Energy and power are different

Daily energy in kilowatt-hours measures consumption accumulated over time. Peak load in kilowatts measures simultaneous power. A household can use 15 kWh per day yet briefly demand 8 kW when a kettle, geyser, oven and pump operate together. Panels and batteries address energy, while inverter and wiring must also handle power and surge.

Use interval data from a smart meter or energy monitor where available. A monthly bill divided by days gives average energy but hides timing. Record which loads must operate during an outage and which can be shifted or switched off.

PV array estimate

PV array kilowatt-peak is daily energy divided by peak-sun hours and overall efficiency. With 15 kWh, five peak-sun hours and 75% efficiency, the estimate is 4 kWp. Peak-sun hours are an energy-equivalent solar resource, not the number of daylight hours.

The efficiency factor combines temperature, inverter, wiring, soiling, mismatch and other losses. It is a scenario rather than a measured site performance ratio. Roof orientation, tilt and shading can change yield by hour and season, so a professional simulation should use the location and array layout.

Seasonal and weather variation

A system sized from an annual-average sun value can underperform during a cloudy season or high-demand winter period. Compare monthly solar resource with monthly load. Off-grid design needs a reliability criterion and weather sequence, not only an average day.

Grid-connected households may import during low generation and export during surplus, subject to tariff and approval. Backup systems must decide how long essential loads should survive without solar or grid. A larger array, more battery, generator or load control can address different parts of that problem.

Battery energy

Nominal battery energy is daily covered energy times autonomy days divided by usable depth of discharge. At 15 kWh, one day and 80% usable depth, the estimate is 18.75 kWh nominal. This arithmetic does not include battery round-trip loss, reserve state of charge or degradation margin unless they are reflected in inputs.

Battery manufacturers specify usable energy, continuous and surge power, cycle conditions, temperature limits, warranty throughput and compatible inverters. Two batteries with equal kWh can support different loads. Use the approved system configuration and battery-management requirements.

Amp-hour benchmark

Amp-hours are estimated by multiplying battery kWh by 1000 and dividing by nominal voltage. This can help interpret low-voltage battery specifications, but it does not make packs at different voltages interchangeable. Series and parallel configuration changes voltage and amp-hours under strict compatibility rules.

Modern lithium systems are often best compared by usable kWh and power within a compatible ecosystem. Do not design battery strings from this Ah number. Incorrect wiring can cause fire, shock, equipment damage or uncontrolled current.

Inverter sizing

The page applies 25% headroom to entered simultaneous peak load as a benchmark. Real selection must consider continuous rating at temperature, motor and compressor surge, power factor, phase arrangement, neutral requirements and whether grid pass-through has a separate limit.

Add actual concurrent loads rather than appliance nameplate values that never operate together, but do not omit automatic loads such as pumps or refrigeration. Load shedding through smart relays can reduce inverter size while preserving essentials. Test the changeover behaviour with sensitive equipment.

Geysers, stoves and heating

Resistance heating consumes large energy and power. Moving water heating to solar thermal, heat pump or controlled daytime operation can reduce battery demand. Gas and other fuels introduce separate ventilation, safety and cost considerations and should not be adopted solely to shrink an electrical quote.

Decide which high-power loads are excluded during an outage and communicate that to every user. A bypassed essential-load board can reconnect them unexpectedly. The electrical design and labels should match the operating plan.

Roof and site assessment

A roof must have usable unshaded area, suitable structure, safe access and compliant mounting. Nearby trees, chimneys and parapets can create string mismatch and hot spots. Future construction or tree growth can change shading. Ground arrays add land, security and cable considerations.

Panel wattage alone does not determine how many fit. Module dimensions, fire access, edge setbacks, roof zones and maintenance paths matter. A structural and electrical professional should approve the final layout, especially on old, damaged or lightweight roofs.

South African compliance

Grid-connected generation can require municipal or utility application, approved equipment, metering changes and commissioning. National and local requirements can differ and change. An installation that can technically export should not be connected without the required approval and protection.

Use appropriately registered professionals and obtain certificates, drawings, settings, warranties and commissioning data. Confirm insurance requirements. Do not accept exposed battery terminals, undocumented protection changes or an inverter connected through an unsafe plug arrangement.

Quote comparison

Give installers the same load profile, essential-load list, autonomy target and site constraints. Ask each quote to state expected monthly yield, usable battery energy, continuous and surge power, warranty, excluded work and approval scope. Compare delivered capability rather than panel count.

Use the calculator to ask why quotes differ. A larger result may include winter reliability or degradation margin; a smaller result may assume grid support and load shifting. Require those assumptions in writing. The cheapest configuration can be costly if it cannot support the actual peak or loses warranty through incompatibility.

Turn the estimate into a load schedule

List each appliance, rated and measured watts, hours used, starting surge and whether it must run during an outage. Separate daytime loads from overnight energy and identify loads that can be deferred. Give this schedule, recent electricity data and site photographs to a competent designer so protection, cable sizing and equipment limits can be checked.