Independent engineering · Cornwall & Devon
Energy planning: understand the numbers
Compare useful energy, power and timing before choosing equipment. These worked examples explain the calculators; they are not installation results or current tariff offers.
By Energetika Ltd · Examples checked 2 November 2026. Change the inputs for your site.
When does a heat pump cost less to run?
Compare the cost of the same useful heat. Heat-pump electricity is useful heat divided by SCOP. Boiler fuel is useful heat divided by operating efficiency. SCOP describes performance over the period; it is not the COP at one favourable operating point.
Illustrative example: 10,000kWh of useful heat, electricity at 26p/kWh, gas at 8p/kWh and a 90% efficient boiler. The gas energy cost is about £889. At SCOP 3.5, the heat-pump energy cost is about £743: approximately £146 less. Break-even SCOP is 2.925. These are assumed prices, not a supplier quote or measured saving.
Standing charges, maintenance and installation cost are separate. Cheap-rate tariffs need realistic consumption shares; the cheapest rate alone is not an annual electricity cost. A commercial or agricultural site may need interval modelling.
Compare your heating costs → · Heat-pump design & installation →
Why does outage cover need more storage?
A daily-shifting battery recharges regularly; an outage battery must cover the gap until another source is available. Define delivered electricity demand first, including heating electricity where relevant. A heat pump’s useful heat demand must be divided by its COP for the period to estimate electrical demand.
For an illustrative 20kWh/day delivered load, 95% discharge efficiency, continuous 80W DC inverter self-draw and 90% usable capacity, one day needs about 25.5kWh nominal storage; 14 days need about 357.4kWh with no recharge. This assumes a full starting charge and 10% minimum reserve. Inverter self-draw contributes 1.92kWh/day before the capacity allowance.
If only 75% of that delivered demand is shifted each day, with the same losses and self-draw, the daily-shifting estimate is about 19.7kWh nominal, provided the charging window and supply can recharge it. A generator can reduce energy depletion, but requires a stated running schedule and fuel assumptions.
Capacity in kWh does not prove the inverter can start or run the equipment. Backed-up circuits, safe isolation and compatible equipment need design.
Explore battery capacity and generator assumptions → · Battery storage advice →
Why annual generation is not the same as usable solar energy
Solar helps when its output matches demand, can be stored for later, or can be exported on acceptable terms. An annual kWh total does not establish winter heating supply, outage autonomy or a financial saving for every generated unit.
Roof or ground layout, shading, losses, demand timing, battery operation and export limits matter. A home’s daytime-use assumption should not be copied unchanged to a farm or commercial site. Interval load data and a site-specific yield estimate provide a stronger basis for design.
Explore the annual solar energy balance → · Solar PV design →
Method references: Energy Saving Trust: air-source heat pumps and solar panels. Numerical examples above are explicit arithmetic assumptions, not figures attributed to these sources.