Free Tool
Solar & Battery Sizing Calculator
Add your appliances or enter your daily usage, and find out exactly what size battery bank and solar array your off-grid system needs.
To size an off-grid setup, add up your daily energy use in watt-hours, then size the battery bank by dividing by your usable depth of discharge and multiplying by your days of autonomy, and size the solar array by dividing daily use by UK peak sun hours and a system-loss factor. This calculator does both from your appliances or a usage figure.
Interactive Tool
Solar & Battery Sizing Calculator
Fridges cycle on and off, so 8 hours reflects a typical 24-hour duty cycle rather than constant running.
How many cloudy days the battery must cover without solar input.
LiFePO4 offers 90% usable capacity, AGM lead-acid only 50%.
Winter sizing needs a much bigger array. UK winter days deliver barely a quarter of the solar energy of summer days.
Battery bank
600Wh
50Ah at 12V
Solar array
230W
2 × 200W panels
410Wh per day ÷ 85% inverter efficiency = 482Wh demand. Battery: × 1 day ÷ 90% usable. Solar: ÷ (2.8 sun hours × 75% system efficiency).
Got your battery and array sizes? Next, size your MPPT charge controller to match them.
How this is calculated
Everything starts with your daily consumption. Each appliance uses its power draw in watts multiplied by the hours it runs per day, giving watt hours. We then divide the total by 85% to cover inverter and wiring losses, because energy stored in the battery never reaches your appliances in full.
Days of autonomy is how long your battery must carry the load with no solar input at all, which in the UK usually means a run of heavily overcast days. One day suits weekend and summer use, while two or three days gives a comfortable buffer for permanent off-grid living.
Usable depth of discharge matters just as much as rated capacity. A LiFePO4 battery can safely deliver around 90% of its rating, but an AGM lead-acid battery should only be discharged to 50% to protect its lifespan. That is why the same daily demand needs a far larger lead-acid bank.
For the solar array we use UK peak sun hours, the equivalent number of hours of full-strength sunshine per day. The year-round UK average is about 2.8 hours, rising to roughly 4.5 in summer and falling to around 1.2 in the depths of winter, and we apply a 75% system efficiency factor for panel angle, temperature, charge controller losses and cloud.
Frequently asked questions
How many solar panels do I need?
Divide your daily watt-hours by UK peak sun hours (about 2.8 as a year-round average, far less in winter), then divide by your panel wattage and apply roughly a 75 percent efficiency factor for angle, temperature and cloud. The calculator applies all of this automatically.
What size battery bank should I get?
Take your daily watt-hours, multiply by your days of autonomy, then divide by your battery’s usable depth of discharge. LiFePO4 gives about 90 percent usable, lead-acid only about 50 percent, so the same demand needs a much larger lead-acid bank.
How many days of autonomy do I need?
One day suits weekend or summer use. Two to three days gives a comfortable buffer for permanent off-grid living through a run of overcast UK days. More autonomy means a bigger, costlier battery bank.
Does battery chemistry change the sizing?
Significantly. Because LiFePO4 can be discharged to around 90 percent and lead-acid only to 50 percent, a lead-acid bank needs almost double the rated capacity for the same usable energy. LiFePO4 also lasts far more cycles.
