Aziz Bench

Off-grid solar calculator

List what you need to power, then set the battery and the sun hours of the worst month you want to cover. The calculator sizes the battery, the panels and the charge controller, and shows how the battery copes with a few days without sun.

Battery
54Ah
12 V · 650 Wh · LiFePO4 lithium
Solar panels
200W
2 × 100 W
Charge controller
20A
MPPT · 16.7 A
Daily use
260 Wh
Solar on a design day
433 Wh
Refill after 2 days without sun
3 days
Battery charge: days without sun, then recovery
Loads
W
h
144 Wh
W
h
96 Wh
W
h
20 Wh
System
Battery voltage
%
W

Used only for loads marked AC. The standby draw counts for the whole day, because the inverter stays on.

Battery
%
%
days

Days without sun the battery must cover on its own. Two or three days is common for systems used all year.

Solar
h

Daily solar energy on the panel, in kWh/m², for the worst month you want to cover. Look it up for your location on PVGIS or PVWatts.

%

Heat, dirt, wiring and ageing: 15 to 25 % is typical.

Charge controller
%

Spare power to refill the battery after cloudy days.

Where the energy goes

    Components

    What to consider

    Plain text, ready for a forum post or a quote request.

    Every load is assumed to run from the battery, which slightly oversizes the battery when loads run during the day. Panels are sized to cover the daily use in the design month after losses and battery charge efficiency. The effect of temperature on battery capacity is not modelled. Before buying, check that the panel string voltage stays within the controller’s maximum input voltage.

    How the system is sized

    Everything starts from daily energy: the watts of each load times the hours it runs. Loads on the inverter count a little more because of its losses, and its standby draw adds up over 24 hours. The battery must cover that energy for the days without sun you choose, using only the share of its capacity it can safely deliver. The panels must put the same energy back in an average day of the worst month, after losses.

    E_day   = Σ (W × h) + AC loads ÷ inverter efficiency + inverter standby × 24 h
battery = E_day × days of autonomy ÷ usable depth            (Wh, then ÷ V for Ah)
panels  = E_day ÷ charge efficiency ÷ (sun hours × (1 − losses) × controller factor) × (1 + margin)
MPPT    = panel watts ÷ battery voltage                           (A)

    Peak sun hours: the number that matters most

    Peak sun hours are the daily solar energy on the panel expressed as hours of full 1000 W/m² sun. 3 peak sun hours means the panel collects as much as 3 hours at its rated irradiance, spread over the whole day. The value changes a lot with season and latitude: in winter it can fall below 2 hours in central and northern Europe, while in summer it is often 5 to 6.

    Size for the worst month you want the system to cover. For a cabin used in summer, that might be September; for a sensor station that must run all year, December. PVGIS, from the European Commission, and PVWatts, from the US Department of Energy, both cover most of the world and give monthly values for your exact location and panel tilt.

    LiFePO4 or lead-acid

    A LiFePO4 battery can deliver 80 % or more of its rated capacity for thousands of cycles, while lead-acid batteries last much longer if they rarely go below 50 %. That is why a lead-acid bank needs to be rated at roughly 1.6 times the size of a LiFePO4 bank for the same usable energy. Lead still has two advantages: it costs less up front and it can be charged below freezing, which LiFePO4 can’t.

    MPPT or PWM

    A PWM controller connects the panel straight to the battery, so a 36-cell panel works at about 13 V instead of its 18 V maximum power point and can lose up to about a quarter of its output. An MPPT controller converts the panel’s higher voltage down to the battery voltage and keeps nearly all of it. PWM is still fine for a single small panel; above about 200 W, or with 60- and 72-cell panels, MPPT pays for itself.

    Typical consumption of common loads

    LoadPowerHours a dayEnergy a daySupply
    4G router6 W24 h144 WhDC
    IP camera4 W24 h96 WhDC
    LED light5 W4 h20 WhDC
    Raspberry Pi 43.5 W24 h84 WhDC
    ESP32 board, Wi-Fi always on0.5 W24 h12 WhDC
    LoRaWAN gateway4 W24 h96 WhDC
    12 V compressor fridge (average)15 W24 h360 WhDC
    12 V water pump60 W0.5 h30 WhDC
    Phone charging10 W2 h20 WhDC
    Laptop charger45 W3 h135 WhAC (inverter)
    Wi-Fi router on its mains adapter8 W24 h192 WhAC (inverter)
    TV40 W3 h120 WhAC (inverter)

    Typical values. Read the label of your devices or measure them with a DC power meter, because consumption varies a lot between models.

    Frequently asked questions

    How many solar panels do I need to run my loads?

    Add up the daily watt-hours of your loads and divide by the peak sun hours of the worst month you want to cover, then allow for about 20 % of losses and some spare margin. For example, 260 Wh a day with 3 sun hours needs roughly 150 W of panels with an MPPT controller, which in practice means two 100 W panels.

    How big should my off-grid battery be?

    Multiply your daily use by the days of autonomy you want, then divide by the usable depth of discharge. 260 Wh a day for 2 days on LiFePO4 at 80 % usable needs 650 Wh, about 54 Ah at 12 V. The same system on lead-acid at 50 % needs about 87 Ah.

    Should I choose an MPPT or a PWM charge controller?

    MPPT for most systems: it can harvest up to about a quarter more energy from the same panels, and it also works with higher-voltage panels, as long as they stay within the controller’s input limit. PWM only makes sense for a single small 36-cell panel on a 12 V battery, where the lower price outweighs the lost energy.

    What are peak sun hours?

    They are the daily solar energy on the panel, in kWh per square metre, expressed as hours of full sun at 1000 W/m². They depend on location, season and panel tilt, and they are the most important input for sizing the panels.

    Why does the inverter use so much energy?

    An inverter draws power as long as it is on, even with nothing connected. A small one at 8 W uses 192 Wh a day, which can be more than the loads it powers. Run small loads on DC with 12 V adapters and switch the inverter off when you don’t need it.

    Can I use a 24 V battery instead of 12 V?

    Yes, and above about 1 kW of panels or inverter it is the better choice: the current halves, so cables, fuses and the controller can be smaller. The energy in watt-hours stays the same, so a 24 V battery needs half the amp-hours of a 12 V one.