatalf("failed loading demo config: %v", err)
+	}
+
+	return conf
+}
diff --git a/cmd/dump.go b/cmd/dump.go
index 0ec76bc31..45c481fa3 100644
--- a/cmd/dump.go
+++ b/cmd/dump.go
@@ -26,7 +26,10 @@ func runDump(cmd *cobra.Command, args []string) {
 	log.INFO.Printf("evcc %s (%s)", server.Version, server.Commit)
 
 	// load config
-	conf := loadConfigFile(cfgFile)
+	conf, err := loadConfigFile(cfgFile)
+	if err != nil {
+		log.FATAL.Fatal(err)
+	}
 
 	// setup mqtt
 	if conf.Mqtt.Broker != "" {
diff --git a/cmd/meter.go b/cmd/meter.go
index d8d7f2a5b..73c67d5eb 100644
--- a/cmd/meter.go
+++ b/cmd/meter.go
@@ -24,7 +24,10 @@ func runMeter(cmd *cobra.Command, args []string) {
 	log.INFO.Printf("evcc %s (%s)", server.Version, server.Commit)
 
 	// load config
-	conf := loadConfigFile(cfgFile)
+	conf, err := loadConfigFile(cfgFile)
+	if err != nil {
+		log.FATAL.Fatal(err)
+	}
 
 	// setup mqtt
 	if conf.Mqtt.Broker != "" {
diff --git a/cmd/root.go b/cmd/root.go
index 9c3a74fb3..1d0ce7867 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -142,7 +142,12 @@ func run(cmd *cobra.Command, args []string) {
 	log.INFO.Printf("evcc %s (%s)", server.Version, server.Commit)
 
 	// load config and re-configure logging after reading config file
-	conf := loadConfigFile(cfgFile)
+	conf, err := loadConfigFile(cfgFile)
+	if err != nil {
+		log.ERROR.Println("missing evcc config - switching into demo mode")
+		conf = demoConfig()
+	}
+
 	util.LogLevel(viper.GetString("log"), viper.GetStringMapString("levels"))
 
 	uri := viper.GetString("uri")
@@ -153,6 +158,9 @@ func run(cmd *cobra.Command, args []string) {
 		configureMQTT(conf.Mqtt)
 	}
 
+	// setup javascript VMs
+	configureJavascript(conf.Javascript)
+
 	// start broadcasting values
 	tee := &Tee{}
 
diff --git a/cmd/setup.go b/cmd/setup.go
index 9060fbbfd..834264324 100644
--- a/cmd/setup.go
+++ b/cmd/setup.go
@@ -9,6 +9,7 @@ import (
 
 	"github.com/andig/evcc/core"
 	"github.com/andig/evcc/hems"
+	"github.com/andig/evcc/provider/javascript"
 	"github.com/andig/evcc/provider/mqtt"
 	"github.com/andig/evcc/push"
 	"github.com/andig/evcc/server"
@@ -57,6 +58,13 @@ func configureMQTT(conf mqttConfig) {
 	}
 }
 
+// setup javascript
+func configureJavascript(conf map[string]interface{}) {
+	if err := javascript.Configure(conf); err != nil {
+		log.FATAL.Fatalf("failed configuring javascript: %v", err)
+	}
+}
+
 // setup HEMS
 func configureHEMS(conf typedConfig, site *core.Site, cache *util.Cache, httpd *server.HTTPd) hems.HEMS {
 	hems, err := hems.NewFromConfig(conf.Type, conf.Other, site, cache, httpd)
@@ -131,15 +139,15 @@ func configureLoadPoints(conf config, cp *ConfigProvider) (loadPoints []*core.Lo
 	return loadPoints, nil
 }
 
-func loadConfigFile(cfgFile string) (conf config) {
+func loadConfigFile(cfgFile string) (conf config, err error) {
 	if cfgFile != "" {
 		log.INFO.Println("using config file", cfgFile)
 		if err := viper.UnmarshalExact(&conf); err != nil {
 			log.FATAL.Fatalf("failed parsing config file %s: %v", cfgFile, err)
 		}
 	} else {
-		log.FATAL.Fatal("missing evcc config")
+		err = errors.New("missing evcc config")
 	}
 
-	return conf
+	return conf, err
 }
diff --git a/cmd/vehicle.go b/cmd/vehicle.go
index 5c07f6da4..d9d00b90a 100644
--- a/cmd/vehicle.go
+++ b/cmd/vehicle.go
@@ -24,7 +24,10 @@ func runVehicle(cmd *cobra.Command, args []string) {
 	log.INFO.Printf("evcc %s (%s)", server.Version, server.Commit)
 
 	// load config
-	conf := loadConfigFile(cfgFile)
+	conf, err := loadConfigFile(cfgFile)
+	if err != nil {
+		log.FATAL.Fatal(err)
+	}
 
 	// setup mqtt
 	if conf.Mqtt.Broker != "" {
diff --git a/go.mod b/go.mod
index 7825ad6b2..0fcbf05d8 100644
--- a/go.mod
+++ b/go.mod
@@ -57,6 +57,7 @@ require (
 	github.com/tv42/httpunix v0.0.0-20191220191345-2ba4b9c3382c
 	github.com/volkszaehler/mbmd v0.0.0-20201205173745-5106cb0b334e
 	golang.org/x/net v0.0.0-20200904194848-62affa334b73
+	golang.org/x/sys v0.0.0-20201231184435-2d18734c6014 // indirect
 	gopkg.in/ini.v1 v1.62.0
 	gopkg.in/sourcemap.v1 v1.0.5 // indirect
 	gopkg.in/yaml.v3 v3.0.0-20200615113413-eeeca48fe776
diff --git a/go.sum b/go.sum
index c1cb64e3f..6a5803c11 100644
--- a/go.sum
+++ b/go.sum
@@ -435,6 +435,8 @@ golang.org/x/sys v0.0.0-20200728102440-3e129f6d46b1 h1:sIky/MyNRSHTrdxfsiUSS4WIA
 golang.org/x/sys v0.0.0-20200728102440-3e129f6d46b1/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f h1:+Nyd8tzPX9R7BWHguqsrbFdRx3WQ/1ib8I44HXV5yTA=
 golang.org/x/sys v0.0.0-20200930185726-fdedc70b468f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20201231184435-2d18734c6014 h1:joucsQqXmyBVxViHCPFjG3hx8JzIFSaym3l3MM/Jsdg=
+golang.org/x/sys v0.0.0-20201231184435-2d18734c6014/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
diff --git a/provider/javascript.go b/provider/javascript.go
index b877733a8..4426a2dc9 100644
--- a/provider/javascript.go
+++ b/provider/javascript.go
@@ -1,6 +1,9 @@
 package provider
 
 import (
+	"strings"
+
+	"github.com/andig/evcc/provider/javascript"
 	"github.com/andig/evcc/util"
 	"github.com/robertkrimen/otto"
 )
@@ -19,6 +22,7 @@ func init() {
 // NewJavascriptProviderFromConfig creates a HTTP provider
 func NewJavascriptProviderFromConfig(other map[string]interface{}) (IntProvider, error) {
 	cc := struct {
+		VM     string
 		Script string
 	}{}
 
@@ -28,9 +32,11 @@ func NewJavascriptProviderFromConfig(other map[string]interface{}) (IntProvider,
 
 	log := util.NewLogger("js")
 
+	vm := javascript.RegisteredVM(strings.ToLower(cc.VM))
+
 	p := &Javascript{
 		log:    log,
-		vm:     otto.New(),
+		vm:     vm,
 		script: cc.Script,
 	}
 
@@ -85,10 +91,21 @@ func (p *Javascript) BoolGetter() func() (bool, error) {
 	}
 }
 
+func (p *Javascript) setParam(param string, val interface{}) error {
+	err := p.vm.Set(param, val)
+	if err == nil {
+		err = p.vm.Set("param", param)
+	}
+	if err == nil {
+		err = p.vm.Set("val", val)
+	}
+	return err
+}
+
 // IntSetter sends int request
 func (p *Javascript) IntSetter(param string) func(int64) error {
 	return func(val int64) error {
-		err := p.vm.Set(param, val)
+		err := p.setParam(param, val)
 		if err == nil {
 			_, err = p.vm.Eval(p.script)
 		}
@@ -99,7 +116,7 @@ func (p *Javascript) IntSetter(param string) func(int64) error {
 // StringSetter sends string request
 func (p *Javascript) StringSetter(param string) func(string) error {
 	return func(val string) error {
-		err := p.vm.Set(param, val)
+		err := p.setParam(param, val)
 		if err == nil {
 			_, err = p.vm.Eval(p.script)
 		}
@@ -110,7 +127,7 @@ func (p *Javascript) StringSetter(param string) func(string) error {
 // BoolSetter sends bool request
 func (p *Javascript) BoolSetter(param string) func(bool) error {
 	return func(val bool) error {
-		err := p.vm.Set(param, val)
+		err := p.setParam(param, val)
 		if err == nil {
 			_, err = p.vm.Eval(p.script)
 		}
diff --git a/provider/javascript/init.go b/provider/javascript/init.go
new file mode 100644
index 000000000..0c644d9e1
--- /dev/null
+++ b/provider/javascript/init.go
@@ -0,0 +1,56 @@
+package javascript
+
+import (
+	"github.com/andig/evcc/util"
+	"github.com/robertkrimen/otto"
+)
+
+// Configure initializes JS VMs
+func Configure(other map[string]interface{}) error {
+	cc := []struct {
+		VM     string
+		Script string
+	}{}
+
+	if err := util.DecodeOther(other, &cc); err != nil {
+		return err
+	}
+
+	// init all VMs that require it
+	for _, conf := range cc {
+		if conf.Script == "" {
+			continue
+		}
+
+		if _, ok := registry[conf.VM]; !ok {
+			vm := otto.New()
+
+			_, err := vm.Run(conf.Script)
+			if err != nil {
+				return err
+			}
+
+			registry[conf.VM] = vm
+		}
+	}
+
+	return nil
+}
+
+var registry = make(map[string]*otto.Otto)
+
+// RegisteredVM returns a JS VM. If name is not empty, it will return a shared instance.
+func RegisteredVM(name string) *otto.Otto {
+	vm, ok := registry[name]
+
+	// create new VM
+	if !ok {
+		vm = otto.New()
+
+		if name != "" {
+			registry[name] = vm
+		}
+	}
+
+	return vm
+}